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# Programming-Idioms.org

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Rust
1
Print a literal string on standard output
``println!("Hello World");``
2
Loop to execute some code a constant number of times
``for _ in 0..10 { println!("Hello"); }``
Alternative implementation:
``print!("{}", "Hello\n".repeat(10));``
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
``````fn finish(name: &str) {
println!("My job here is done. Goodbye {}", name);
}``````
4
Create a function which returns the square of an integer
``fn square(x : u32) -> u32 { x * x }``
5
Declare a container type for two floating-point numbers x and y
``````struct Point {
x: f64,
y: f64,
}``````
Alternative implementation:
``struct Point(f64, f64);``
6
Do something with each item x of the list (or array) items, regardless indexes.
``````for x in items {
do_something(x);
}``````
Alternative implementation:
``items.into_iter().for_each(|x| do_something(x));``
7
Print each index i with its value x from an array-like collection items
``````for (i, x) in items.iter().enumerate() {
println!("Item {} = {}", i, x);
}``````
Alternative implementation:
``````items.iter().enumerate().for_each(|(i, x)| {
println!("Item {} = {}", i, x);
})``````
8
Create a new map object x, and provide some (key, value) pairs as initial content.
``````let mut x = BTreeMap::new();
x.insert("one", 1);
x.insert("two", 2);``````
Alternative implementation:
``````let x: HashMap<&str, i32> = [
("one", 1),
("two", 2),
].into_iter().collect();``````
9
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
``````struct BinTree<T> {
value: T,
left: Option<Box<BinTree<T>>>,
right: Option<Box<BinTree<T>>>,
}``````
10
Generate a random permutation of the elements of list x
``````let mut rng = StdRng::new().unwrap();
rng.shuffle(&mut x);``````
Alternative implementation:
``````let mut rng = thread_rng();
x.shuffle(&mut rng);``````
11
The list x must be non-empty.
``x[rand::thread_rng().gen_range(0..x.len())]``
Alternative implementation:
``````let mut rng = rand::thread_rng();
let choice = x.choose(&mut rng).unwrap();``````
12
Check if the list contains the value x.
list is an iterable finite container.
``list.contains(&x);``
Alternative implementation:
``list.iter().any(|v| v == &x)``
Alternative implementation:
``(&list).into_iter().any(|v| v == &x)``
Alternative implementation:
``list.binary_search(&x).is_ok()``
13
Access each key k with its value x from an associative array mymap, and print them.
``````for (k, x) in &mymap {
println!("Key={key}, Value={val}", key=k, val=x);
}``````
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
``thread_rng().gen_range(a..b);``
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
``````fn pick(a: i32, b: i32) -> i32 {
let between = Range::new(a, b);
let mut rng = rand::thread_rng();
between.ind_sample(&mut rng)
}
``````
Alternative implementation:
``Uniform::new_inclusive(a, b).sample(&mut rand::thread_rng())``
16
Call a function f on every node of binary tree bt, in depth-first infix order
``````fn depthFirstTraverse<T>(bt: &mut BiTree<T>, f: fn(&mut BiTree<T>)) {
if let Some(left) = &mut bt.left {
depthFirstTraverse(left, f);
}

f(bt);

if let Some(right) = &mut bt.right {
depthFirstTraverse(right, f);
}
}``````
17
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent.
``````struct Node<T> {
value: T,
children: Vec<Node<T>>,
}``````
18
Call a function f on every node of a tree, in depth-first prefix order
``````pub struct Tree<V> {
children: Vec<Tree<V>>,
value: V
}

impl<V> Tree<V> {
pub fn dfs<F: Fn(&V)>(&self, f: F) {
self.dfs_helper(&f);
}
fn dfs_helper<F: Fn(&V)>(&self, f: &F) {
(f)(&self.value);
for child in &self.children {
child.dfs_helper(f)
}
}
// ...
}``````
19
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
``let y: Vec<_> = x.into_iter().rev().collect();``
Alternative implementation:
``x.reverse();``
20
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
``````fn search<T: Eq>(m: &Vec<Vec<T>>, x: &T) -> Option<(usize, usize)> {
for (i, row) in m.iter().enumerate() {
for (j, column) in row.iter().enumerate() {
if *column == *x {
return Some((i, j));
}
}
}

None
}``````
21
Swap the values of the variables a and b
``````std::mem::swap(&mut a, &mut b);
``````
Alternative implementation:
``let (a, b) = (b, a);``
22
Extract the integer value i from its string representation s (in radix 10)
``let i = s.parse::<i32>().unwrap();``
Alternative implementation:
``let i: i32 = s.parse().unwrap_or(0);``
Alternative implementation:
``````let i = match s.parse::<i32>() {
Ok(i) => i,
Err(_e) => -1,
};``````
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
``let s = format!("{:.2}", x);``
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
``let s = "ネコ";``
25
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
``````let (send, recv) = channel();

loop {
let msg = recv.recv().unwrap();
println!("Hello, {:?}", msg);
}
});

