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• (instead of printing) Dart
1
Print a literal string on standard output
`print("Hello World");`
2
Loop to execute some code a constant number of times
```print("Hello\n" * 10);
```
Alternative implementation:
```for (var i = 0; i < 10; i++)
print("Hello");```
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
```void finish(String name) {
print("My job here is done. Goodbye \$name.");
}```
4
Create a function which returns the square of an integer
`int square(int x) => x * x;`
5
Declare a container type for two floating-point numbers x and y
```class Point {
double x, y;
}```
Alternative implementation:
```class Point {
double x, y;
Point(this.x, this.y);
}```
6
Do something with each item x of an array-like collection items, regardless indexes.
`items.forEach(doSomething);`
7
Print each index i with its value x from an array-like collection items
```for (var i = 0; i < items.length; i++) {
print('index=\$i, value=\${items[i]}');
}```
Alternative implementation:
```items.asMap().forEach((i, x) {
print('index=\$i, value=\$x');
});```
Alternative implementation:
```items.forEachIndexed((i, x) {
print('index=\$i, value=\$x');
});```
8
Create a new map object x, and provide some (key, value) pairs as initial content.
```var x = {
"one": 1,
"two": 2
};```
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.
```class Node<T> {
final T value;
Node<T> left, right;
Node(this.value, {this.left, this.right});
}```
10
Generate a random permutation of the elements of list x
`x.shuffle();`
Alternative implementation:
`var shuffled = x.toList()..shuffle();`
11
The list x must be non-empty.
`x[new Random().nextInt(x.length)];`
12
Check if the list contains the value x.
list is an iterable finite container.
`list.contains(x);`
13
Access each key k with its value x from an associative array mymap, and print them.
`mymap.forEach((k, v) => print('Key=\$k, Value=\$v'));`
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
`double pick(num a, num b) => a + new Random().nextDouble() * (b - a);`
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
`int pick(int a, int b) => a + Random().nextInt(b - a + 1);`
16
Call a function f on every node of binary tree bt, in depth-first infix order
```traverse(Node bt, f(value)) {
if (bt == null) return;
traverse(bt.left, f);
f(bt.value);
traverse(bt.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.
```class Node<T> {
final T value;
final List<Node<T>> children;
Node(this.value, this.children);
}
```
18
Call a function f on every node of a tree, in depth-first prefix order
```traverse(Tree node, f(value)) {
f(node.value);
for (var child in node.children) {
traverse(child, f);
}
}```
19
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
`x = x.reversed.toList();`
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.
```class Position {
int i, j;
Position(this.i, this.j);
}
Position search(List<List> m, x) {
for (var i = 0; i < m.length; i++) {
var line = m[i];
for (var j = 0; j < line.length; j++) {
if (line[j] == x) {
return new Position(i, j);
}
}
}
return null;
}```
21
Swap the values of the variables a and b
```var tmp = a;
a = b;
b = tmp;```
22
Extract the integer value i from its string representation s (in radix 10)
`var i = int.parse(s);`
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
`var s = x.toStringAsFixed(2);`
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
`var s = "ネコ";`
25
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
`sendPort.send("value");`
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
`var x = new List.generate(m, (_) => new List(n));`
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
```var x = new List.generate(m, (_) =>
new List.generate(n, (_) =>
new List.filled(p, 0.0),
growable: false),
growable: false);```
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((a, b) => Comparable.compare(a.p, b.p));`
Alternative implementation:
`items.sort((a, b) => (a.p).compareTo(b.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.removeAt(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.
```for (int i = 1; i <= 1000; i++) {
Isolate.spawn(f,i);
}```
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
`f(i) => (i == 0) ? 1 : i * f(i - 1);`
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
```int exp(int x, int n) {
if (n == 0) return 1;
if (n == 1) return x;
var r = exp(x * x, n ~/ 2);
if (n % 2 == 1) r *= x;
return r;
}```
Alternative implementation:
```int exp(int x, int n) {
var r = 1;
while (true) {
if (n.isOdd) r *= x;
n ~/= 2;
if (n == 0) break;
x *= x;
}
return r;
}```
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.
