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Class: Array

Relationships & Source Files
Super Chains via Extension / Inclusion / Inheritance
Instance Chain:
self, ::Enumerable
Inherits: Object
Defined in: array.c,
pack.c

Overview

Arrays are ordered, integer-indexed collections of any object.

Array indexing starts at 0, as in C or Java. A negative index is assumed to be relative to the end of the array—that is, an index of -1 indicates the last element of the array, -2 is the next to last element in the array, and so on.

Creating Arrays

A new array can be created by using the literal constructor .[]. Arrays can contain different types of objects. For example, the array below contains an ::Integer, a ::String and a ::Float:

ary = [1, "two", 3.0] #=> [1, "two", 3.0]

An array can also be created by explicitly calling .new with zero, one (the initial size of the Array) or two arguments (the initial size and a default object).

ary = Array.new    #=> []
Array.new(3)       #=> [nil, nil, nil]
Array.new(3, true) #=> [true, true, true]

Note that the second argument populates the array with references to the same object. Therefore, it is only recommended in cases when you need to instantiate arrays with natively immutable objects such as Symbols, numbers, true or false.

To create an array with separate objects a block can be passed instead. This method is safe to use with mutable objects such as hashes, strings or other arrays:

Array.new(4) { Hash.new }  #=> [{}, {}, {}, {}]
Array.new(4) {|i| i.to_s } #=> ["0", "1", "2", "3"]

This is also a quick way to build up multi-dimensional arrays:

empty_table = Array.new(3) { Array.new(3) }
#=> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

An array can also be created by using the Array() method, provided by ::Kernel, which tries to call #to_ary, then #to_a on its argument.

Array({:a => "a", :b => "b"}) #=> [[:a, "a"], [:b, "b"]]

Example Usage

In addition to the methods it mixes in through the ::Enumerable module, the Array class has proprietary methods for accessing, searching and otherwise manipulating arrays.

Some of the more common ones are illustrated below.

Accessing Elements

Elements in an array can be retrieved using the #[] method. It can take a single integer argument (a numeric index), a pair of arguments (start and length) or a range. Negative indices start counting from the end, with -1 being the last element.

arr = [1, 2, 3, 4, 5, 6]
arr[2]    #=> 3
arr[100]  #=> nil
arr[-3]   #=> 4
arr[2, 3] #=> [3, 4, 5]
arr[1..4] #=> [2, 3, 4, 5]
arr[1..-3] #=> [2, 3, 4]

Another way to access a particular array element is by using the #at method

arr.at(0) #=> 1

The #slice method works in an identical manner to #[].

To raise an error for indices outside of the array bounds or else to provide a default value when that happens, you can use #fetch.

arr = ['a', 'b', 'c', 'd', 'e', 'f']
arr.fetch(100) #=> IndexError: index 100 outside of array bounds: -6...6
arr.fetch(100, "oops") #=> "oops"

The special methods #first and #last will return the first and last elements of an array, respectively.

arr.first #=> 1
arr.last  #=> 6

To return the first n elements of an array, use #take

arr.take(3) #=> [1, 2, 3]

#drop does the opposite of #take, by returning the elements after n elements have been dropped:

arr.drop(3) #=> [4, 5, 6]

Obtaining Information about an Array

Arrays keep track of their own length at all times. To query an array about the number of elements it contains, use #length, #count or #size.

browsers = ['Chrome', 'Firefox', 'Safari', 'Opera', 'IE']
browsers.length #=> 5
browsers.count #=> 5

To check whether an array contains any elements at all

browsers.empty? #=> false

To check whether a particular item is included in the array

browsers.include?('Konqueror') #=> false

Adding Items to Arrays

Items can be added to the end of an array by using either #push or #<<

arr = [1, 2, 3, 4]
arr.push(5) #=> [1, 2, 3, 4, 5]
arr << 6    #=> [1, 2, 3, 4, 5, 6]

#unshift will add a new item to the beginning of an array.

arr.unshift(0) #=> [0, 1, 2, 3, 4, 5, 6]

With #insert you can add a new element to an array at any position.

arr.insert(3, 'apple')  #=> [0, 1, 2, 'apple', 3, 4, 5, 6]

Using the #insert method, you can also insert multiple values at once:

arr.insert(3, 'orange', 'pear', 'grapefruit')
#=> [0, 1, 2, "orange", "pear", "grapefruit", "apple", 3, 4, 5, 6]

Removing Items from an Array

The method #pop removes the last element in an array and returns it:

arr =  [1, 2, 3, 4, 5, 6]
arr.pop #=> 6
arr #=> [1, 2, 3, 4, 5]

To retrieve and at the same time remove the first item, use #shift:

arr.shift #=> 1
arr #=> [2, 3, 4, 5]

To delete an element at a particular index:

arr.delete_at(2) #=> 4
arr #=> [2, 3, 5]

To delete a particular element anywhere in an array, use #delete:

arr = [1, 2, 2, 3]
arr.delete(2) #=> 2
arr #=> [1,3]

A useful method if you need to remove nil values from an array is #compact:

arr = ['foo', 0, nil, 'bar', 7, 'baz', nil]
arr.compact  #=> ['foo', 0, 'bar', 7, 'baz']
arr          #=> ['foo', 0, nil, 'bar', 7, 'baz', nil]
arr.compact! #=> ['foo', 0, 'bar', 7, 'baz']
arr          #=> ['foo', 0, 'bar', 7, 'baz']

Another common need is to remove duplicate elements from an array.

It has the non-destructive #uniq, and destructive method #uniq!

arr = [2, 5, 6, 556, 6, 6, 8, 9, 0, 123, 556]
arr.uniq #=> [2, 5, 6, 556, 8, 9, 0, 123]

Iterating over Arrays

Like all classes that include the ::Enumerable module, Array has an each method, which defines what elements should be iterated over and how. In case of Array's #each, all elements in the Array instance are yielded to the supplied block in sequence.