send.send("Alan").unwrap();``````
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
``let mut x = vec![vec![0.0f64; N]; M];``
Alternative implementation:
``let mut x = [[0.0; N] ; M];``
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
``let x = vec![vec![vec![0.0f64; p]; n]; m];``
Alternative implementation:
``let x = [[[0.0f64; P]; N]; M];``
28
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
``items.sort_by(|a,b| a.p.cmp(&b.p));``
Alternative implementation:
``items.sort_by_key(|x| x.p);``
29
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
``items.remove(i)``
30
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
``````let threads: Vec<_> = (0..1000).map(|i| {
}).collect();

}``````
Alternative implementation:
``(0..1000).into_par_iter().for_each(f);``
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
``````fn f(n: u32) -> u32 {
if n < 2 {
1
} else {
n * f(n - 1)
}
}``````
Alternative implementation:
``````fn factorial(num: u64) -> u64 {
match num {
0 | 1 => 1,
_ => factorial(num - 1) * num,
}
}``````
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
``````fn exp(x: u64, n: u64) -> u64 {
match n {
0 => 1,
1 => x,
i if i % 2 == 0 => exp(x * x, n / 2),
_ => x * exp(x * x, (n - 1) / 2),
}
}``````
Alternative implementation:
``````fn exp(x: u64, n: u32) -> u64 {
x.pow(n)
}``````
33
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write.
``````let mut x = x.lock().unwrap();
*x = f(x);``````
34
Declare and initialize a set x containing unique objects of type T.
``let x: HashSet<T> = HashSet::new();``
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f
``````fn compose<'a, A, B, C, G, F>(f: F, g: G) -> Box<Fn(A) -> C + 'a>
where F: 'a + Fn(A) -> B, G: 'a + Fn(B) -> C
{
Box::new(move |x| g(f(x)))
}``````
Alternative implementation:
``````fn compose<A, B, C>(f: impl Fn(A) -> B, g: impl Fn(B) -> C) -> impl Fn(A) -> C {
move |x| g(f(x))
}``````
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
``````fn compose<'a, A, B, C, G, F>(f: F, g: G) -> Box<Fn(A) -> C + 'a>
where F: 'a + Fn(A) -> B, G: 'a + Fn(B) -> C
{
Box::new(move |x| g(f(x)))
}``````
Alternative implementation:
``````fn compose<A, B, C>(f: impl Fn(A) -> B, g: impl Fn(B) -> C) -> impl Fn(A) -> C {
move |x| g(f(x))
}``````
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
``````fn add(a: u32, b: u32) -> u32 {
a + b
}

let add5 = move |x| add(5, x);
``````
38
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
``let t = s.graphemes(true).skip(i).take(j - i).collect::<String>();``
Alternative implementation:
``let t = s.substring(i, j);``
Alternative implementation:
``````let mut iter = s.grapheme_indices(true);
let i_idx = iter.nth(i).map(|x|x.0).unwrap_or(0);
let j_idx = iter.nth(j-i).map(|x|x.0).unwrap_or(0);
let t = s[i_idx..j_idx];``````
39
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise.
``let ok = s.contains(word);``
41
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
``let t = s.chars().rev().collect::<String>();``
Alternative implementation:
``let t: String = s.chars().rev().collect();``
42
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b.
``````'outer: for va in &a {
for vb in &b {
if va == vb {
continue 'outer;
}
}
println!("{}", va);
}
``````
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
``````'outer: for v in m {
'inner: for i in v {
if i < 0 {
println!("Found {}", i);
break 'outer;
}
}
}``````
44
Insert the element x at position i in the list s. Further elements must be shifted to the right.
``s.insert(i, x);``
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
``thread::sleep(time::Duration::from_secs(5));``
46
Create the string t consisting of the 5 first characters of the string s.
Make sure that multibyte characters are properly handled.
``let t = s.char_indices().nth(5).map_or(s, |(i, _)| &s[..i]);``
Alternative implementation:
``let t = s.chars().take(5).collect::<String>();``
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
``````let last5ch = s.chars().count() - 5;
let t: String = s.chars().skip(last5ch).collect();``````
Alternative implementation:
``````let s = "a̐éö̲\r\n";
let t = s.grapheme_indices(true).rev().nth(5).map_or(s, |(i,_)|&s[i..]);``````
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
``````let s = "line 1
line 2
line 3";``````
Alternative implementation:
``````let s = r#"Huey
Dewey
Louie"#;``````
49
Build list chunks consisting in substrings of the string s, separated by one or more space characters.
``let chunks: Vec<_> = s.split_whitespace().collect();``
Alternative implementation:
``let chunks: Vec<_> = s.split_ascii_whitespace().collect();``
Alternative implementation:
``let chunks: Vec<_> = s.split(' ').collect();``
50
Write a loop that has no end clause.
``````loop {
// Do something
}``````
51
Determine whether the map m contains an entry for the key k
``m.contains_key(&k)``
52
Determine whether the map m contains an entry with the value v, for some key.
``let does_contain = m.values().any(|&val| *val == v);``
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
``let y = x.join(", ");``
54
Calculate the sum s of the integer list or array x.
``x.iter().sum()``
Alternative implementation:
``let s = x.iter().sum::<i32>();``
55
Create the string representation s (in radix 10) of the integer value i.
``let s = i.to_string();``
Alternative implementation:
``let s = format!("{}", i);``
56
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished".
``````let threads: Vec<_> = (0..1000).map(|i| thread::spawn(move || f(i))).collect();

for t in threads {
t.join();
}
``````
57
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
``let y: Vec<_> = x.iter().filter(p).collect();``
58
Create the string lines from the content of the file with filename f.
``````let mut file = File::open(f)?;
let mut lines = String::new();
Alternative implementation:
``let lines = fs::read_to_string(f).expect("Can't read file.");``
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
``eprintln!("{} is negative", x);``
60
Assign to x the string value of the first command line parameter, after the program name.
``````let first_arg = env::args().skip(1).next();

let fallback = "".to_owned();
let x = first_arg.unwrap_or(fallback);``````
Alternative implementation:
``let x = env::args().nth(1).unwrap_or("".to_string());``
61
Assign to the variable d the current date/time value, in the most standard type.
``let d = time::now();``
Alternative implementation:
``let d = SystemTime::now();``
62
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x.
``let i = x.find(y);``
63
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping.
``let x2 = x.replace(&y, &z);``
64
Assign to x the value 3^247
``````let a = 3.to_bigint().unwrap();
let x = num::pow(a, 247);``````
65
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%"
``let s = format!("{:.1}%", 100.0 * x);``
66
Calculate the result z of x power n, where x is a big integer and n is a positive integer.
``let z = num::pow(x, n);``
67
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
``````fn binom(n: u64, k: u64) -> BigInt {
let mut res = BigInt::one();
for i in 0..k {
res = (res * (n - i).to_bigint().unwrap()) /
(i + 1).to_bigint().unwrap();
}
res
}``````
68
Create an object x to store n bits (n being potentially large).
``let mut x = vec![false; n];``
69
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time.
``````let s = 32;
let mut rng = StdRng::seed_from_u64(s);``````
70
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
``````let d = SystemTime::now()
.duration_since(SystemTime::UNIX_EPOCH)
.expect("Duration since UNIX_EPOCH failed");
let mut rng = StdRng::seed_from_u64(d.as_secs());``````
71
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
``println!("{}", env::args().skip(1).collect::<Vec<_>>().join(" "));``
Alternative implementation:
``println!("{}", std::env::args().skip(1).format(" "));``
73
Create a factory named fact for any sub class of Parent and taking exactly one string str as constructor parameter.
``````fn fact<Parent: std::str::FromStr>(str: String, _: Parent) -> Parent where <Parent as FromStr>::Err: Debug
{
return str.parse::<Parent>().unwrap();
}``````
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
``let x = a.gcd(&b);``
75
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
``let x = a.lcm(&b);``
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
``let s = format!("{:b}", x);``
Alternative implementation:
``let s = format!("{x:b}");``
77
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
``````let mut x = Complex::new(-2, 3);
x *= Complex::i();``````
78
Execute a block once, then execute it again as long as boolean condition c is true.
``````loop {
doStuff();
if !c { break; }
}``````
Alternative implementation:
``````while {
doStuff();
c
} { /* EMPTY */ }``````
79
Declare the floating point number y and initialize it with the value of the integer x .
``let y = x as f64;``
80
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser).
``let y = x as i32;``
81
Declare the integer y and initialize it with the rounded value of the floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
``let y = x.round() as i64;``
82
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
``let c = s.matches(t).count();``
83
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
``let r = Regex::new(r"htt+p").unwrap();``
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
``let c = i.count_ones();``
85
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value.
``````fn adding_will_overflow(x: usize, y: usize) -> bool {
}``````
86
Write the boolean function multiplyWillOverflow which takes two integers x, y and returns true if (x*y) overflows.