`x = f(x);`
34
Declare and initialize a set x containing unique objects of type T.
`var x = new Set<T>();`
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f
```typedef R Func<T,R>(T arg);

Func<A,C> compose(B f(A x), C g(B x)) => (A 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.
`compose(f, g) => (x) => g(f(x));`
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
`curry(f(a, b), a) => (b) => f(a, b);`
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.
`var t = s.substring(i, j);`
39
Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise.
`var 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.
`var t = new String.fromCharCodes(s.runes.toList().reversed);`
Alternative implementation:
```var t = s.split('').reversed.join();

```
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.
```main()  {
var a = [1, 2, 3];
var b = [2, 3];

outer: for (var v in a) {
for (var w in b) {
if (w == v) continue outer;
}
print(v);
}
}
```
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
```OUTER: for (var i = 0; i < m.length; i++) {
for (var j = 0; j < m[i].length; j++) {
if (m[i][j] < 0) {
print("Negative value found at \$i,\$j: \${m[i][j]}");
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.
`sleep(const Duration(seconds: 5));`
Alternative implementation:
`await new Future.delayed(const Duration(seconds : 5));`
46
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled.
`var t = s.substring(0, 5);`
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
```var n = s.length;
var t = s.substring(n-5, n);```
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
```var s = '''A
multi-line
string''';```
Alternative implementation:
```var s = """A
multi-line
string""";```
49
Build list chunks consisting in substrings of input string s, separated by one or more space characters.
`s.split(new RegExp('\\s+'))`
50
Write a loop that has no end clause.
```while (true) {
// do something
}```
Alternative implementation:
```for (;;) {

}```
51
Determine whether the map m contains an entry for the key k
`m.containsKey(k)`
52
Determine whether the map m contains an entry with the value v, for some key.
`m.containsValue(v);`
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
`y = x.join(', ');`
54
Calculate the sum s of the integer list or array x.
`var s = x.fold(0, (a, b) => a + b);`
55
Create the string representation s (in radix 10) of the integer value i.
`var s = "\$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".
```List<Future> tasks = [];
for (int i = 1; i <= 1000; i++) {
}
print("finished");```
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.
`var y = x.where(p).toList();`
58
Create string lines from the content of the file with filename f.
`var lines = new File(f).readAsStringSync();`
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
`stderr.write("\$x is negative");`
60
Assign to x the string value of the first command line parameter, after the program name.
```main(args) {
var x = args;
}```
61
Assign to the variable d the current date/time value, in the most standard type.
```var d = DateTime.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.
`var i = x.indexOf(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.
`var x2 = x.replaceAll(y, z);`
64
Assign to x the value 3^247
`var x = pow(3, 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%"
```var s = "\${(x * 100).toStringAsFixed(1)}%";
```
66
Calculate the result z of x power n, where x is a big integer and n is a positive integer.
`var z = pow(x, n);`
67
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
```int binom(int n, int k) {
int result = 1;
for (int i = 0; i < k; i++) {
result = result * (n - i) ~/ (i + 1);
}
return result;
}```
68
Create an object x to store n bits (n being potentially large).
`final x = FixedBitArray(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.
```var r = new Random(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.
`var r = new Random(new DateTime.now().millisecondsSinceEpoch);`
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.
```main(List<String> args) {
print(args.join(' '));
}```
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
```int gcd(int a, int b) {
while (b != 0) {
var t = b;
b = a % t;
a = t;
}
return a;
}```
75
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
```x = lcm(a, b);

int lcm(int a, int b) => (a * b) ~/ gcd(a, b);

int gcd(int a, int b) {
while (b != 0) {
var t = b;
b = a % t;
a = t;
}
return a;
}```
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
`var s = x.toRadixString(2);`
78
Execute a block once, then execute it again as long as boolean condition c is true.
```do {
someThing();
someOtherThing();
} while(c);```
79
Declare the floating point number y and initialize it with the value of the integer x .
`double y = x.toDouble();`
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).
`int y = x.toInt()`
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).
```var y = x.round();
```
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
`var c = i.toRadixString(2).replaceAll('0','').length;`
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
`exit(0);`
88
Create a new bytes buffer buf of size 1,000,000.