Note that this operation leaves the array unchanged.

arr = [1, 2, 3, 4, 5]
arr.each { |a| print a -= 10, " " }
# prints: -9 -8 -7 -6 -5
#=> [1, 2, 3, 4, 5]

Another sometimes useful iterator is #reverse_each which will iterate over the elements in the array in reverse order.

words = %w[first second third fourth fifth sixth]
str = ""
words.reverse_each { |word| str += "#{word} " }
p str #=> "sixth fifth fourth third second first "

The #map method can be used to create a new array based on the original array, but with the values modified by the supplied block:

arr.map { |a| 2*a }   #=> [2, 4, 6, 8, 10]
arr                   #=> [1, 2, 3, 4, 5]
arr.map! { |a| a**2 } #=> [1, 4, 9, 16, 25]
arr                   #=> [1, 4, 9, 16, 25]

Selecting Items from an Array

Elements can be selected from an array according to criteria defined in a block. The selection can happen in a destructive or a non-destructive manner. While the destructive operations will modify the array they were called on, the non-destructive methods usually return a new array with the selected elements, but leave the original array unchanged.

Non-destructive Selection

arr = [1, 2, 3, 4, 5, 6]
arr.select { |a| a > 3 }     #=> [4, 5, 6]
arr.reject { |a| a < 3 }     #=> [3, 4, 5, 6]
arr.drop_while { |a| a < 4 } #=> [4, 5, 6]
arr                          #=> [1, 2, 3, 4, 5, 6]

Destructive Selection

#select! and #reject! are the corresponding destructive methods to #select and #reject

Similar to #select vs. #reject, #delete_if and #keep_if have the exact opposite result when supplied with the same block:

arr.delete_if { |a| a < 4 } #=> [4, 5, 6]
arr                         #=> [4, 5, 6]

arr = [1, 2, 3, 4, 5, 6]
arr.keep_if { |a| a < 4 } #=> [1, 2, 3]
arr                       #=> [1, 2, 3]

Class Method Summary

Instance Attribute Summary

::Enumerable - Included

#all?

Passes each element of the collection to the given block.
#any?

Passes each element of the collection to the given block.
#none?

Passes each element of the collection to the given block.
#one?

Passes each element of the collection to the given block.

Instance Method Summary

::Enumerable - Included

#chunk

Enumerates over the items, chunking them together based on the return value of the block.
#chunk_while

Creates an enumerator for each chunked elements.
#collect

Alias for Enumerable#map.
#collect_concat

Alias for Enumerable#flat_map.
#count

Returns the number of items in enum through enumeration.
#cycle

Calls block for each element of enum repeatedly n times or forever if none or nil is given.
#detect

Alias for Enumerable#find.
#drop

Drops first n elements from enum, and returns rest elements in an array.
#drop_while

Drops elements up to, but not including, the first element for which the block returns nil or false and returns an array containing the remaining elements.
#each_cons

Iterates the given block for each array of consecutive <n> elements.
#each_entry

Calls block once for each element in self, passing that element as a parameter, converting multiple values from yield to an array.
#each_slice

Iterates the given block for each slice of <n> elements.
#each_with_index

Calls block with two arguments, the item and its index, for each item in enum.
#each_with_object

Iterates the given block for each element with an arbitrary object given, and returns the initially given object.
#entries

Alias for Enumerable#to_a.
#find

Passes each entry in enum to block.
#find_all

Alias for Enumerable#select.
#find_index

Compares each entry in enum with value or passes to block.
#first

Returns the first element, or the first n elements, of the enumerable.
#flat_map

Returns a new array with the concatenated results of running block once for every element in enum.
#grep

Returns an array of every element in enum for which Pattern === element.
#grep_v

Inverted version of Enumerable#grep.
#group_by

Groups the collection by result of the block.
#include?

Alias for Enumerable#member?.
#inject

Combines all elements of enum by applying a binary operation, specified by a block or a symbol that names a method or operator.
#lazy

Returns a lazy enumerator, whose methods map/collect, flat_map/collect_concat, select/find_all, reject, grep, grep_v, zip, take, take_while, drop, and drop_while enumerate values only on an as-needed basis.
#map

Returns a new array with the results of running block once for every element in enum.
#max

Returns the object in enum with the maximum value.
#max_by

Returns the object in enum that gives the maximum value from the given block.
#member?

Returns true if any member of enum equals obj.
#min

Returns the object in enum with the minimum value.
#min_by

Returns the object in enum that gives the minimum value from the given block.
#minmax

Returns a two element array which contains the minimum and the maximum value in the enumerable.
#minmax_by

Returns a two element array containing the objects in enum that correspond to the minimum and maximum values respectively from the given block.
#partition

Returns two arrays, the first containing the elements of enum for which the block evaluates to true, the second containing the rest.
#reduce

Alias for Enumerable#inject.
#reject

Returns an array for all elements of enum for which the given block returns false.
#reverse_each

Builds a temporary array and traverses that array in reverse order.
#select

Returns an array containing all elements of enum for which the given block returns a true value.
#slice_after

Creates an enumerator for each chunked elements.
#slice_before

Creates an enumerator for each chunked elements.
#slice_when

Creates an enumerator for each chunked elements.
#sort

Returns an array containing the items in enum sorted, either according to their own #<=> method, or by using the results of the supplied block.
#sort_by

Sorts enum using a set of keys generated by mapping the values in enum through the given block.
#take

Returns first n elements from enum.
#take_while

Passes elements to the block until the block returns nil or false, then stops iterating and returns an array of all prior elements.
#to_a

Returns an array containing the items in enum.
#to_h

Returns the result of interpreting enum as a list of [key, value] pairs.
#zip

Takes one element from enum and merges corresponding elements from each args.

Constructor Details

.new(size = 0, default = nil) .new(array) .new(size) {|index| ... }

Returns a new array.

In the first form, if no arguments are sent, the new array will be empty. When a #size and an optional default are sent, an array is created with #size copies of default. Take notice that all elements will reference the same object default.