An overflow may reach above the max positive value, or below the min negative value.
``````fn multiply_will_overflow(x: i64, y: i64) -> bool {
x.checked_mul(y).is_none()
}``````
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
``std::process::exit(0);``
88
Create a new bytes buffer buf of size 1,000,000.
``let buf: Vec<u8> = Vec::with_capacity(1000000);``
89
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem.
``````enum CustomError { InvalidAnswer }

fn do_stuff(x: i32) -> Result<i32, CustomError> {
if x != 42 {
} else {
Ok(x)
}
}``````
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
``````struct Foo {
x: usize
}

impl Foo {
pub fn new(x: usize) -> Self {
Foo { x }
}

pub fn x<'a>(&'a self) -> &'a usize {
&self.x
}

pub fn bar(&mut self) {
self.x += 1;
}
}``````
91
Read from the file data.json and write its content into the object x.
Assume the JSON data is suitable for the type of x.
``let x = ::serde_json::from_reader(File::open("data.json")?)?;``
92
Write the contents of the object x into the file data.json.
``::serde_json::to_writer(&File::create("data.json")?, &x)?``
93
Implement the procedure control which receives one parameter f, and runs f.
``````fn control(f: impl Fn()) {
f();
}``````
94
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages.
``````fn type_of<T>(_: &T) -> &'static str {
std::intrinsics::type_name::<T>()
}

println!("{}", type_of(&x));``````
95
Assign to variable x the length (number of bytes) of the local file at path.
``let x = fs::metadata(path)?.len();``
Alternative implementation:
``let x = path.metadata()?.len();``
96
Set the boolean b to true if string s starts with prefix prefix, false otherwise.
``let b = s.starts_with(prefix);``
97
Set boolean b to true if string s ends with string suffix, false otherwise.
``let b = s.ends_with(suffix);``
98
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00
``let d = NaiveDateTime::from_timestamp(ts, 0);``
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
``Utc::today().format("%Y-%m-%d")``
Alternative implementation:
``````let format = format_description!("[year]-[month]-[day]");
let x = d.format(&format).expect("Failed to format the date");``````
100
Sort elements of array-like collection items, using a comparator c.
``items.sort_by(c);``
101
Make an HTTP request with method GET to the URL u, then store the body of the response in the string s.
``````let client = Client::new();
let s = client.get(u).send().and_then(|res| res.text())?;``````
Alternative implementation:
``let s = ureq::get(u).call().into_string()?;``
Alternative implementation:
``````let mut response = reqwest::blocking::get(u)?;
let mut s = String::new();
102
Make an HTTP request with method GET to the URL u, then store the body of the response in the file result.txt. Try to save the data as it arrives if possible, without having all its content in memory at once.
``````let client = Client::new();
match client.get(&u).send() {
Ok(res) => {
let file = File::create("result.txt")?;
::std::io::copy(res, file)?;
},
Err(e) => eprintln!("failed to send request: {}", e),
};``````
105
Assign to the string s the name of the currently executing program (but not its full path).
``````fn get_exec_name() -> Option<String> {
std::env::current_exe()
.ok()
.and_then(|pb| pb.file_name().map(|s| s.to_os_string()))
.and_then(|s| s.into_string().ok())
}

fn main() -> () {
let s = get_exec_name().unwrap();
println!("{}", s);
}``````
Alternative implementation:
``````let s = std::env::current_exe()
.expect("Can't get the exec path")
.file_name()
.expect("Can't get the exec name")
.to_string_lossy()
.into_owned();``````
106
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
``let dir = env::current_dir().unwrap();``
107
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.)
``````let dir = std::env::current_exe()?
.canonicalize()
.expect("the current exe should exist")
.parent()
.expect("the current exe should be a file")
.to_string_lossy()
.to_owned();``````
109
Set n to the number of bytes of a variable t (of type T).
``let n = std::mem::size_of::<T>();``
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
``let blank = s.trim().is_empty();``
Alternative implementation:
``let blank = s.chars().all(|c| c.is_whitespace());``
111
From current process, run program x with command-line parameters "a", "b".
``````let output = Command::new("x")
.args(&["a", "b"])
.spawn()
.expect("failed to execute process");``````
Alternative implementation:
``````let output = Command::new("x")
.args(&["a", "b"])
.output()
.expect("failed to execute process");``````
Alternative implementation:
``````let output = Command::new("x")
.args(&["a", "b"])
.status()
.expect("failed to execute process");``````
112
Print each key k with its value x from an associative array mymap, in ascending order of k.
``````for (k, x) in mymap {
println!("({}, {})", k, x);
}``````
113
Print each key k with its value x from an associative array mymap, in ascending order of x.
Multiple entries may exist for the same value x.
``````for (k, x) in mymap.iter().sorted_by_key(|x| x.1) {
println!("[{},{}]", k, x);
}``````
Alternative implementation:
``````let mut items: Vec<_> = mymap.iter().collect();
items.sort_by_key(|item| item.1);
for (k, x) in items {
println!("[{},{}]", k, x);
}``````
114
Set boolean b to true if objects x and y contain the same values, recursively comparing all referenced elements in x and y.
Tell if the code correctly handles recursive types.
``let b = x == y;``
115
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise.
``let b = d1 < d2;``
116
Remove all occurrences of string w from string s1, and store the result in s2.
``s2 = s1.replace(w, "");``
Alternative implementation:
``let s2 = str::replace(s1, w, "");``
117
Set n to the number of elements of the list x.
``````let n = x.len();

``````
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
``let y: HashSet<_> = x.into_iter().collect();``
119
Remove duplicates from the list x.
Explain if the original order is preserved.
``````x.sort();
x.dedup();``````
Alternative implementation:
``let dedup: Vec<_> = x.iter().unique().collect();``
120
Read an integer value from the standard input into the variable n
``````fn get_input() -> String {
let mut buffer = String::new();
buffer
}

let n = get_input().trim().parse::<i64>().unwrap();``````
Alternative implementation:
``````let mut input = String::new();
let n: i32 = input.trim().parse().unwrap();``````
Alternative implementation:
``````let n: i32 = std::io::stdin()
.lock()
.lines()
.next()
.expect("stdin should be available")
.expect("couldn't read from stdin")
.trim()
.parse()
.expect("input was not an integer");``````
Alternative implementation:
``````let n: i32 = read!();
``````
121
Listen UDP traffic on port p and read 1024 bytes into buffer b.
``````let mut b = [0 as u8; 1024];
let sock = UdpSocket::bind(("localhost", p)).unwrap();
sock.recv_from(&mut b).unwrap();``````
122
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
``````enum Suit {
Hearts,
Diamonds,
Clubs,
}``````
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
``assert!(is_consistent);``
124
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists.
``a.binary_search(&x).unwrap_or(-1);``
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
``````let start = Instant::now();
foo();
let duration = start.elapsed();
println!("{}", duration);
``````
126
Write a function foo that returns a string and a boolean value.
``````fn foo() -> (String, bool) {
(String::from("bar"), true)
}``````
127
Import the source code for the function foo body from a file "foobody.txt".
``````fn main() {
include!("foobody.txt");
}
``````
128
Call a function f on every node of a tree, in breadth-first prefix order
``````struct Tree<V> {
children: Vec<Tree<V>>,
value: V
}

