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Ruby
1
Print a literal string on standard output
puts 'Hello World'
2
Loop to execute some code a constant number of times
10.times do
  puts "Hello"
end
Alternative implementation:
10.times { puts 'Hello' }
Alternative implementation:
puts "Hello\n" * 10
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
def finish( name )
  puts "My job here is done. Goodbye #{name}"
end
4
Create a function which returns the square of an integer
def square(x)
  x*x
end
Alternative implementation:
def square(x) = x*x
5
Declare a container type for two floating-point numbers x and y
Point = Struct.new(:x, :y)
6
Do something with each item x of an array-like collection items, regardless indexes.
items.each{|x| do_something( x )}
Alternative implementation:
items.each do |x|
  do_something( x )
end
7
Print each index i with its value x from an array-like collection items
items.each_with_index do |x, i| 
  puts "Item #{i} = #{x}"
end
Alternative implementation:
items.each_index{|i| puts "Item %d = %s" % [i, items[i]]}
8
Create a new map object x, and provide some (key, value) pairs as initial content.
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.
Node = Struct.new(:left, :right, :value)
parent = Node.new(Node.new, Node.new)
10
Generate a random permutation of the elements of list x
x.shuffle
Alternative implementation:
shuffled_list = x.shuffle
11
The list x must be non-empty.
x.sample
12
Check if the list contains the value x.
list is an iterable finite container.
list.include? x
13
Access each key k with its value x from an associative array mymap, and print them.
mymap.each {|k, x| puts "Key= #{k}  Value=#{x}"}
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
rand(a...b)
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
rand(a..b)
16
Call a function f on every node of binary tree bt, in depth-first infix order
def dfs(f, bt)
  dfs(f, bt.left) if bt.left
  f.(bt)
  dfs(f, bt.right) if bt.right
end
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.
Node = Struct.new(:children)
parent = Node.new([])
parent.children << Node.new([])
18
Call a function f on every node of a tree, in depth-first prefix order
def dfs(f, node)
  f.(node)
  node.children.each do |child|
    dfs(f, child)
  end
end
19
Reverse the order of the elements of list x.
This may reverse "in-place" and destroy the original ordering.
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.
def search(m, x)
  m.each_with_index do |row, i|
    row.each_with_index do |value, j|
      return i, j if value == x
    end
  end
  nil
end
21
Swap the values of the variables a and b
a, b = b, a
22
Extract the integer value i from its string representation s (in radix 10)
i = s.to_i
Alternative implementation:
contrato_nf['items']['issueDate'] = data
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
s = "%.2f" % x
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
s = "ネコ"
25
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
queue = Queue.new
thread = Thread.new do
  puts queue.pop
end
queue << "Alan"
thread.join
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
x = Array.new(m) { Array.new(n) }
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
x = Array.new(m) { Array.new(n) { Array.new(p) } }
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(&: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.delete_at(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.
threads = 1000.times.map do |i|
  Thread.new { f(i) }
end
threads.join
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
f = Hash.new { |hash, i| hash[i] = i * hash[i -1] }
f[0] = 1
Alternative implementation:
fac = Hash.new {|h, i| h[i] = i * h[i-1] }.tap {|h| h[0] = 1 }
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
def exp(x, n)
  x ** n
end
Alternative implementation:
def exp(x, n)
  return 1 if n == 0
  return x if n == 1
  return exp(x*x, n/2) if n.even?
  x * exp(x*x, (n-1)/2)
end
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 = Atomic.new(0)
x.update { |x| f(x) }
34
Declare and initialize a set x containing unique objects of type T.
x = Set.new
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns composition function g ∘ f
def compose(f, g)
  -> x { g.(f.(x)) }
end
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
def compose(f, g)
  -> x { g.(f.(x)) }
end
Alternative implementation:
def compose(f, g)
  f >> g
end
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
adder = -> a, b { a + b }
add_two = adder.curry.(2)
add_two.(5) # => 7
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.
t = s[i..j-1] 
39
Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise.
ok = s.include?(word)
40
Declare a Graph data structure in which each Vertex has a collection of its neighbouring vertices.
Vertex = Struct.new(:x, :y, :z)
Graph = Struct.new(:vertex, :neighbours)