The second form creates a copy of the array passed as a parameter (the array is generated by calling to_ary on the parameter).

first_array = ["Matz", "Guido"]

second_array = Array.new(first_array) #=> ["Matz", "Guido"]

first_array.equal? second_array       #=> false

In the last form, an array of the given size is created. Each element in this array is created by passing the element's index to the given block and storing the return value.

Array.new(3){ |index| index ** 2 }
# => [0, 1, 4]

Common gotchas

When sending the second parameter, the same object will be used as the value for all the array elements:

a = Array.new(2, Hash.new)
# => [{}, {}]

a[0]['cat'] = 'feline'
a # => [{"cat"=>"feline"}, {"cat"=>"feline"}]

a[1]['cat'] = 'Felix'
a # => [{"cat"=>"Felix"}, {"cat"=>"Felix"}]

Since all the Array elements store the same hash, changes to one of them will affect them all.

If multiple copies are what you want, you should use the block version which uses the result of that block each time an element of the array needs to be initialized:

a = Array.new(2) { Hash.new }
a[0]['cat'] = 'feline'
a # => [{"cat"=>"feline"}, {}]

Class Method Details

.[](*args)

Returns a new array populated with the given objects.

Array.[]( 1, 'a', /^A/ ) # => [1, "a", /^A/]
Array[ 1, 'a', /^A/ ]    # => [1, "a", /^A/]
[ 1, 'a', /^A/ ]         # => [1, "a", /^A/]

.try_convert(obj) ⇒ Array?

Tries to convert obj into an array, using #to_ary method. Returns the converted array or nil if obj cannot be converted for any reason. This method can be used to check if an argument is an array.

Array.try_convert([1])   #=> [1]
Array.try_convert("1")   #=> nil

if tmp = Array.try_convert(arg)
  # the argument is an array
elsif tmp = String.try_convert(arg)
  # the argument is a string
end

Instance Attribute Details

#any? {|obj| ... } ⇒ Boolean (readonly) #any? ⇒ Boolean

See also Enumerable#any?

#empty? ⇒ Boolean (readonly)

Returns true if self contains no elements.

[].empty?   #=> true

#frozen? ⇒ Boolean (readonly)

Return true if this array is frozen (or temporarily frozen while being sorted). See also Object#frozen?

Instance Method Details

#&(other_ary) ⇒ Array

Set Intersection — Returns a new array containing elements common to the two arrays, excluding any duplicates. The order is preserved from the original array.

It compares elements using their #hash and #eql? methods for efficiency.

[ 1, 1, 3, 5 ] & [ 1, 2, 3 ]                 #=> [ 1, 3 ]
[ 'a', 'b', 'b', 'z' ] & [ 'a', 'b', 'c' ]   #=> [ 'a', 'b' ]

See also #uniq.

#*(int) ⇒ Array #*(str) ⇒ String

Repetition — With a ::String argument, equivalent to ary.join(str).

Otherwise, returns a new array built by concatenating the int copies of self.

[ 1, 2, 3 ] * 3    #=> [ 1, 2, 3, 1, 2, 3, 1, 2, 3 ]
[ 1, 2, 3 ] * ","  #=> "1,2,3"

#+(other_ary) ⇒ Array

Concatenation — Returns a new array built by concatenating the two arrays together to produce a third array.

[ 1, 2, 3 ] + [ 4, 5 ]    #=> [ 1, 2, 3, 4, 5 ]
a = [ "a", "b", "c" ]
c = a + [ "d", "e", "f" ]
c                         #=> [ "a", "b", "c", "d", "e", "f" ]
a                         #=> [ "a", "b", "c" ]

Note that

x += y

is the same as

x = x + y

This means that it produces a new array. As a consequence, repeated use of += on arrays can be quite inefficient.

See also #concat.

#-(other_ary) ⇒ Array

Array Difference

Returns a new array that is a copy of the original array, removing any items that also appear in other_ary. The order is preserved from the original array.

It compares elements using their #hash and #eql? methods for efficiency.

[ 1, 1, 2, 2, 3, 3, 4, 5 ] - [ 1, 2, 4 ]  #=>  [ 3, 3, 5 ]

If you need set-like behavior, see the library class Set.

#<<(obj) ⇒ Array

Append—Pushes the given object on to the end of this array. This expression returns the array itself, so several appends may be chained together.

[ 1, 2 ] << "c" << "d" << [ 3, 4 ]
        #=>  [ 1, 2, "c", "d", [ 3, 4 ] ]

#<=>(other_ary) ⇒ 1, ...

Comparison — Returns an integer (-1, 0, or +1) if this array is less than, equal to, or greater than other_ary.

Each object in each array is compared (using the <=> operator).

Arrays are compared in an “element-wise” manner; the first element of ary is compared with the first one of other_ary using the <=> operator, then each of the second elements, etc… As soon as the result of any such comparison is non zero (i.e. the two corresponding elements are not equal), that result is returned for the whole array comparison.

If all the elements are equal, then the result is based on a comparison of the array lengths. Thus, two arrays are “equal” according to <=> if, and only if, they have the same length and the value of each element is equal to the value of the corresponding element in the other array.

nil is returned if the other_ary is not an array or if the comparison of two elements returned nil.

[ "a", "a", "c" ]    <=> [ "a", "b", "c" ]   #=> -1
[ 1, 2, 3, 4, 5, 6 ] <=> [ 1, 2 ]            #=> +1
[ 1, 2 ]             <=> [ 1, :two ]         #=> nil

#==(other_ary) ⇒ Boolean

Equality — Two arrays are equal if they contain the same number of elements and if each element is equal to (according to Object#==) the corresponding element in other_ary.

[ "a", "c" ]    == [ "a", "c", 7 ]     #=> false
[ "a", "c", 7 ] == [ "a", "c", 7 ]     #=> true
[ "a", "c", 7 ] == [ "a", "d", "f" ]   #=> false

#[](index) ⇒ Object? #[](start, length) ⇒ Array? #[](range) ⇒ Array? #slice(index) ⇒ Object? #slice(start, length) ⇒ Array? #slice(range) ⇒ Array?
Also known as: #slice

Element Reference — Returns the element at #index, or returns a subarray starting at the start index and continuing for #length elements, or returns a subarray specified by range of indices.