impl<V> Tree<V> {
fn bfs(&self, f: impl Fn(&V)) {
let mut q = VecDeque::new();
q.push_back(self);

while let Some(t) = q.pop_front() {
(f)(&t.value);
for child in &t.children {
q.push_back(child);
}
}
}
}``````
129
Call the function f on every vertex accessible from the vertex start, in breadth-first prefix order
``````struct Vertex<V> {
value: V,
neighbours: Vec<Weak<RefCell<Vertex<V>>>>,
}

// ...

fn bft(start: Rc<RefCell<Vertex<V>>>, f: impl Fn(&V)) {
let mut q = vec![start];
let mut i = 0;
while i < q.len() {
let v = Rc::clone(&q[i]);
i += 1;
(f)(&v.borrow().value);
for n in &v.borrow().neighbours {
let n = n.upgrade().expect("Invalid neighbour");
if q.iter().all(|v| v.as_ptr() != n.as_ptr()) {
q.push(n);
}
}
}
}``````
130
Call th function f on every vertex accessible from the vertex v, in depth-first prefix order
``````struct Vertex<V> {
value: V,
neighbours: Vec<Weak<RefCell<Vertex<V>>>>,
}

// ...

fn dft_helper(start: Rc<RefCell<Vertex<V>>>, f: &impl Fn(&V), s: &mut Vec<*const Vertex<V>>) {
s.push(start.as_ptr());
(f)(&start.borrow().value);
for n in &start.borrow().neighbours {
let n = n.upgrade().expect("Invalid neighbor");
if s.iter().all(|&p| p != n.as_ptr()) {
Self::dft_helper(n, f, s);
}
}
}``````
131
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
``if c1 { f1() } else if c2 { f2() } else if c3 { f3() }``
Alternative implementation:
``````match true {
_ if c1 => f1(),
_ if c2 => f2(),
_ if c3 => f3(),
_ => (),
}``````
132
Run the procedure f, and return the duration of the execution of f.
``````let start = Instant::now();
f();
let duration = start.elapsed();``````
133
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise.
``````let re = Regex::new(&format!("(?i){}", regex::escape(word))).unwrap();
let ok = re.is_match(&s);``````
Alternative implementation:
``````let re =
RegexBuilder::new(&regex::escape(word))
.case_insensitive(true)
.build().unwrap();

let ok = re.is_match(s);``````
Alternative implementation:
``let ok = s.to_ascii_lowercase().contains(&word.to_ascii_lowercase());``
134
Declare and initialize a new list items, containing 3 elements a, b, c.
``let items = vec![a, b, c];``
135
Remove at most 1 item from list items, having the value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged.
``````if let Some(i) = items.first(&x) {
items.remove(i);
}``````
Alternative implementation:
``````if let Some(i) = items.iter().position(|item| *item == x) {
items.remove(i);
}``````
136
Remove all occurrences of the value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
``items = items.into_iter().filter(|&item| item != x).collect();``
Alternative implementation:
``items.retain(|&item| item != x);``
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
``````let chars_are_numeric: Vec<bool> = s.chars()
.map(|c|c.is_numeric())
.collect();
let b = !chars_are_numeric.contains(&false);``````
Alternative implementation:
``let b = s.chars().all(char::is_numeric);``
Alternative implementation:
``let b = s.bytes().all(|c| c.is_ascii_digit());``
138
Create a new temporary file on the filesystem.
``````let temp_dir = TempDir::new("prefix")?;
let temp_file = File::open(temp_dir.path().join("file_name"))?;``````
139
Create a new temporary folder on filesystem, for writing.
``let tmp = TempDir::new("prefix")?;``
140
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
``m.remove(&k);``
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
``````for i in item1.iter().chain(item2.iter()) {
print!("{} ", i);
}``````
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
``let s = format!("{:X}", x);``
143
Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
``````for pair in izip!(&items1, &items2) {
println!("{}", pair.0);
println!("{}", pair.1);
}``````
144
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS.
``````let b = std::path::Path::new(fp).exists();
``````
145
Print message msg, prepended by current date and time.

Explain what behavior is idiomatic: to stdout or stderr, and what the date format is.
``eprintln!("[{}] {}", humantime::format_rfc3339_seconds(std::time::SystemTime::now()), msg);``
146
Extract floating point value f from its string representation s
``let f = s.parse::<f32>().unwrap();``
Alternative implementation:
``let f: f32 = s.parse().unwrap();``
147
Create string t from string s, keeping only ASCII characters
``let t = s.replace(|c: char| !c.is_ascii(), "");``
Alternative implementation:
``let t = s.chars().filter(|c| c.is_ascii()).collect::<String>();``
148
Read a list of integer numbers from the standard input, until EOF.
``````let mut string = String::new();
let result = string
.lines()
.map(i32::from_str)
.collect::<Result<Vec<_>, _>>();``````
150
Remove the last character from the string p, if this character is a forward slash /
``if p.ends_with('/') { p.pop(); }``
151
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string!
``````let p = if ::std::path::is_separator(p.chars().last().unwrap()) {
&p[0..p.len()-1]
} else {
p
};``````
Alternative implementation:
``p = p.strip_suffix(std::path::is_separator).unwrap_or(p);``
152
Create string s containing only the character c.
``let s = c.to_string();``
153
Create the string t as the concatenation of the string s and the integer i.
``let t = format!("{}{}", s, i);``
Alternative implementation:
``let t = format!("{s}{i}");``
154
Find color c, the average between colors c1, c2.

c, c1, c2 are strings of hex color codes: 7 chars, beginning with a number sign # .
Assume linear computations, ignore gamma corrections.
``"Too long for text box, see online demo"``
155
Delete from filesystem the file having path filepath.
``let r = fs::remove_file(filepath);``
156
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i1000.
``let s = format!("{:03}", i);``
157
Initialize a constant planet with string value "Earth".
``const PLANET: &str = "Earth";``
158
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.
``````let mut rng = &mut rand::thread_rng();
let y = x.choose_multiple(&mut rng, k).cloned().collect::<Vec<_>>();``````
159
Define a Trie data structure, where entries have an associated value.
(Not all nodes are entries)
``````struct Trie {
val: String,
nodes: Vec<Trie>
}``````
160
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection.
``````match std::mem::size_of::<&char>() {
4 => f32(),
8 => f64(),
_ => {}
}``````
Alternative implementation:
``````#[cfg(target_pointer_width = "64")]
f64();