`var buf = BytesBuilder().addByte(1000000);`
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
```class Foo{
int _x = 0;
int get x => _x;
}```
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.
`Map x = json.jsonDecode(await new File('data.json').readAsString());`
Alternative implementation:
`Map x = json.jsonDecode(new File('data.json').readAsStringSync());`
92
Write the contents of the object x into the file data.json.
```class JsonModel {
// Create Field
int id;
String qrText, licen, trans, name;

//   // Constructor
//   JsonModel(
//       int idInt, String nameString, String userString, String passwordString) {
// //     id=idInt;
// // name =nameString;
// // user =userString;
//   }

JsonModel(this.id, this.name, this.licen, this.trans, this.qrText);

JsonModel.fromJson(Map<String, dynamic> parseJSON) {
id = int.parse(parseJSON['id']);
licen = parseJSON['Li```
93
Implement procedure control which receives one parameter f, and runs f.
`control(Function f) => 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.
`print(x.runtimeType);`
95
Assign to variable x the length (number of bytes) of the local file at path.
`final x = (await File(path).readAsBytes()).length;`
Alternative implementation:
`var x = File(path).lengthSync();`
96
Set boolean b to true if string s starts with prefix prefix, false otherwise.
`var b = s.startsWith(prefix);`
97
Set boolean b to true if string s ends with string suffix, false otherwise.
`var b = s.endsWith(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
`var d = new DateTime.fromMillisecondsSinceEpoch(ts, isUtc: true);`
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
`x = DateFormat('YYYY-MM-DD').format(d);`
100
Sort elements of array-like collection items, using a comparator c.
`items.sort(c);`
105
Assign to the string s the name of the currently executing program (but not its full path).
`var s = File(Platform.resolvedExecutable).uri.pathSegments.last;`
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
`final blank = s == null || s.trim() == '';`
Alternative implementation:
`bool blank = s?.trim()?.isEmpty ?? true;`
112
Print each key k with its value x from an associative array mymap, in ascending order of k.
```var sortedKeys = myMap.keys.toList()..sort();
for (var k in sortedKeys) {
print("Key=\$k Value=\${myMap[k]}");
}```
113
Print each key k with its value x from an associative array mymap, in ascending order of x.
Note that multiple entries may exist for the same value x.
```mymap.entries.toList()
..sort((a, b) => a.value.compareTo(b.value))
..forEach(print);```
115
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise.
`bool b = d1.isBefore(d2);`
116
Remove all occurrences of string w from string s1, and store the result in s2.
`var s2 = s1.replaceAll(w,"");`
117
Set n to the number of elements of the list x.
`int n = x.length;`
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
`var y = x.toSet();`
119
Remove duplicates from the list x.
Explain if the original order is preserved.
`x = x.toSet().toList();`
120
Read an integer value from the standard input into the variable n
```String? s = stdin.readLineSync();
int? n = int.tryParse(s ?? "");```
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(isConsistent);`
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.binarySearch(x);`
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
```var t = Stopwatch();
t.start();
foo();
t.stop();
print(t.elapsedMicroseconds * 1000);```
126
Write a function foo that returns a string and a boolean value.
`foo() => ['a string', true];`
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;
}```
132
Run procedure f, and return the duration of the execution of f.
```var s = Stopwatch()..start();
f();
s.stop();
print(s.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.
`bool ok = s.toUpperCase().contains(word.toUpperCase());`
134
Declare and initialize a new list items, containing 3 elements a, b, c.
`var items = [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.
```items.remove(x);
```
136
Remove all occurrences of value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
`items.removeWhere((y)=>y==x);`
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
`final b = RegExp(r'^[0-9]+\$').hasMatch(s);`
140
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
```Map myMap = {};
myMap.remove(key)```
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
```items1.forEach(print);
items2.forEach(print);```
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
`var s = x.toRadixString(16);`
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.
`var b = await File(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.