Negative indices count backward from the end of the array (-1 is the last element). For start and range cases the starting index is just before an element. Additionally, an empty array is returned when the starting index for an element range is at the end of the array.

Returns nil if the index (or starting index) are out of range.

a = [ "a", "b", "c", "d", "e" ]
a[2] +  a[0] + a[1]    #=> "cab"
a[6]                   #=> nil
a[1, 2]                #=> [ "b", "c" ]
a[1..3]                #=> [ "b", "c", "d" ]
a[4..7]                #=> [ "e" ]
a[6..10]               #=> nil
a[-3, 3]               #=> [ "c", "d", "e" ]
# special cases
a[5]                   #=> nil
a[6, 1]                #=> nil
a[5, 1]                #=> []
a[5..10]               #=> []

#[]=(index, obj) ⇒ Object #[]=(start, length, obj or other_ary or nil) ⇒ Object, ... #[]=(range, obj or other_ary or nil) ⇒ Object, ...

Element Assignment — Sets the element at #index, or replaces a subarray from the start index for #length elements, or replaces a subarray specified by the range of indices.

If indices are greater than the current capacity of the array, the array grows automatically. Elements are inserted into the array at start if #length is zero.

Negative indices will count backward from the end of the array. For start and range cases the starting index is just before an element.

An IndexError is raised if a negative index points past the beginning of the array.

See also #push, and #unshift.

a = Array.new
a[4] = "4";                 #=> [nil, nil, nil, nil, "4"]
a[0, 3] = [ 'a', 'b', 'c' ] #=> ["a", "b", "c", nil, "4"]
a[1..2] = [ 1, 2 ]          #=> ["a", 1, 2, nil, "4"]
a[0, 2] = "?"               #=> ["?", 2, nil, "4"]
a[0..2] = "A"               #=> ["A", "4"]
a[-1]   = "Z"               #=> ["A", "Z"]
a[1..-1] = nil              #=> ["A", nil]
a[1..-1] = []               #=> ["A"]
a[0, 0] = [ 1, 2 ]          #=> [1, 2, "A"]
a[3, 0] = "B"               #=> [1, 2, "A", "B"]

#assoc(obj) ⇒ element_ary?

Searches through an array whose elements are also arrays comparing obj with the first element of each contained array using obj.==.

Returns the first contained array that matches (that is, the first associated array), or nil if no match is found.

See also #rassoc

s1 = [ "colors", "red", "blue", "green" ]
s2 = [ "letters", "a", "b", "c" ]
s3 = "foo"
a  = [ s1, s2, s3 ]
a.assoc("letters")  #=> [ "letters", "a", "b", "c" ]
a.assoc("foo")      #=> nil

#at(index) ⇒ Object?

Returns the element at #index. A negative index counts from the end of self. Returns nil if the index is out of range. See also #[].

a = [ "a", "b", "c", "d", "e" ]
a.at(0)     #=> "a"
a.at(-1)    #=> "e"

#bsearch {|x| ... } ⇒ elem

By using binary search, finds a value from this array which meets the given condition in O(log n) where n is the size of the array.

You can use this method in two use cases: a find-minimum mode and a find-any mode. In either case, the elements of the array must be monotone (or sorted) with respect to the block.

In find-minimum mode (this is a good choice for typical use case), the block must return true or false, and there must be an index i (0 <= i <= ary.size) so that:

  • the block returns false for any element whose index is less than i, and

  • the block returns true for any element whose index is greater than or equal to i.

This method returns the i-th element. If i is equal to ary.size, it returns nil.

ary = [0, 4, 7, 10, 12]
ary.bsearch {|x| x >=   4 } #=> 4
ary.bsearch {|x| x >=   6 } #=> 7
ary.bsearch {|x| x >=  -1 } #=> 0
ary.bsearch {|x| x >= 100 } #=> nil

In find-any mode (this behaves like libc's bsearch(3)), the block must return a number, and there must be two indices i and j (0 <= i <= j <= ary.size) so that:

  • the block returns a positive number for ary if 0 <= k < i,

  • the block returns zero for ary if i <= k < j, and

  • the block returns a negative number for ary if j <= k < ary.size.

Under this condition, this method returns any element whose index is within i…j. If i is equal to j (i.e., there is no element that satisfies the block), this method returns nil.

ary = [0, 4, 7, 10, 12]
# try to find v such that 4 <= v < 8
ary.bsearch {|x| 1 - x / 4 } #=> 4 or 7
# try to find v such that 8 <= v < 10
ary.bsearch {|x| 4 - x / 2 } #=> nil

You must not mix the two modes at a time; the block must always return either true/false, or always return a number. It is undefined which value is actually picked up at each iteration.

#bsearch_index {|x| ... } ⇒ Integer?

By using binary search, finds an index of a value from this array which meets the given condition in O(log n) where n is the size of the array.

It supports two modes, depending on the nature of the block and they are exactly the same as in the case of #bsearch method with the only difference being that this method returns the index of the element instead of the element itself. For more details consult the documentation for #bsearch.

#clear ⇒ Array

Removes all elements from self.

a = [ "a", "b", "c", "d", "e" ]
a.clear    #=> [ ]

#collect {|item| ... } ⇒ Array #map {|item| ... } ⇒ Array #collect ⇒ Enumerator #map ⇒ Enumerator

Alias for #map.

#collect! {|item| ... } ⇒ Array #map! {|item| ... } ⇒ Array #collect! ⇒ Enumerator #map! ⇒ Enumerator

Alias for #map!.

#combination(n) {|c| ... } ⇒ Array #combination(n) ⇒ Enumerator

When invoked with a block, yields all combinations of length n of elements from the array and then returns the array itself.

The implementation makes no guarantees about the order in which the combinations are yielded.

If no block is given, an ::Enumerator is returned instead.