#[cfg(target_pointer_width = "32")]
f32();	``````
161
Multiply all the elements of the list elements by a constant c
``let elements = elements.into_iter().map(|x| c * x).collect::<Vec<_>>();``
Alternative implementation:
``elements.iter_mut().for_each(|x| *x *= c);``
162
execute bat if b is a program option and fox if f is a program option.
``````if let Some(arg) = ::std::env::args().nth(1) {
if &arg == "f" {
fox();
} else if &arg = "b" {
bat();
} else {
eprintln!("invalid argument: {}", arg),
}
} else {
eprintln!("missing argument");
}``````
Alternative implementation:
``````if let Some(arg) = ::std::env::args().nth(1) {
match arg.as_str() {
"f" => fox(),
"b" => box(),
_ => eprintln!("invalid argument: {}", arg),
};
} else {
eprintln!("missing argument");
}``````
163
Print all the list elements, two by two, assuming list length is even.
``````for pair in list.chunks(2) {
println!("({}, {})", pair[0], pair[1]);
}``````
164
Open the URL s in the default browser.
Set the boolean b to indicate whether the operation was successful.
``````webbrowser::open(s).expect("failed to open URL");
``````
165
Assign to the variable x the last element of the list items.
``let x = items[items.len()-1];``
Alternative implementation:
``let x = items.last().unwrap();``
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
``let ab = [a, b].concat();``
167
Create the string t consisting of the string s with its prefix p removed (if s starts with p).
``let t = s.trim_start_matches(p);``
Alternative implementation:
``let t = if s.starts_with(p) { &s[p.len()..] } else { s };``
Alternative implementation:
``let t = s.strip_prefix(p).unwrap_or(s);``
168
Create string t consisting of string s with its suffix w removed (if s ends with w).
``let t = s.trim_end_matches(w);``
Alternative implementation:
``let t = s.strip_suffix(w).unwrap_or(s);``
169
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
``let n = s.chars().count();``
170
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
``let n = mymap.len();``
171
Append the element x to the list s.
``s.push(x);``
172
Insert value v for key k in map m.
``m.insert(k, v);``
173
Number will be formatted with a comma separator between every group of thousands.
``println!("{}", 1000.separated_string());``
174
Make a HTTP request with method POST to the URL u
``````let client = reqwest::blocking::Client::new();
let mut response = client.post(u).body("abc").send()?;``````
175
From the array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit).
``let s = a.encode_hex::<String>();``
Alternative implementation:
``````let mut s = String::with_capacity(2 * n);
for byte in a {
write!(s, "{:02X}", byte)?;
}``````
Alternative implementation:
``let s = HEXLOWER.encode(&a);``
Alternative implementation:
``````fn byte_to_hex(byte: u8) -> (u8, u8) {
static HEX_LUT: [u8; 16] = [b'0', b'1', b'2', b'3', b'4', b'5', b'6', b'7', b'8', b'9', b'a', b'b', b'c', b'd', b'e', b'f'];

let upper = HEX_LUT[(byte >> 4) as usize];
let lower = HEX_LUT[(byte & 0xF) as usize];
(lower, upper)
}

let utf8_bytes: Vec<u8> = a.iter().copied().flat_map(|byte| {
let (lower, upper) = byte_to_hex(byte);
[upper, lower]
}).collect();
let s = unsafe { String::from_utf8_unchecked(utf8_bytes) };``````
176
From hex string s of 2n digits, build the equivalent array a of n bytes.
Each pair of hexadecimal characters (16 possible values per digit) is decoded into one byte (256 possible values).
``let a: Vec<u8> = Vec::from_hex(s).expect("Invalid Hex String");``
Alternative implementation:
``let a: Vec<u8> = decode(s).expect("Hex string should be valid");``
Alternative implementation:
``let a: Vec<u8> = HEXLOWER_PERMISSIVE.decode(&s.into_bytes()).unwrap();``
177
Construct a list L that contains all filenames that have the extension ".jpg" , ".jpeg" or ".png" in directory D and all its subdirectories.
``````let d = Path::new("/path/to/D");
let l: Vec<PathBuf> = d
.filter_map(|f| f.ok())
.filter(|f| match f.path().extension() {
None => false,
Some(ex) => ex == "jpg" || ex == "jpeg" || ex == "png"
})
.map(|f| f.path())
.collect();``````
178
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
``````struct Rect {
x1: i32,
x2: i32,
y1: i32,
y2: i32,
}

impl Rect {
fn contains(&self, x: i32, y: i32) -> bool {
return self.x1 < x && x < self.x2 && self.y1 < y && y < self.y2;
}
}``````
179
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
``````struct Rectangle {
x1: f64,
y1: f64,
x2: f64,
y2: f64,
}

impl Rectangle {
pub fn center(&self) -> (f64, f64) {
((self.x1 + self.x2) / 2.0, (self.y1 + self.y2) / 2.0)
}
}``````
180
Create the list x containing the contents of the directory d.

x may contain files and subfolders.
No recursive subfolder listing.
``let x = std::fs::read_dir(d).unwrap();``
Alternative implementation:
``let x = std::fs::read_dir(d)?.collect::<Result<Vec<_>, _>>()?;``
182
Output the source of the program.
``````fn main() {
let x = "fn main() {\n    let x = ";
let y = "print!(\"{}{:?};\n    let y = {:?};\n    {}\", x, x, y, y)\n}\n";
print!("{}{:?};
let y = {:?};
{}", x, x, y, y)
}``````
Alternative implementation:
``fn main(){print!("{},{0:?})}}","fn main(){print!(\"{},{0:?})}}\"")}``
Alternative implementation:
``````fn main() {
print!("{}", include_str!("main.rs"))
}
``````
184
Assign to variable t a string representing the day, month and year of the day after the current date.
``let t = chrono::Utc::now().date().succ().to_string();``
185
Schedule the execution of f(42) in 30 seconds.
``````sleep(Duration::new(30, 0));
f(42);
``````
186
Exit a program cleanly indicating no error to OS
``exit(0);``
188
Perform matrix multiplication of a real matrix a with nx rows and ny columns, a real matrix b with ny rows and nz columns and assign the value to a real matrix c with nx rows and nz columns.
``let c = a.dot(&b);``
189
Produce a new list y containing the result of the function T applied to all elements e of the list x that match the predicate P.
``````let y = x.iter()
.filter(P)
.map(T)
.collect::<Vec<_>>();``````
190
Declare an external C function with the prototype

void foo(double *a, int n);

and call it, passing an array (or a list) of size 10 to a and 10 to n.

Use only standard features of your language.
``````extern "C" {
/// # Safety
///
/// `a` must point to an array of at least size 10
fn foo(a: *mut libc::c_double, n: libc::c_int);
}