v = Vertex.new(1, 2, 3)
neighbours = [ Vertex.new(2, 3, 4), Vertex.new(4, 5, 6) ]
graph = Graph.new(v, neighbours)
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.
t = s.reverse
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.
a.each do |v|
  catch :matched do
    b.each do |u|
      throw :matched if v == u
    end
    puts v
  end  
end
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
negative_value = catch :negative do
  matrix.each do |row|
    row.each do |value|
      throw :negative, value if value < 0
    end
  end
end

puts negative_value
44
Insert element x at position i in 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 5
46
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled.
t = s[0, 5]
Alternative implementation:
t = s.slice(0, 5)
Alternative implementation:
t = s.slice(0...5)
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
t = s[-5..-1]
Alternative implementation:
t = s[-5..]
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
s = "Spanning
string
works"
49
Build list chunks consisting in substrings of input string s, separated by one or more space characters.
chunks = s.split
50
Write a loop that has no end clause.
loop do
  # endless joy
end
51
Determine whether the map m contains an entry for the key k
m.include?(k)
Alternative implementation:
m.key?(k)
Alternative implementation:
m.has_key?(k)
52
Determine whether the map m contains an entry with the value v, for some key.
m.value?(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.
s = x.sum
Alternative implementation:
s = x.reduce(:+)
55
Create the string representation s (in radix 10) of the integer value i.
s = i.to_s
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".
threads = 1000.times.map do |i|
  Thread.new { f(i) }
end
threads.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.
y = x.select(&:p)
58
Create string lines from the content of the file with filename f.
lines = File.read(f)
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
warn "#{x} is negative"
$stderr.puts "%d is negative" % x
60
Assign to x the string value of the first command line parameter, after the program name.
x = ARGV.first
61
Assign to variable d the current date/time value, in the most standard type.
d = Time.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.
i = x.index(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.
x2 = x.gsub(y, z)
64
Assign to x the value 3^247
x = 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%"
s = "%.1f%%" % (100 * x)
66
Calculate the result z of x power n, where x is a big integer and n is a positive integer.
z = x ** n
67
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
def binom(n,k)
  (1+n-k..n).inject(:*)/(1..k).inject(:*)
end
68
Create an object x to store n bits (n being potentially large).
x = 0
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.
r = Random.new(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.
Random.new
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.
printf("%s\n", ARGV.join(' '))
Alternative implementation:
puts ARGV.join(' ')
73
Create a factory named fact for any sub class of Parent and taking exactly one string str as constructor parameter.
def fact(klass, str)
  klass.new(str) if klass.is_a?(Parent)
end
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
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.
x = a.lcm(b)
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
s = x.to_s(2)
77
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
x = 3i - 2
x *= 1i
78
Execute a block once, then execute it again as long as boolean condition c is true.
begin
  # code
end while c
79
Declare the floating point number y and initialize it with the value of the integer x .
y = x.to_f
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).
y = x.to_i
81
Declare integer y and initialize it with the rounded value of floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
y = (x + 1/2r).floor
82
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
s.scan(t).size
83
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
r = /htt+p/
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
c = i.digits(2).count(1)
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.
def addingWillOverflow(x,y)
  false
end
86
Write boolean function multiplyWillOverflow 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.
def multiplyWillOverflow(x,y)
  false
end
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
exit
88
Create a new bytes buffer buf of size 1,000,000.
(' ' * 1_000_000).bytes.to_a
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.
raise ArgumentError, "invalid value #{x}."
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
class Foo

  def initialize
    @x = rand(10)
  end

  def x
    @x
  end

end

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.
x = JSON.parse(File.read('data.json'))
92
Write the contents of the object x into the file data.json.
x = {:hello => "goodbye"}