`print('\${DateTime.now()} msg');`
146
Extract floating point value f from its string representation s
`f = num.parse(s);`
147
Create string t from string s, keeping only ASCII characters
```var t = '';
for(var c in s.runes) {
if(c < 128){
t = t + String.fromCharCode(c);
}
}```
150
Remove the last character from the string p, if this character is a forward slash /
`if (p.endsWith("/")) p = p.substring(0, p.length - 1);`
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!
`if (p.endsWith(path.separator)) p = p.substring(0, p.length - 1);`
152
Create string s containing only the character c.
`var s=c;`
153
Create the string t as the concatenation of the string s and the integer i.
`var t = '\$s\$i';`
155
Delete from filesystem the file having path filepath.
`File(filepath).deleteSync();`
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.
`var s = i.toString().padLeft(3,'0');`
157
Initialize a constant planet with string value "Earth".
`const String _planet = "Earth";`
161
Multiply all the elements of the list elements by a constant c
`elements = [for (var e in elements) e * c];`
Alternative implementation:
`elements = elements.map((e) => e * c).toList();`
162
execute bat if b is a program option and fox if f is a program option.
```if (args.contains("b")) bat();
if (args.contains("f")) fox();```
163
Print all the list elements, two by two, assuming list length is even.
```for(int i = 0; i < list.length; i += 2) {
print("\${list[i]}, \${list[i + 1]}");
}```
165
Assign to the variable x the last element of the list items.
`x = items.last;`
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
```var a = [1,2,3];
var b = [3,2,4];
var ab = a + b;```
167
Create string t consisting of string s with its prefix p removed (if s starts with p).
`var t = s.startsWith(p) ? s.replaceFirst(p,"") : s;`
Alternative implementation:
`var t = s.startsWith(p) ? s.substring(p.length, s.length) : 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.
`int n = s.length;`
170
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
`var n = mymap.length;`
171
Append element x to the list s.
`s.add(x);`
172
Insert value v for key k in map m.
`m[k] = v;`
174
Make a HTTP request with method POST to the URL u
`await post(u, body: body, headers: headers);`
175
From 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).
```var s = a.buffer
.asUint8List()
.join();```
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).
`var a = hex.decode(s);`
177
Construct a list L that contains all filenames that have the extension ".jpg" , ".jpeg" or ".png" in directory D and all it's subdirectories.
```var extensions = [".jpg", "jpeg", ".png"];
var L = Directory(D)
.listSync(recursive: true)
.where((file) => extensions.contains(p.extension(file.path)))
.toList();
```
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.
```var r = Rectangle.fromPoints(Point(x1,y1),Point(x2,y2));
var b = r.containsPoint(Point(x,y));```
179
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
```var r = MutableRectangle.fromPoints(Point(x1, y1), Point(x2, y2));
var center = Point(r.width / 2, r.height / 2);```
180
Create list x containing the contents of directory d.

x may contain files and subfolders.
No recursive subfolder listing.
`var x = Directory(d).listSync();`
184
Assign to variable t a string representing the day, month and year of the day after the current date.
```var now = new DateTime.now();
var t = now.add(new Duration(days: 1));```
185
Schedule the execution of f(42) in 30 seconds.
`Future.delayed(const Duration(seconds: 30), () => f(42));`
186
Exit a program cleanly indicating no error to OS
`exit(0);`
189
Produce a new list y containing the result of function T applied to all elements e of list x that match the predicate P.
`var y = x.where(P).map(T).toList();`
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.any((value) => value > x)) f();`
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.
```var a = List.filled(n,0);
foo(a.take(m).toList());
foo(List<int> a) {
a.fillRange(0, a.length, 42);
}```
197
Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file.
`var lines = File(path).readAsLinesSync();`
198
Abort program execution with error condition x (where x is an integer value)
`exit(x);`
199
Truncate a file F at the given file position.
`File(F).openSync(mode: FileMode.write).truncateSync(position);`
200
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
`var h = sqrt(x * x + y * y);`
202
Calculate the sum of squares s of data, an array of floating point values.
`var s = data.map((v) => v * v).reduce((sum, v) => sum + v);`
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".
`var foo = Platform.environment["FOO"] ?? "none";`
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.
```var myProc = {"foo": foo,
"bar": bar,
"baz": baz,
"barfl": barfl};

myProc[str]?.call();```
211
Create the folder at path on the filesystem
`Directory(path).create();`
212
Set boolean b to true if path exists on the filesystem and is a directory; false otherwise.