let mut a = [0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0];
let n = 10;
unsafe {
foo(a.as_mut_ptr(), n);
}``````
191
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case
``````if a.iter().any(|&elem| elem > x) {
f()
}``````
192
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time.
``let a = Decimal::from_str("1234567890.123456789012345").unwrap();``
193
Declare two two-dimensional arrays a and b of dimension n*m and m*n, respectively. Assign to b the transpose of a (i.e. the value with index interchange).
``````let a = DMatrix::<u8>::from_fn(n, m, |_, _| rand::thread_rng().gen());
let b = a.transpose();``````
195
Pass an array a of real numbers to the procedure (resp. function) foo. Output the size of the array, and the sum of all its elements when each element is multiplied with the array indices i and j (assuming they start from one).
``````fn foo(a: Vec<Vec<usize>>) {
println!(
"Length of array: {}",
a.clone()
.into_iter()
.flatten()
.collect::<Vec<usize>>()
.len()
);

let mut sum = 0;
for (i, j) in izip!(&a[0], &a[1]) {
sum += i * j
}

println!("Sum of all products of indices: {}", sum);
}``````
196
Given an integer array a of size n, pass the first, third, fifth and seventh, ... up to the m th element to a routine foo which sets all these elements to 42.
``````fn foo(el: &mut i32) {
*el = 42;
}
a.iter_mut().take(m).step_by(2).for_each(foo);``````
197
Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file.
``````let lines = BufReader::new(File::open(path).unwrap())
.lines()
.collect::<Vec<_>>();``````
198
Abort program execution with error condition x (where x is an integer value)
``process::exit(x);``
199
Truncate a file F at the given file position.
``````let pos = f.stream_position()?;
f.set_len(pos)?;``````
200
Compute the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
``````fn hypot(x:f64, y:f64)-> f64 {
let num = x.powi(2) + y.powi(2);
num.powf(0.5)
}``````
Alternative implementation:
``let h = x.hypot(y);``
201
Calculate n, the Euclidean norm of data (an array or list of floating point values).
``````fn euclidean(data: Vec<f64>) -> f64 {
let mut n = 0.0;
for i in data {
n += i*i;
}
return sqrt(n as f64)
}
let n = euclidean(data);``````
202
Calculate the sum of squares s of data, an array of floating point values.
``let s = data.iter().map(|x| x.powi(2)).sum::<f32>();``
204
Given a real number a, print the fractional part and the exponent of the internal representation of that number. For 3.14, this should print (approximately)

0.785 2
``````let sign = if a < 0.0 { a = -a; -1 } else { 1 };
let exponent = (a + f64::EPSILON).log2().ceil() as i32;
let fraction = a / 2.0f64.powi(exponent);``````
205
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
``````let foo = match env::var("FOO") {
Ok(val) => val,
Err(_e) => "none".to_string(),
};``````
Alternative implementation:
``let foo = env::var("FOO").unwrap_or("none".to_string());``
Alternative implementation:
``````let foo = match env::var("FOO") {
Ok(val) => val,
Err(_e) => "none".to_string(),
};``````
Alternative implementation:
``````if let Ok(tnt_root) = env::var("TNT_ROOT") {
//
}``````
206
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language.
``````match str {
"foo" => foo(),
"bar" => bar(),
"baz" => baz(),
"barfl" => barfl(),
_ => {}
}
``````
207
Allocate a list a containing n elements (n assumed to be too large for a stack) that is automatically deallocated when the program exits the scope it is declared in.
``let a = vec![0; n];``
208
Given the arrays a,b,c,d of equal length and the scalar e, calculate a = e*(a+b*c+cos(d)).
Store the results in a.
``````for i in 0..a.len() {
a[i] = e * (a[i] + b[i] * c[i] + d[i].cos());
}``````
209
Declare a type t which contains a string s and an integer array n with variable size, and allocate a variable v of type t. Allocate v.s and v.n and set them to the values "Hello, world!" for s and [1,4,9,16,25], respectively. Deallocate v, automatically deallocating v.s and v.n (no memory leaks).
``````struct T {
s: String,
n: Vec<usize>,
}

fn main() {
let v = T {
s: "Hello, world!".into(),
n: vec![1,4,9,16,25]
};
}``````
211
Create the folder at path on the filesystem
``fs::create_dir(path)?;``
Alternative implementation:
``fs::create_dir_all(path)?;``
212
Set the boolean b to true if path exists on the filesystem and is a directory; false otherwise.
``let b: bool = Path::new(path).is_dir();``
213
Compare four strings in pair-wise variations. The string comparison can be implemented with an equality test or a containment test, must be case-insensitive and must apply Unicode casefolding.
``````for x in strings
.iter()
.combinations(2)
.filter(|x| x[0].to_lowercase() == x[1].to_lowercase())
{
println!("{:?} == {:?}", x[0], x[1])
}``````
214
Append extra character c at the end of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
``let out = s.pad_to_width_with_char(m, c);``
215
Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
``````if let Some(columns_short) = m.checked_sub(s.width()) {
let padding_width = c
.width()
.filter(|n| *n > 0)
.expect("padding character should be visible");
// Saturate the columns_short
let padding_needed = columns_short + padding_width - 1 / padding_width;
let mut t = String::with_capacity(s.len() + padding_needed);
t.push_str(&s);
s = t;
}``````
216
Add the extra character c at the beginning and ending of string s to make sure its length is at least m.
After the padding the original content of s should be at the center of the result.
The length is the number of characters, not the number of bytes.

E.g. with s="abcd", m=10 and c="X" the result should be "XXXabcdXXX".
``````let n = (m + s.len())/2;
s = format!("{s:X>n\$}");
s = format!("{out:X<m\$}");``````
Alternative implementation:
``s = s.pad(m, c, Alignment::Middle, true);``
217
Create a zip-file with filename name and add the files listed in list to that zip-file.
``````let path = std::path::Path::new(_name);
let file = std::fs::File::create(&path).unwrap();
let mut zip = zip::ZipWriter::new(file); zip.start_file("readme.txt", FileOptions::default())?;
zip.write_all(b"Hello, World!\n")?;
zip.finish()?;``````
Alternative implementation:
``````fn zip(_name: &str, _list: Vec<&str>) -> zip::result::ZipResult<()>
{
let path = std::path::Path::new(_name);
let file = std::fs::File::create(&path).unwrap();
let mut zip = zip::ZipWriter::new(file);
for i in _list.iter() {
zip.start_file(i as &str, FileOptions::default())?;
}
zip.finish()?;
Ok(())
}``````
218
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter.
``````let unique_a = a.iter().collect::<HashSet<_>>();
let unique_b = b.iter().collect::<HashSet<_>>();

let c = unique_a.intersection(&unique_b).collect::<Vec<_>>();``````
Alternative implementation:
``````let set_a: HashSet<_> = a.into_iter().collect();
let set_b: HashSet<_> = b.into_iter().collect();
let c = set_a.intersection(&set_b);``````
219
Create the string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
``````let re = Regex::new(r"\s+").unwrap();
let t = re.replace_all(s, " ");``````
220
Create t consisting of 3 values having different types.