File.open("data.json", "w") do |f|
  f.puts(x.to_json)
end
93
Implement procedure control which receives one parameter f, and runs f.
def control
    yield
end
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.
puts x.class
95
Assign to variable x the length (number of bytes) of the local file at path.
x = File.size(path)
96
Set boolean b to true if string s starts with prefix prefix, false otherwise.
b = s.start_with?(prefix)
97
Set boolean b to true if string s ends with string suffix, false otherwise.
b = s.end_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
d = DateTime.strptime(ts, '%s')
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
d = Date.today
x = d.to_s
100
Sort elements of array-like collection items, using a comparator c.
items.sort!{|a,b| a-b }
Alternative implementation:
items.sort!(&c)
101
Make an HTTP request with method GET to URL u, then store the body of the response in string s.
u = URI("http://example.com/index.html")
s = Net::HTTP.get_response(u).body
102
Make an HTTP request with method GET to URL u, then store the body of the response in file result.txt. Try to save the data as it arrives if possible, without having all its content in memory at once.
u = URI('http://example.com/large_file')

Net::HTTP.start(u.host, u.port) do |http|
  request = Net::HTTP::Get.new(u)
  http.request(request) do |response|
    open('result.txt', 'w') do |file|
      response.read_body do |chunk|
        file.write(chunk)
      end
    end
  end
end
104
Write the contents of the object x into the file data.xml.
class Person
  include XML::Mapping
  attr_accessor :name, :surname, :age, :children

  text_node :from, "@from"
  text_node :name, "Name"
  text_node :surname, "Surname"

  def initialize(name, surname, ..)
    # ...
  end
end

x = Person.new(..)
x.save_to_xml.write($stdout,2) # nicely prints
x.save_to_file('data.xml')
105
Assign to the string s the name of the currently executing program (but not its full path).
s = __FILE__
Alternative implementation:
s = $0
106
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
dir = Dir.pwd
107
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.)
dir = __dir__
108
Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.)
puts x if defined?(x)
109
Set n to the number of bytes of a variable t (of type T).
n = ObjectSpace.memsize_of(t)
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
blank = s.strip.empty?
111
From current process, run program x with command-line parameters "a", "b".
`x a b`
112
Print each key k with its value x from an associative array mymap, in ascending order of k.
my_map.sort.each{|k,x| puts "#{k}: #{x}"}
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.
h.sort_by{|k,x| x}.each{|k,x| puts "#{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.
b = x == y
115
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise.
b = d1 < d2
116
Remove all occurrences of string w from string s1, and store the result in s2.
s2 = s1.gsub(w, "")
117
Set n to the number of elements of the list x.
n = x.length
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
y = x.to_set
119
Remove duplicates from the list x.
Explain if the original order is preserved.
x.uniq!
120
Read an integer value from the standard input into the variable n
n = $stdin.gets.to_i
121
Listen UDP traffic on port p and read 1024 bytes into buffer b.
require 'socket'

p = 4913
u1 = UDPSocket.new
u1.bind("127.0.0.1", p)
u1.send "message-to-self", 0, "127.0.0.1", p

b = u1.recvfrom(1024).first
122
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
class Colors
  include Ruby::Enum

  define :SPADES, "spades"
  define :HEARTS, "hearts"
  define :DIAMONDS, "diamonds"
  define :CLUBS, "clubs"
end
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
raise unless 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.
def binary_search(a, el)
  a.bsearch_index{|x| x == el} || -1
end
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
t1 = Time.now
foo
p (Time.now - t1)*1000000
126
Write a function foo that returns a string and a boolean value.
def foo
  string, boolean  = "bar", false
  [string, boolean]
end
127
Import the source code for the function foo body from a file "foobody.txt".
def foo
  eval File.read "foobody.txt"
end
128
Call a function f on every node of a tree, in breadth-first prefix order
class Tree
  attr_accessor :value, :children