Examples:

a = [1, 2, 3, 4]
a.combination(1).to_a  #=> [[1],[2],[3],[4]]
a.combination(2).to_a  #=> [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
a.combination(3).to_a  #=> [[1,2,3],[1,2,4],[1,3,4],[2,3,4]]
a.combination(4).to_a  #=> [[1,2,3,4]]
a.combination(0).to_a  #=> [[]] # one combination of length 0
a.combination(5).to_a  #=> []   # no combinations of length 5

#compact ⇒ Array

Returns a copy of self with all nil elements removed.

[ "a", nil, "b", nil, "c", nil ].compact
                  #=> [ "a", "b", "c" ]

#compact! ⇒ Array?

Removes nil elements from the array.

Returns nil if no changes were made, otherwise returns the array.

[ "a", nil, "b", nil, "c" ].compact! #=> [ "a", "b", "c" ]
[ "a", "b", "c" ].compact!           #=> nil

#concat(other_ary) ⇒ Array

Appends the elements of other_ary to self.

[ "a", "b" ].concat( ["c", "d"] ) #=> [ "a", "b", "c", "d" ]
a = [ 1, 2, 3 ]
a.concat( [ 4, 5 ] )
a                                 #=> [ 1, 2, 3, 4, 5 ]

See also #+.

#count ⇒ Integer #count(obj) ⇒ Integer #count {|item| ... } ⇒ Integer

Returns the number of elements.

If an argument is given, counts the number of elements which equal obj using #==.

If a block is given, counts the number of elements for which the block returns a true value.

ary = [1, 2, 4, 2]
ary.count                  #=> 4
ary.count(2)               #=> 2
ary.count { |x| x%2 == 0 } #=> 3

#cycle(n = nil) {|obj| ... } ⇒ nil #cycle(n = nil) ⇒ Enumerator

Calls the given block for each element n times or forever if nil is given.

Does nothing if a non-positive number is given or the array is empty.

Returns nil if the loop has finished without getting interrupted.

If no block is given, an ::Enumerator is returned instead.

a = ["a", "b", "c"]
a.cycle { |x| puts x }     # print, a, b, c, a, b, c,.. forever.
a.cycle(2) { |x| puts x }  # print, a, b, c, a, b, c.

#delete(obj) ⇒ item? #delete(obj) ⇒ item, result of block

Deletes all items from self that are equal to obj.

Returns the last deleted item, or nil if no matching item is found.

If the optional code block is given, the result of the block is returned if the item is not found. (To remove nil elements and get an informative return value, use #compact!)

a = [ "a", "b", "b", "b", "c" ]
a.delete("b")                   #=> "b"
a                               #=> ["a", "c"]
a.delete("z")                   #=> nil
a.delete("z") { "not found" }   #=> "not found"

#delete_at(index) ⇒ Object?

Deletes the element at the specified #index, returning that element, or nil if the #index is out of range.

See also #slice!

a = ["ant", "bat", "cat", "dog"]
a.delete_at(2)    #=> "cat"
a                 #=> ["ant", "bat", "dog"]
a.delete_at(99)   #=> nil

#delete_if {|item| ... } ⇒ Array #delete_if ⇒ Enumerator

Deletes every element of self for which block evaluates to true.

The array is changed instantly every time the block is called, not after the iteration is over.

See also #reject!

If no block is given, an ::Enumerator is returned instead.

scores = [ 97, 42, 75 ]
scores.delete_if {|score| score < 80 }   #=> [97]

#dig(idx, ...) ⇒ Object

Extracts the nested value specified by the sequence of idx objects by calling dig at each step, returning nil if any intermediate step is nil.

a = [[1, [2, 3]]]

a.dig(0, 1, 1)                    #=> 3
a.dig(1, 2, 3)                    #=> nil
a.dig(0, 0, 0)                    #=> NoMethodError, undefined method `dig' for 1:Fixnum
[42, {foo: :bar}].dig(1, :foo)    #=> :bar

#drop(n) ⇒ Array

Drops first n elements from ary and returns the rest of the elements in an array.

If a negative number is given, raises an ::ArgumentError.

See also #take

a = [1, 2, 3, 4, 5, 0]
a.drop(3)             #=> [4, 5, 0]

#drop_while {|obj| ... } ⇒ Array #drop_while ⇒ Enumerator

Drops elements up to, but not including, the first element for which the block returns nil or false and returns an array containing the remaining elements.

If no block is given, an ::Enumerator is returned instead.

See also #take_while

a = [1, 2, 3, 4, 5, 0]
a.drop_while {|i| i < 3 }   #=> [3, 4, 5, 0]

#each {|item| ... } ⇒ Array #each ⇒ Enumerator

Calls the given block once for each element in self, passing that element as a parameter. Returns the array itself.

If no block is given, an ::Enumerator is returned.

a = [ "a", "b", "c" ]
a.each {|x| print x, " -- " }

produces:

a -- b -- c --

#each_index {|index| ... } ⇒ Array #each_index ⇒ Enumerator

Same as #each, but passes the #index of the element instead of the element itself.

An Enumerator is returned if no block is given.

a = [ "a", "b", "c" ]
a.each_index {|x| print x, " -- " }

produces:

0 -- 1 -- 2 --

#eql?(other) ⇒ Boolean

Returns true if self and other are the same object, or are both arrays with the same content (according to Object#eql?).

#fetch(index) ⇒ Object #fetch(index, default) ⇒ Object #fetch(index) {|index| ... } ⇒ Object

Tries to return the element at position #index, but throws an ::IndexError exception if the referenced #index lies outside of the array bounds. This error can be prevented by supplying a second argument, which will act as a default value.