Explain if the elements of t are strongly typed or not.
``let t = (2.5, "hello", -1);``
221
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
``let t: String = s.chars().filter(|c| c.is_digit(10)).collect();``
222
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x.
``````let opt_i = items.iter().position(|y| *y == x);

let i = match opt_i {
Some(index) => index as i32,
None => -1
};``````
Alternative implementation:
``let i = items.iter().position(|y| *y == x).map_or(-1, |n| n as i32);``
223
Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.
``````let mut found = false;
for item in items {
if item == &"baz" {
println!("found it");
found = true;
break;
}
}
if !found {
println!("never found it");
}``````
Alternative implementation:
``````if let None = items.iter().find(|&&item| item == "rockstar programmer") {
println!("NotFound");
};``````
Alternative implementation:
``````items
.iter()
.find(|&&item| item == "rockstar programmer")
.or_else(|| {
println!("NotFound");
Some(&"rockstar programmer")
});``````
Alternative implementation:
``````if 'search: loop {
for i in items {
if i == &"qux" {
println!("found it");
break 'search false;
}
}
break 'search true
} {
}``````
Alternative implementation:
``````'label: {
for &item in items {
if item == "baz" {
println!("found");
break 'label;
}
}
}``````
224
Insert the element x at the beginning of the list items.
``items.push_front(x);``
225
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
``````fn f(x: Option<()>) {
match x {
Some(x) => println!("Present {}", x),
None => println!("Not present"),
}
}``````
226
Remove the last element from the list items.
``items.pop();``
227
Create the new list y containing the same elements as the list x.

Subsequent modifications of y must not affect x (except for the contents referenced by the elements themselves if they contain pointers).
``let y = x.clone();``
228
Copy the file at path src to dst.
``fs::copy(src, dst).unwrap();``
230
Cancel an ongoing processing p if it has not finished after 5s.
``timeout(Duration::from_secs(5), p()).await;``
231
Set b to true if the byte sequence s consists entirely of valid UTF-8 character code points, false otherwise.
``let b = std::str::from_utf8(&bytes).is_ok();``
232
Print "verbose is true" if the flag -v was passed to the program command line, "verbose is false" otherwise.
``````let matches = App::new("My Program")
.arg(Arg::with_name("verbose")
.short("v")
.takes_value(false))
.get_matches();

if matches.is_present("verbose") {
println!("verbose is true")
} else {
println!("verbose is false")
}``````
234
Assign to the string s the standard base64 encoding of the byte array data, as specified by RFC 4648.
``let s = base64::encode(data);``
235
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648.
``````let bytes = base64::decode(s).unwrap();
``````
237
Assign to c the result of (a xor b)
``let c = a ^ b;``
238
Write in a new byte array c the xor result of byte arrays a and b.

a and b have the same size.
``let c: Vec<_> = a.iter().zip(b).map(|(x, y)| x ^ y).collect();``
239
Assign to string x the first word of string s consisting of exactly 3 digits, or the empty string if no such match exists.

A word containing more digits, or 3 digits as a substring fragment, must not match.
``````let re = Regex::new(r"\b\d\d\d\b").expect("failed to compile regex");
let x = re.find(s).map(|x| x.as_str()).unwrap_or("");``````
240
Lists a and b have the same length. Apply the same permutation to a and b to have them sorted based on the values of a.
``````let mut tmp: Vec<_> = a.iter().zip(b).collect();
tmp.as_mut_slice().sort_by_key(|(&x, _y)| x);
let (aa, bb): (Vec<i32>, Vec<i32>) = tmp.into_iter().unzip();
``````
241
Explicitly decrease the priority of the current process, so that other execution threads have a better chance to execute now. Then resume normal execution and call the function busywork.
``````::std::thread::yield_now();
busywork();``````
242
Call a function f on each element e of a set x.
``````for item in &x {
f(item);
}``````
243
Print the contents of the list or array a on the standard output.
``println!("{:?}", a)``
244
Print the contents of the map m to the standard output: keys and values.
``println!("{:?}", m);``
245
Print the value of object x having custom type T, for log or debug.
``println!("{:?}", x);``
246
Set c to the number of distinct elements in the list items.
``let c = items.iter().unique().count();``
247
Remove all the elements from list x that don't satisfy the predicate p, without allocating a new list.
Keep all the elements that do satisfy p.

For languages that don't have mutable lists, refer to idiom #57 instead.
``````let mut j = 0;
for i in 0..x.len() {
if p(x[i]) {
x[j] = x[i];
j += 1;
}
}
x.truncate(j);``````
Alternative implementation:
``x.retain(p);``
248
Construct the "double precision" (64-bit) floating point number d from the mantissa m, the exponent e and the sign flag s (true means the sign is negative).
``let d = if s { -1.0 } else { 1.0 } * m as f64 * 2.0f64.powf(e as f64);``
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
``let (a, b, c) = (42, "hello", 5.0);``
250
Choose a value x from map m.
m must not be empty. Ignore the keys.
``````let mut rng = rand::thread_rng();
let x = m.values().choose(&mut rng).expect("m is empty");``````
251
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
``let i = i32::from_str_radix(s, 2).expect("Not a binary number!");``
252
Assign to the variable x the string value "a" if calling the function condition returns true, or the value "b" otherwise.
``x = if condition() { "a" } else { "b" };``
253
Print the stack frames of the current execution thread of the program.
``````let bt = Backtrace::new();
println!("{:?}", bt);``````
254
Replace all exact occurrences of "foo" with "bar" in the string list x
``````for v in &mut x {
if *v == "foo" {
*v = "bar";
}
}``````
255
Print the values of the set x to the standard output.
The order of the elements is irrelevant and is not required to remain the same next time.
``println!("{:?}", x);``
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
``````(0..=5).rev().for_each(|i| println!("{}", i));
``````
Alternative implementation:
``````for i in (0..=5).rev() {
println!("{}", i);
}``````
257
Print each index i and value x from the list items, from the last down to the first.
``````for (i, item) in items.iter().enumerate().rev() {
println!("{} = {}", i, *item);
}``````
Alternative implementation:
``````  for (i, x) in items.iter().rev().enumerate() {
println!("{i} = {x}");
}``````
258
Convert the string values from list a into a list of integers b.
``let b: Vec<i64> = a.iter().map(|x| x.parse::<i64>().unwrap()).collect();``
Alternative implementation:
``````let b: Vec<i32> = a.iter().flat_map(|s| s.parse().ok()).collect();
``````
259
Build the list parts consisting of substrings of the input string s, separated by any of the characters ',' (comma), '-' (dash), '_' (underscore).
``let parts: Vec<_> = s.split(&[',', '-', '_'][..]).collect();``
260
Declare a new list items of string elements, containing zero elements
``let items: Vec<String> = vec![];``
261
Assign to the string x the value of fields (hours, minutes, seconds) of the date d, in format HH:MM:SS.
``````let format = format_description!("[hour]:[minute]:[second]");
let x = d.format(&format).expect("Failed to format the time");``````
262
Assign to t the number of trailing 0 bits in the binary representation of the integer n.

E.g. for n=112, n is 1110000 in base 2 ⇒ t=4
``let t = n.trailing_zeros();``
263
Write two functions log2d and log2u, which calculate the binary logarithm of their argument n rounded down and up, respectively. n is assumed to be positive. Print the result of these functions for numbers from 1 to 12.
``````fn log2d(n: f64) -> f64 {
n.log2().floor()
}