  def initialize(value)
    @value = value
    @children = []
  end

  def traverse_breadth_first(f)
    queue = []
    queue.unshift(self)
    while !(queue.empty?)
      node = queue.pop
      method(f).call(node.value)
      node.children.each { |child| queue.unshift(child) }
    end
  end
end
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.
case
  when c1
    f1
  when c2
    f2
  when c3
    f3
end
132
Run procedure f, and return the duration of the execution of f.
def clock 
  t = Time.now
  yield
  Time.now - t
end

clock{ f }
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.
ok = s.match?( /#{word}/i )
134
Declare and initialize a new list items, containing 3 elements a, b, c.
items = [a, b, c]
135
Remove at most 1 item from list items, having 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.
i = items.index(x)
items.delete_at(i) unless i.nil?
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.delete(x)
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
b = s.count("^0-9").zero?
138
Create a new temporary file on the filesystem.
file = Tempfile.new('foo') 
139
Create a new temporary folder on filesystem, for writing.
td = Dir.mktmpdir
140
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
m.delete(k)
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
[items1, items2].each{|ar| ar.each{|item| p item }}
Alternative implementation:
items1.chain(items2).each{|item| puts item}
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
s = x.to_s(16)
143
Iterate alternatively over the elements of the list items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
items1.zip(items2){|pair| puts pair}
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.
b = File.exist?(fp)
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.
logger = Logger.new('logfile.log') # or STDOUT or STDERR
logger.info(msg)
146
Extract floating point value f from its string representation s
f = s.to_f
147
Create string t from string s, keeping only ASCII characters
t = s.gsub(/[^[:ascii:]]/, "")
Alternative implementation:
t = s.gsub(/[[:^ascii:]]/, "")
148
Read a list of integer numbers from the standard input, until EOF.
STDIN.read.split.map(&:to_i)
150
Remove last character from string p, if this character is a slash /.
p.chomp!("/")
Alternative implementation:
p = p.chomp("/")
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!
p.chomp!("/")
152
Create string s containing only the character c.
s = ?c
153
Create the string t as the concatenation of the string s and the integer i.
t = s + i.to_s
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.
rgbs = c1[1..-1].scan(/../), c2[1..-1].scan(/../)
c = "#%02X%02X%02X" % rgbs.transpose.map{|h1, h2| (h1.hex + h2.hex)/2 }
155
Delete from filesystem the file having path filepath.
File.delete(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.
s = "%03d" % i
157
Initialize a constant planet with string value "Earth".
Planet = '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.
y = x.sample(k)
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.
case 1.size
  when 8 then f64
  when 4 then f32
end
161
Multiply all the elements of the list elements by a constant c
elements.map { |el| el * c }
162
execute bat if b is a program option and fox if f is a program option.
bat if ARGV.include?("b")
fox if ARGV.include?("f")
163
Print all the list elements, two by two, assuming list length is even.
list.each_slice(2){|slice| p slice}
164
Open the URL s in the default browser.
Set boolean b to indicate whether the operation was successful.
cmd = case  RbConfig::CONFIG['host_os']
  when  /mswin|mingw|cygwin/ then "start "
  when  /darwin/ then "open "
  when  /linux|bsd/ then "xdg-open "
  else raise "No OS detected"
end
    
b = system cmd + s
165
Assign to variable x the last element of list items.
x = items.last
Alternative implementation:
x = items[-1]
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
ab = a + b
167
Create string t consisting of string s with its prefix p removed (if s starts with p).
t = s.sub(/\A#{p}/, "")
Alternative implementation:
t = s.delete_prefix(p)
168
Create string t consisting of string s with its suffix w removed (if s ends with w).
t = s.sub(/#{w}\z/, "")
Alternative implementation:
t = s.delete_suffix(w)
169
Assign to integer n the number of characters of string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
n = s.length
Alternative implementation:
n = s.size
170
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
n = mymap.size
171
Append element x to the list s.
s << x
172
Insert value v for key k in map m.
m[k] = v
173
Number will be formatted with a comma separator between every group of thousands.
'1000'.gsub(/\B(?=(...)*\b)/, ',')
Alternative implementation:
1000.to_s(:delimited)
Alternative implementation:
'1000'.reverse.scan(/.{1,3}/).join(',').reverse
174
Make a HTTP request with method POST to URL u
Net::HTTP.post(u, content)
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).
s = a.unpack("H*")
Alternative implementation:
s = a.pack("c*").unpack("H*").first
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).
[s].pack("H*").unpack("C*")
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.
L = Dir.glob(File.join("**", "*.{jpg,jpeg,png}"), base: D)
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.
Point = Struct.new(:x, :y)