Alternatively, if a block is given it will only be executed when an invalid #index is referenced. Negative values of #index count from the end of the array.

a = [ 11, 22, 33, 44 ]
a.fetch(1)               #=> 22
a.fetch(-1)              #=> 44
a.fetch(4, 'cat')        #=> "cat"
a.fetch(100) { |i| puts "#{i} is out of bounds" }
                         #=> "100 is out of bounds"

#fill(obj) ⇒ Array #fill(obj, start [, length]) ⇒ Array #fill(obj, range) ⇒ Array #fill {|index| ... } ⇒ Array #fill(start [, length] ) {|index| ... } ⇒ Array #fill(range) {|index| ... } ⇒ Array

The first three forms set the selected elements of self (which may be the entire array) to obj.

A start of nil is equivalent to zero.

A #length of nil is equivalent to the length of the array.

The last three forms fill the array with the value of the given block, which is passed the absolute index of each element to be filled.

Negative values of start count from the end of the array, where -1 is the last element.

a = [ "a", "b", "c", "d" ]
a.fill("x")              #=> ["x", "x", "x", "x"]
a.fill("z", 2, 2)        #=> ["x", "x", "z", "z"]
a.fill("y", 0..1)        #=> ["y", "y", "z", "z"]
a.fill { |i| i*i }       #=> [0, 1, 4, 9]
a.fill(-2) { |i| i*i*i } #=> [0, 1, 8, 27]

#find_index(obj) ⇒ Integer? #find_index {|item| ... } ⇒ Integer? #find_index ⇒ Enumerator #index(obj) ⇒ Integer? #index {|item| ... } ⇒ Integer? #index ⇒ Enumerator

Alias for #index.

#first ⇒ Object? #first(n) ⇒ Array

Returns the first element, or the first n elements, of the array. If the array is empty, the first form returns nil, and the second form returns an empty array. See also #last for the opposite effect.

a = [ "q", "r", "s", "t" ]
a.first     #=> "q"
a.first(2)  #=> ["q", "r"]

#flatten ⇒ Array #flatten(level) ⇒ Array

Returns a new array that is a one-dimensional flattening of self (recursively).

That is, for every element that is an array, extract its elements into the new array.

The optional level argument determines the level of recursion to flatten.

s = [ 1, 2, 3 ]           #=> [1, 2, 3]
t = [ 4, 5, 6, [7, 8] ]   #=> [4, 5, 6, [7, 8]]
a = [ s, t, 9, 10 ]       #=> [[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10]
a.flatten                 #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
a = [ 1, 2, [3, [4, 5] ] ]
a.flatten(1)              #=> [1, 2, 3, [4, 5]]

#flatten! ⇒ Array? #flatten!(level) ⇒ Array?

Flattens self in place.

Returns nil if no modifications were made (i.e., the array contains no subarrays.)

The optional level argument determines the level of recursion to flatten.

a = [ 1, 2, [3, [4, 5] ] ]
a.flatten!   #=> [1, 2, 3, 4, 5]
a.flatten!   #=> nil
a            #=> [1, 2, 3, 4, 5]
a = [ 1, 2, [3, [4, 5] ] ]
a.flatten!(1) #=> [1, 2, 3, [4, 5]]

#hash ⇒ Fixnum

Compute a hash-code for this array.

Two arrays with the same content will have the same hash code (and will compare using #eql?).

See also Object#hash.

#include?(object) ⇒ Boolean

Returns true if the given object is present in self (that is, if any element #== object), otherwise returns false.

a = [ "a", "b", "c" ]
a.include?("b")   #=> true
a.include?("z")   #=> false

#find_index(obj) ⇒ Integer? #find_index {|item| ... } ⇒ Integer? #find_index ⇒ Enumerator #index(obj) ⇒ Integer? #index {|item| ... } ⇒ Integer? #index ⇒ Enumerator
Also known as: #find_index

Returns the index of the first object in ary such that the object is #== to obj.

If a block is given instead of an argument, returns the index of the first object for which the block returns true. Returns nil if no match is found.

See also #rindex.

An Enumerator is returned if neither a block nor argument is given.

a = [ "a", "b", "c" ]
a.index("b")              #=> 1
a.index("z")              #=> nil
a.index { |x| x == "b" }  #=> 1

#replace(other_ary) ⇒ Array #initialize_copy(other_ary) ⇒ Array

Alias for #replace.

#insert(index, obj...) ⇒ Array

Inserts the given values before the element with the given #index.

Negative indices count backwards from the end of the array, where -1 is the last element. If a negative index is used, the given values will be inserted after that element, so using an index of -1 will insert the values at the end of the array.

a = %w{ a b c d }
a.insert(2, 99)         #=> ["a", "b", 99, "c", "d"]
a.insert(-2, 1, 2, 3)   #=> ["a", "b", 99, "c", 1, 2, 3, "d"]

#inspect ⇒ String #to_s ⇒ String

Alias for #to_s.

#join(separator = $,) ⇒ String

Returns a string created by converting each element of the array to a string, separated by the given separator. If the separator is nil, it uses current $,. If both the separator and $, are nil, it uses empty string.

[ "a", "b", "c" ].join        #=> "abc"
[ "a", "b", "c" ].join("-")   #=> "a-b-c"

#keep_if {|item| ... } ⇒ Array #keep_if ⇒ Enumerator

Deletes every element of self for which the given block evaluates to false.

See also #select!

If no block is given, an ::Enumerator is returned instead.

a = %w{ a b c d e f }
a.keep_if { |v| v =~ /[aeiou]/ }  #=> ["a", "e"]

#last ⇒ Object? #last(n) ⇒ Array

Returns the last element(s) of self. If the array is empty, the first form returns nil.

See also #first for the opposite effect.

a = [ "w", "x", "y", "z" ]
a.last     #=> "z"
a.last(2)  #=> ["y", "z"]

#length ⇒ Integer Also known as: #size

Returns the number of elements in self. May be zero.

[ 1, 2, 3, 4, 5 ].length   #=> 5
[].length                  #=> 0

#collect {|item| ... } ⇒ Array #map {|item| ... } ⇒ Array #collect ⇒ Enumerator #map ⇒ Enumerator
Also known as: #collect

Invokes the given block once for each element of self.

Creates a new array containing the values returned by the block.

See also Enumerable#collect.