fn log2u(n: f64) -> f64 {
n.log2().ceil()
}

fn main() {
for n in 1..=12 {
let f = f64::from(n);
println!("{} {} {}", n, log2d(f), log2u(f));
}
}``````
264
Pass a two-dimensional integer array a to a procedure foo and print the size of the array in each dimension. Do not pass the bounds manually. Call the procedure with a two-dimensional array.
``````fn foo(matrix: &[Vec<i32>]) {
let iter = matrix.iter();
let (vertical, _) = iter.size_hint();
let horizontal = iter
.max()
.expect("empty array!")
.len();
println!("{horizontal} by {vertical}");
}

fn main() {
let matrix = vec![
vec![1, 2, 3],
vec![4, 5, 6],
];
foo(&matrix);
}``````
Alternative implementation:
``````fn foo<const X: usize, const Y: usize>(_: [[i32;X];Y]) {
println!("{} {}", Y, X);
}

let a = [
[1, 2, 3],
[4, 5, 6],
];
foo(a);``````
265
Calculate the parity p of the integer variable i : 0 if it contains an even number of bits set, 1 if it contains an odd number of bits set.
``````let i = 42i32;
let p = i.count_ones() % 2;``````
266
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
``let s = v.repeat(n);``
267
Declare an argument x to a procedure foo that can be of any type. If the type of the argument is a string, print it, otherwise print "Nothing."

Test by passing "Hello, world!" and 42 to the procedure.
``````fn foo(x: &dyn Any) {
if let Some(s) = x.downcast_ref::<String>() {
println!("{}", s);
} else {
println!("Nothing")
}
}

fn main() {
foo(&"Hello, world!".to_owned());
foo(&42);
}``````
268
Define a type vector containing three floating point numbers x, y, and z. Write a user-defined operator x that calculates the cross product of two vectors a and b.
``````struct Vector {
x: f32,
y: f32,
z: f32,
}

impl Mul for Vector {
type Output = Self;

fn mul(self, rhs: Self) -> Self {
Self {
x: self.y * rhs.z - self.z * rhs.y,
y: self.z * rhs.x - self.x * rhs.z,
z: self.x * rhs.y - self.y * rhs.x,
}
}
}
``````
269
Given the enumerated type t with 3 possible values: bike, car, horse.
Set the enum value e to one of the allowed values of t.
Set the string s to hold the string representation of e (so, not the ordinal value).
Print s.
``````let e = t::bike;
let s = format!("{:?}", e);

println!("{}", s);``````
271
If a variable x passed to procedure tst is of type foo, print "Same type." If it is of a type that extends foo, print "Extends type." If it is neither, print "Not related."
``````fn type_of<T>(_: &T) -> &str {
std::any::type_name::<T>()
}

if type_of(&x) == type_of(&foo) {
println!("x & foo -> same type");
} else {
println!("x & foo -> not related");
}``````
272
Fizz buzz is a children's counting game, and a trivial programming task used to affirm that a programmer knows the basics of a language: loops, conditions and I/O.

The typical fizz buzz game is to count from 1 to 100, saying each number in turn. When the number is divisible by 3, instead say "Fizz". When the number is divisible by 5, instead say "Buzz". When the number is divisible by both 3 and 5, say "FizzBuzz"
``````for i in 1..101 {
match i {
i if (i % 15) == 0 => println!("FizzBuzz"),
i if (i % 3) == 0 => println!("Fizz"),
i if (i % 5) == 0 => println!("Buzz"),
_ => println!("{i}"),
}
}
``````
273
Set the boolean b to true if the directory at filepath p is empty (i.e. doesn't contain any other files and directories)
``let b = fs::read_dir(p).unwrap().count() == 0;``
274
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc.
``let t: String = s.chars().filter(|c| !c.is_whitespace()).collect();``
275
From the string s consisting of 8n binary digit characters ('0' or '1'), build the equivalent array a of n bytes.
Each chunk of 8 binary digits (2 possible values per digit) is decoded into one byte (256 possible values).
``````let a: Vec<u8> = s.as_bytes()
.chunks(8)
.map(|chunk| unsafe {
let chunk_str = std::str::from_utf8_unchecked(chunk);
})
.collect();``````
276
Insert an element e into the set x.
``x.insert(e);``
277
Remove the element e from the set x.

Explains what happens if e was already absent from x.
``x.remove(e);``
Alternative implementation:
``x.take(e)``
278
Read one line into the string line.

Explain what happens if EOF is reached.
``````let mut buffer = String::new();
let mut stdin = io::stdin();
279
Read all the lines (until EOF) into the list of strings lines.
``let lines = std::io::stdin().lock().lines().map(|x| x.unwrap()).collect::<Vec<String>>();``
280
Remove all the elements from the map m that don't satisfy the predicate p.
Keep all the elements that do satisfy p.

Explain if the filtering happens in-place, i.e. if m is reused or if a new map is created.
``m.retain(|_, &mut v| p(v));``
281
You have a Point with integer coordinates x and y. Create a map m with key type Point (or equivalent) and value type string. Insert "Hello" at position (42, 5).
``````let mut map: HashMap<Point, String> = HashMap::new();
map.insert(Point { x: 42, y: 5 }, "Hello".into());
``````
283
Build the list parts consisting of substrings of input string s, separated by the string sep.
``let parts = s.split(sep);``
Alternative implementation:
``let parts = s.split(sep).collect::<Vec<&str>>();``
Alternative implementation:
``let parts: Vec<&str> = s.split(sep).collect();``
284
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
``let a = vec![0; n];``
285
Given two floating point variables a and b, set a to a to a quiet NaN and b to a signalling NaN. Use standard features of the language only, without invoking undefined behavior.
``let a: f64 = f64::NAN;``