Rect  = Struct.new(:x1, :y1, :x2, :y2) do
  def contains?(point)
    point.x.between?(x1,x2) && point.y.between?(y1,y2)
  end
end

b = Rect.new(0,0,2,5).contains?(Point.new(0,0))
179
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
Point = Struct.new(:x, :y)

Rect  = Struct.new(:x1, :y1, :x2, :y2) do
  def center  
    Point.new((x1+x2)/2.0, (y1+y2)/2.0)
  end
end

c = Rect.new(0,0,2,5).center
180
Create list x containing the contents of directory d.

x may contain files and subfolders.
No recursive subfolder listing.
x = Dir.children(d)
182
Output the source of the program.
eval s="print 'eval s=';p s"
183
Make a HTTP request with method PUT to URL u
http = Net::HTTP.new(u)
response = http.send_request('PUT', path, body)
184
Assign to variable t a string representing the day, month and year of the day after the current date.
t = (Date.today + 1).to_s
Alternative implementation:
t = 1.day.since.to_s
Alternative implementation:
t = Date.tomorrow.to_s
185
Schedule the execution of f(42) in 30 seconds.
Thread.new do
  sleep 30
  f(42)
end
186
Exit a program cleanly indicating no error to OS
exit
187
Disjoint Sets hold elements that are partitioned into a number of disjoint (non-overlapping) sets.
a.disjoint?(b)
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.
c = a * b
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.
y = x.each_with_object([]){|e, ar| ar << t(e) if p(e)}
Alternative implementation:
y = x.filter_map{|e| t(e) if p(e)}
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
f if a.any?{|v| v > x }
192
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time.
a = BigDecimal('1234567890.12345678901')
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).
a = [[1,2], [3,4], [5,6]]
b = a.transpose
194
Given an array a, set b to an array which has the values of a along its second dimension shifted by n. Elements shifted out should come back at the other end.
b  = a.map{|ar| ar.rotate(n) }
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).
def foo(ar)
  puts "Array size: #{ar.size}, #{ar.max.size}."
  ar.each.with_index(1).sum do |a, i|
    a.each.with_index(1).sum do |el, j|
      el*i*j
    end
  end
end

puts foo(array)
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.
# @param a [Array<Integer>]
# 
# @return [Array<Integer>]
# 
def foo(a)
  a.fill(42)
end

foo(arry.select(&:odd?))

# For older versions of ruby:
# foo(arry.select { |x| x.odd? })
197
Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file.
lines = open(path).readlines
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.
F.truncate(F.pos)
200
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
h = hypot(x, y)
201
Calculate n, the Euclidean norm of data (an array or list of floating point values).
data = Vector[5.0, 4.0, 3.0, 2.0, 1.0]
n = data.norm
202
Calculate the sum of squares s of data, an array of floating point values.
s = data.sum{|i| i**2}
203
Calculate the mean m and the standard deviation s of the list of floating point values data.
m  = data.sum / data.length.to_f
sd = Math.sqrt data.sum { |n| (m-n)**2 } / data.length.to_f 
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
puts Math::frexp(a)
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".
foo = ENV["FOO"]
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.
method(str).call if ["foo", "bar", "baz", "barfl"].include?(str)
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.
a = Array.new(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.
a = a.zip(b,c,d).map{|i,j,k,l| e*(i+j*k+Math::cos(l)) }
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).
T = Struct.new(:s, :n)
v = T.new("Hello, world", [1, 4, 9, 16, 25])
v = nil
210
Assign, at runtime, the compiler version and the options the program was compilerd with to variables version and options, respectively, and print them. For interpreted languages, substitute the version of the interpreter.