If no block is given, an ::Enumerator is returned instead.

a = [ "a", "b", "c", "d" ]
a.collect { |x| x + "!" }         #=> ["a!", "b!", "c!", "d!"]
a.map.with_index { |x, i| x * i } #=> ["", "b", "cc", "ddd"]
a                                 #=> ["a", "b", "c", "d"]

#collect! {|item| ... } ⇒ Array #map! {|item| ... } ⇒ Array #collect! ⇒ Enumerator #map! ⇒ Enumerator
Also known as: #collect!

Invokes the given block once for each element of self, replacing the element with the value returned by the block.

See also Enumerable#collect.

If no block is given, an ::Enumerator is returned instead.

a = [ "a", "b", "c", "d" ]
a.map! {|x| x + "!" }
a #=>  [ "a!", "b!", "c!", "d!" ]
a.collect!.with_index {|x, i| x[0...i] }
a #=>  ["", "b", "c!", "d!"]

#pack(aTemplateString) ⇒ aBinaryString

Packs the contents of arr into a binary sequence according to the directives in aTemplateString (see the table below) Directives “A,'' “a,'' and “Z'' may be followed by a count, which gives the width of the resulting field. The remaining directives also may take a count, indicating the number of array elements to convert. If the count is an asterisk (“*''), all remaining array elements will be converted. Any of the directives “sSiIlL'' may be followed by an underscore (“_'') or exclamation mark (“!'') to use the underlying platform's native size for the specified type; otherwise, they use a platform-independent size. Spaces are ignored in the template string. See also String#unpack.

a = [ "a", "b", "c" ]
n = [ 65, 66, 67 ]
a.pack("A3A3A3")   #=> "a  b  c  "
a.pack("a3a3a3")   #=> "a\000\000b\000\000c\000\000"
n.pack("ccc")      #=> "ABC"

Directives for pack.

Integer      | Array   |
Directive    | Element | Meaning
---------------------------------------------------------------------------
   C         | Integer | 8-bit unsigned (unsigned char)
   S         | Integer | 16-bit unsigned, native endian (uint16_t)
   L         | Integer | 32-bit unsigned, native endian (uint32_t)
   Q         | Integer | 64-bit unsigned, native endian (uint64_t)
   J         | Integer | pointer width unsigned, native endian (uintptr_t)
             |         | (J is available since Ruby 2.3.)
             |         |
   c         | Integer | 8-bit signed (signed char)
   s         | Integer | 16-bit signed, native endian (int16_t)
   l         | Integer | 32-bit signed, native endian (int32_t)
   q         | Integer | 64-bit signed, native endian (int64_t)
   j         | Integer | pointer width signed, native endian (intptr_t)
             |         | (j is available since Ruby 2.3.)
             |         |
   S_, S!    | Integer | unsigned short, native endian
   I, I_, I! | Integer | unsigned int, native endian
   L_, L!    | Integer | unsigned long, native endian
   Q_, Q!    | Integer | unsigned long long, native endian (ArgumentError
             |         | if the platform has no long long type.)
             |         | (Q_ and Q! is available since Ruby 2.1.)
   J!        | Integer | uintptr_t, native endian (same with J)
             |         | (J! is available since Ruby 2.3.)
             |         |
   s_, s!    | Integer | signed short, native endian
   i, i_, i! | Integer | signed int, native endian
   l_, l!    | Integer | signed long, native endian
   q_, q!    | Integer | signed long long, native endian (ArgumentError
             |         | if the platform has no long long type.)
             |         | (q_ and q! is available since Ruby 2.1.)
   j!        | Integer | intptr_t, native endian (same with j)
             |         | (j! is available since Ruby 2.3.)
             |         |
   S> L> Q>  | Integer | same as the directives without ">" except
   J> s> l>  |         | big endian
   q> j>     |         | (available since Ruby 1.9.3)
   S!> I!>   |         | "S>" is same as "n"
   L!> Q!>   |         | "L>" is same as "N"
   J!> s!>   |         |
   i!> l!>   |         |
   q!> j!>   |         |
             |         |
   S< L< Q<  | Integer | same as the directives without "<" except
   J< s< l<  |         | little endian
   q< j<     |         | (available since Ruby 1.9.3)
   S!< I!<   |         | "S<" is same as "v"
   L!< Q!<   |         | "L<" is same as "V"
   J!< s!<   |         |
   i!< l!<   |         |
   q!< j!<   |         |
             |         |
   n         | Integer | 16-bit unsigned, network (big-endian) byte order
   N         | Integer | 32-bit unsigned, network (big-endian) byte order
   v         | Integer | 16-bit unsigned, VAX (little-endian) byte order
   V         | Integer | 32-bit unsigned, VAX (little-endian) byte order
             |         |
   U         | Integer | UTF-8 character
   w         | Integer | BER-compressed integer

Float        |         |
Directive    |         | Meaning
---------------------------------------------------------------------------
   D, d      | Float   | double-precision, native format
   F, f      | Float   | single-precision, native format
   E         | Float   | double-precision, little-endian byte order
   e         | Float   | single-precision, little-endian byte order
   G         | Float   | double-precision, network (big-endian) byte order
   g         | Float   | single-precision, network (big-endian) byte order

String       |         |
Directive    |         | Meaning
---------------------------------------------------------------------------
   A         | String  | arbitrary binary string (space padded, count is width)
   a         | String  | arbitrary binary string (null padded, count is width)
   Z         | String  | same as ``a'', except that null is added with *
   B         | String  | bit string (MSB first)
   b         | String  | bit string (LSB first)
   H         | String  | hex string (high nibble first)
   h         | String  | hex string (low nibble first)
   u         | String  | UU-encoded string
   M         | String  | quoted printable, MIME encoding (see RFC2045)
   m         | String  | base64 encoded string (see RFC 2045, count is width)
             |         | (if count is 0, no line feed are added, see RFC 4648)
   P         | String  | pointer to a structure (fixed-length string)
   p         | String  | pointer to a null-terminated string

Misc.        |         |
Directive    |         | Meaning
---------------------------------------------------------------------------
   @         | ---     | moves to absolute position
   X         | ---     | back up a byte
   x         | ---     | null byte

#permutation {|p| ... } ⇒ Array #permutation ⇒ Enumerator #permutation(n) {|p| ... } ⇒ Array #permutation(n) ⇒ Enumerator

When invoked with a block, yield all permutations of length n of the elements of the array, then return the array itself.