`var b = await Directory(path).exists();`
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.
`s = s.padRight(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.
`s = s.padLeft(m, c);`
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.
`var t = s.replaceAll(RegExp(r"\s+"), " ");`
221
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
```var t = String.fromCharCodes(
s.codeUnits.where((x) => (x ^ 0x30) <= 9) );```
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.
`i=items.indexOf(x)`
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.
```print(items.any((element) => element == "baz")
? "found it"
: "did not find it");```
224
Insert the element x at the beginning of the list items.
`items = [x, ...items];`
225
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
`void f({int? x}) => print(x == null ? "Not present" : "Present");`
226
Remove the last element from list items.
`items.removeLast();`
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).
`y = [...x];`
228
Copy the file at path src to dst.
`File(src).copySync(dst);`
232
Print "verbose is true" if the flag -v was passed to the program command line, "verbose is false" otherwise.
`print("verbose is \${args.contains("-v")}");`
234
Assign to string s the standard base64 encoding of the byte array data, as specified by RFC 4648.
`var s = base64.encode(data);`
235
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648.
`var bytes = base64Decode(s);`
237
Assign to c the result of (a xor b)
`var c = a ^ b;`
242
Call a function f on each element e of a set x.
`x.forEach(f);`
243
Print the contents of the list or array a on the standard output.
`print(a);`
244
Print the contents of the map m to the standard output: keys and values.
`print(m);`
245
Print the value of object x having custom type T, for log or debug.
`print(x);`
246
Set c to the number of distinct elements in the list items.
`var c = items.toSet().length;`
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.
`x.retainWhere((e) => p);`
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
`var a = 42, b = "Hello", c = 5.0;`
250
Choose a value x from map m.
m must not be empty. Ignore the keys.
`var x = m.values.toList()[Random().nextInt(m.length)];`
251
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
```var i = int.parse(s, radix: 2);
```
252
Assign to the variable x the value "a" if calling the function condition returns true, or the value "b" otherwise.
`var x = condition() ? "a" : "b";`
254
Replace all exact occurrences of "foo" with "bar" in the string list x
`x = x.map((e) => e == 'foo' ? 'bar' : e).toList();`
Alternative implementation:
```for (var i = 0; i < x.length; i++) {
if (x[i] == 'foo') x[i] = '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.
`print(x);`
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
```for (int i = 5; i >= 0; i--) {
print(i);
}```
257
Print each index i and value x from the list items, from the last down to the first.
```var i = items.length ;
for (var e in items.reversed) {
i--;
print("\$i, \$e");
}```
258
Convert the string values from list a into a list of integers b.
`var b = a.map(int.parse).toList();`
259
Build the list parts consisting of substrings of the input string s, separated by any of the characters ',' (comma), '-' (dash), '_' (underscore).
`var parts = s.split( RegExp(r"[,-_]") );`
260
Declare a new list items of string elements, containing zero elements
`var items = <String>[];`
Alternative implementation:
`List<String> items = [];`
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
```var t = 0;
while (n.isEven && n != 0) {
t++;
n = n >> 1;
}```
Alternative implementation:
```t = n.toRadixString(2)
.split('')
.reversed
.takeWhile((e) => e == '0')
.length;
```
Alternative implementation:
`t = n.bitLength - 1 - n.toRadixString(2).lastIndexOf('1');`
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.
```foo(List<List<int>> a) => print("\${a.length} \${a.length}");
var a = [
[1, 2],
[3, 4],
[5, 6]
];
foo(a);```
265
Set the integer variable i to 42 and calculate its parity (i.e. 0 if it contains an even number of bits, 1 if it contains an odd number of bits).
```var i = 42;
var parity = i.bitLength.isEven ? 0 : 1;
```
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"
`var s = v * 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.
```foo(x) => print(x is String ? x : 'Nothing.');

foo('Hello, world!');
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.
```class Vector {
final double x, y, z;

Vector(this.x, this.y, this.z);

Vector operator *(other) {
return Vector(y * other.z - z * other.y,
z * other.x - x * other.z,
x * other.y - y * other.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.