Example output:

GCC version 10.0.0 20190914 (experimental)
-mtune=generic -march=x86-64
puts version = RUBY_VERSION
211
Create the folder at path on the filesystem
FileUtils.mkpath( path )
212
Set boolean b to true if path exists on the filesystem and is a directory; false otherwise.
b = Dir.exist?( path )
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.
strings = ['ᾲ στο διάολο', 'ὰι στο διάολο', 'Ὰͅ ΣΤΟ ΔΙΆΟΛΟ', 'ᾺΙ ΣΤΟ ΔΙΆΟΛΟ']

strings.combination(2){|a,b| puts "#{a} equals #{b}: #{a.casecmp?(b)}" }
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.ljust(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.rjust(m, c)
216
Add 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".
 s.center(m,c)
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.
c = a & b
219
Create 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.
t = s.squeeze(" ")
220
Create t consisting of 3 values having different types.

Explain if the elements of t are strongly typed or not.
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.
t = s.delete("^0-9")
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.index(x) || -1
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.
items = ['foo', 'bar', 'baz', 'qux']
puts items.any?("baz") ? "found it" : "never found it"
224
Insert element x at the beginning of list items.
items.unshift(x)
Alternative implementation:
items.prepend(x)
225
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
def f( x=nil )
  puts x ? "present" : "not present"
end
226
Remove the last element from 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).
y = x.dup
228
Copy the file at path src to dst.
FileUtils.copy(src, dst)
230
Cancel an ongoing processing p if it has not finished after 5s.
Timeout::timeout(5) { p }
231
Set b to true if the byte sequence s consists entirely of valid UTF-8 character code points, false otherwise.
b = s.force_encoding("UTF-8").valid_encoding?  
234
Assign to string s the standard base64 encoding of the byte array data, as specified by RFC 4648.
s = Base64.strict_encode64(data)
235
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648.
data = Base64.strict_decode64(s)
236
Initialize a quotient q = a/b of arbitrary precision. a and b are large integers.
q = Rational(a, b)
237
Assign to c the result of (a xor b)
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.
c = a.zip(b).map{|aa, bb| aa ^ bb}
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.
x = s.match(/\b\d{3}\b/).to_s
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.
a, b = a.zip(b).sort.transpose
242
Call a function f on each element e of a set x.
x.each { |e| f(e) }
243
Print the contents of the list or array a on the standard output.
puts a
Alternative implementation:
puts a.join(", ")
244
Print the contents of the map m to the standard output: keys and values.
puts m
245
Print the value of object x having custom type T, for log or debug.
puts x
246
Set c to the number of distinct elements in list items.
c = items.uniq.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.
x.select!(&:p)
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
a, b, c = 42, 'hello', 5.0
250
Choose a value x from map m.
m must not be empty. Ignore the keys.
x = m.values.sample
251
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
i = s.to_i(2)
252
Assign to the variable x the value "a" if calling the function condition returns true, or the value "b" otherwise.
x = condition ? "a" : "b"
Alternative implementation:
x = if condition
      "a"
    else
      "b"
    end
253
Print the stack frames of the current execution thread of the program.
puts caller
254
Replace all exact occurrences of "foo" with "bar" in the string list x
x.map!{|el| el == "foo" ? "bar" : el}
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.
puts x
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
5.downto(0){|n| puts n }
257
Print each index i and value x from the list items, from the last down to the first.
items.each_with_index.reverse_each{|x, i| puts "#{i} #{x}" }
258
Convert the string values from list a into a list of integers b.
b = a.map(&:to_i)
259
Build the list parts consisting of substrings of the input string s, separated by any of the characters ',' (comma), '-' (dash), '_' (underscore).
parts = s.split( Regexp.union(",", "-", "_") )
260
Declare a new list items of string elements, containing zero elements
items = []
261
Assign to string x the value of fields (hours, minutes, seconds) of date d, in format HH:MM:SS.
d = Time.now
x = d.strftime("%H:%M:%S")
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
t = n.digits(2).index(1)
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.
def log2d(n) =  n.bit_length - 1

def log2u(n) = 2 ** log2d(n) == n ? log2d(n) : log2d(n) + 1

(1..12).each{|n| puts "#{n}  #{log2d(n)}  #{log2u(n)}" }
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.
def foo(ar)
  puts "#{ar.size} #{ar.first.size}"
end

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).
i = 42
i.digits(2).count(1)[0]
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"
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.
def foo(x)
  puts x.class == String ? x : "Nothing"
end

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.
Vector = Struct.new(:x, :y, :z) do
  def * (other)
    Vector.new(
      y*other.z - z*other.y,
      z*other.x - x*other.z,
      x*other.y - y*other.x)
  end
end
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"
(1..100).each do |i|
  d3 = i % 3 == 0
  d5 = i % 5 == 0

  if d3 && d5
    puts "FizzBuzz"
  elsif d3
    puts "Fizz"
  elsif d5
    puts "Buzz"
  else
    puts "#{i}"
  end
end
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)
b = Dir.empty?(p)
274
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc.
t = s.gsub(/[[:space:]]/, "")
Alternative implementation:
t = s.gsub(/\s/, "")
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).
a = s.scan(/[01]{8}/).map{|slice| slice.to_i(2).to_s(16).rjust(2, "0")}
276
Insert an element e into the set x.
x.add(e)
Alternative implementation:
x << e
277
Remove the element e from the set x.

Explains what happens if e was already absent from x.
x.delete(e)
278
Read one line into the string line.

Explain what happens if EOF is reached.
line = gets
279
Read all the lines (until EOF) into the list of strings lines.
lines = readlines
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.select{|k,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).
Point = Struct.new(:x, :y)
m = {Point.new(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.
Foo = Struct.new(:x, :y)
m = {Foo.new(42, 5) => "Hello"}
283
Build the list parts consisting of substrings of input string s, separated by the string sep.
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.
a = Array.new(n, 0)
Alternative implementation:
a = [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.
a = Float::NAN
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.each_char.with_index{|c, i| puts "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.
n = s.bytesize
288
Set the boolean b to true if the set x contains the element e, false otherwise.
b = x.include? e
289
Create the string s by concatenating the strings a and b.
s = a + b
291
Delete all the elements from index i (included) to index j (excluded) from the list items.
items.slice!(i...j)
292
Write "Hello World and 你好" to standard output in UTF-8.
puts "Hello World and 你好" 
293
Create a new stack s, push an element x, then pop the element into the variable y.
s = []
s.push(x)
y = s.pop
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.
a = [1, 12, 42]
puts a.join(", ")
297
Sort the string list data in a case-insensitive manner.

The sorting must not destroy the original casing of the strings.
data.sort_by(&:downcase)
298
Create the map y by cloning the map x.

y is a shallow copy, not a deep copy.
y = x.dup
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.
def fib(n) = n < 2 ? n : fib(n-1) + fib(n-2)
302
Give the integer x = 8, assign to the string s the value "Our sun has 8 planets", where the number 8 was evaluated from x.
s = "Our sun has #{x} planets."
304
Create the array of bytes data by encoding the string s in UTF-8.
data = s.bytes
305
Calculate and print a^b, and a^n, where a and b are floating point numbers and n is an integer.
puts a ** b, a ** n
307
Create a function that XOR encrypts/decrypts a string
def xor_string(str, key)
  ords = key.chars.map(&:ord).cycle
  str.chars.zip(ords).inject(""){|res, (c,o)| res << (c.ord ^ o) }
end
308
Create the string representation s of the integer value n in base b.

18 in base 3 -> "200"
26 in base 5 -> "101"
121 in base 12 -> "a1"

s = n.to_s(b)