```var e = t.bike;
var s = e.name;
print(s);
```
270
Given a floating point number r1 classify it as follows:
If it is a signaling NaN, print "This is a signaling NaN."
If it is a quiet NaN, print "This s a quiet NaN."
If it is not a NaN, print "This is a number."
```if (r1.isNaN) {
print("This is a quiet NaN.");
} else {
print("This is a number.");
}```
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."
```tst(x) {
if (x.runtimeType == Foo) {
print("Same Type.");
} else if (x is Foo) {
print("Extends type.");
} else {
print("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 (int i = 1; i <= 100; i++) {
var s = StringBuffer();
if (i % 3 == 0) s.write("Fizz");
if (i % 5 == 0) s.write("Buzz");
if (s.isEmpty) s.write(i);
print(s);
}
```
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)
`var b = await Directory(p).list().isEmpty;`
276
Insert an element e into the set x.
`x.add(e);`
277
Remove the element e from the set x.

Explains what happens if e was already absent from x.
`x.remove(e);`
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).
```Map<Point, String> m = {};
m[Point(42, 5)] = "Hello";
```
282
Declare a type Foo, and create a new map with Foo as key type.

Mention the conditions on Foo required to make it a possible map key type.
```class Foo{};
Map<Foo, int> m = {};```
283
Build the list parts consisting of substrings of input string s, separated by the string sep.
`var parts = s.split(sep);`
284
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
`var a = List.filled(n, 0);`
286
Print a line "Char i is c" for each character c of the string s, where i is the character index of c in s (not the byte index).

Make sure that multi-byte characters are properly handled, and count for a single character.
`s.split('').asMap().forEach((i, c) => print('Char \$i is \$c'));`
287
Assign to n the number of bytes in the string s.

This can be different from the number of characters. If n includes more bytes than the characters per se (trailing zero, length field, etc.) then explain it. One byte is 8 bits.
`var n = utf8.encode(s).length;`
288
Set the boolean b to true if the set x contains the element e, false otherwise.
`var b = x.contains(e);`
289
Create the string s by concatenating the strings a and b.
`var s = '\$a\$b';`
Alternative implementation:
`var s = a + b;`
290
Sort the part of the list items from index i (included) to index j (excluded), in place, using the comparator c.

Elements before i and after j must remain unchanged.
`items.setRange(i, j, items.sublist(i, j)..sort(c));`
291
Delete all the elements from index i (included) to index j (excluded) from the list items.
`items.removeRange(i, j);`
293
Create a new stack s, push an element x, then pop the element into the variable y.
```var s = [];
var y = s.removeLast();```
294
Given an array a containing the three values 1, 12, 42, print out
"1, 12, 42" with a comma and a space after each integer except the last one.
`print(a.join(", "));`
295
Given the enumerated type T, create a function TryStrToEnum that takes a string s as input and converts it into an enum value of type T.

Explain whether the conversion is case sensitive or not.
Explain what happens if the conversion fails.
`T.values.byName(s);`
296
Assign to x2 the value of string x with the last occurrence of y replaced by z.
If y is not contained in x, then x2 has the same value as x.
```var position = x.lastIndexOf(y).clamp(0, x.length);
var x2 = x.replaceFirst(y, z, position);```
297
Sort the string list data in a case-insensitive manner.

The sorting must not destroy the original casing of the strings.
`data.sort((a, b) => a.toUpperCase().compareTo(b.toUpperCase()));`
298
Create the map y by cloning the map x.

y is a shallow copy, not a deep copy.
`var y = Map.of(x);`
299
Write a line of comments.

This line will not be compiled or executed.
`// This is a comment`
301
Compute the Fibonacci sequence of n numbers using recursion.

Note that naive recursion is extremely inefficient for this task.
```int fibonacci(int n) => n <= 2 ? 1 : fibonacci(n - 2) + fibonacci(n - 1);
```
302
Given the integer x = 8, assign to the string s the value "Our sun has 8 planets", where the number 8 was evaluated from x.
`var s = "Our sun has \$x planets";`