If n is not specified, yield all permutations of all elements.

The implementation makes no guarantees about the order in which the permutations are yielded.

If no block is given, an ::Enumerator is returned instead.

Examples:

a = [1, 2, 3]
a.permutation.to_a    #=> [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
a.permutation(1).to_a #=> [[1],[2],[3]]
a.permutation(2).to_a #=> [[1,2],[1,3],[2,1],[2,3],[3,1],[3,2]]
a.permutation(3).to_a #=> [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
a.permutation(0).to_a #=> [[]] # one permutation of length 0
a.permutation(4).to_a #=> []   # no permutations of length 4

#pop ⇒ Object? #pop(n) ⇒ Array

Removes the last element from self and returns it, or nil if the array is empty.

If a number n is given, returns an array of the last n elements (or less) just like array.slice!(-n, n) does. See also #push for the opposite effect.

a = [ "a", "b", "c", "d" ]
a.pop     #=> "d"
a.pop(2)  #=> ["b", "c"]
a         #=> ["a"]

#product(other_ary, ...) ⇒ Array #product(other_ary, ...) {|p| ... } ⇒ Array

Returns an array of all combinations of elements from all arrays.

The length of the returned array is the product of the length of self and the argument arrays.

If given a block, #product will yield all combinations and return self instead.

[1,2,3].product([4,5])     #=> [[1,4],[1,5],[2,4],[2,5],[3,4],[3,5]]
[1,2].product([1,2])       #=> [[1,1],[1,2],[2,1],[2,2]]
[1,2].product([3,4],[5,6]) #=> [[1,3,5],[1,3,6],[1,4,5],[1,4,6],
                           #     [2,3,5],[2,3,6],[2,4,5],[2,4,6]]
[1,2].product()            #=> [[1],[2]]
[1,2].product([])          #=> []

#push(obj, ... ) ⇒ Array

Append — Pushes the given object(s) on to the end of this array. This expression returns the array itself, so several appends may be chained together. See also #pop for the opposite effect.

a = [ "a", "b", "c" ]
a.push("d", "e", "f")
        #=> ["a", "b", "c", "d", "e", "f"]
[1, 2, 3,].push(4).push(5)
        #=> [1, 2, 3, 4, 5]

#rassoc(obj) ⇒ element_ary?

Searches through the array whose elements are also arrays.

Compares obj with the second element of each contained array using obj.==.

Returns the first contained array that matches obj.

See also #assoc.

a = [ [ 1, "one"], [2, "two"], [3, "three"], ["ii", "two"] ]
a.rassoc("two")    #=> [2, "two"]
a.rassoc("four")   #=> nil

#reject {|item| ... } ⇒ Array #reject ⇒ Enumerator

Returns a new array containing the items in self for which the given block is not true. The ordering of non-rejected elements is maintained.

See also #delete_if

If no block is given, an ::Enumerator is returned instead.

#reject! {|item| ... } ⇒ Array? #reject! ⇒ Enumerator

Deletes every element of self for which the block evaluates to true, if no changes were made returns nil.

The array may not be changed instantly every time the block is called.

See also Enumerable#reject and #delete_if.

If no block is given, an ::Enumerator is returned instead.

#repeated_combination(n) {|c| ... } ⇒ Array #repeated_combination(n) ⇒ Enumerator

When invoked with a block, yields all repeated combinations of length n of elements from the array and then returns the array itself.

The implementation makes no guarantees about the order in which the repeated combinations are yielded.

If no block is given, an ::Enumerator is returned instead.

Examples:

a = [1, 2, 3]
a.repeated_combination(1).to_a  #=> [[1], [2], [3]]
a.repeated_combination(2).to_a  #=> [[1,1],[1,2],[1,3],[2,2],[2,3],[3,3]]
a.repeated_combination(3).to_a  #=> [[1,1,1],[1,1,2],[1,1,3],[1,2,2],[1,2,3],
                                #    [1,3,3],[2,2,2],[2,2,3],[2,3,3],[3,3,3]]
a.repeated_combination(4).to_a  #=> [[1,1,1,1],[1,1,1,2],[1,1,1,3],[1,1,2,2],[1,1,2,3],
                                #    [1,1,3,3],[1,2,2,2],[1,2,2,3],[1,2,3,3],[1,3,3,3],
                                #    [2,2,2,2],[2,2,2,3],[2,2,3,3],[2,3,3,3],[3,3,3,3]]
a.repeated_combination(0).to_a  #=> [[]] # one combination of length 0

#repeated_permutation(n) {|p| ... } ⇒ Array #repeated_permutation(n) ⇒ Enumerator

When invoked with a block, yield all repeated permutations of length n of the elements of the array, then return the array itself.

The implementation makes no guarantees about the order in which the repeated permutations are yielded.

If no block is given, an ::Enumerator is returned instead.

Examples:

a = [1, 2]
a.repeated_permutation(1).to_a  #=> [[1], [2]]
a.repeated_permutation(2).to_a  #=> [[1,1],[1,2],[2,1],[2,2]]
a.repeated_permutation(3).to_a  #=> [[1,1,1],[1,1,2],[1,2,1],[1,2,2],
                                #    [2,1,1],[2,1,2],[2,2,1],[2,2,2]]
a.repeated_permutation(0).to_a  #=> [[]] # one permutation of length 0

#replace(other_ary) ⇒ Array #initialize_copy(other_ary) ⇒ Array
Also known as: #initialize_copy

Replaces the contents of self with the contents of other_ary, truncating or expanding if necessary.

a = [ "a", "b", "c", "d", "e" ]
a.replace([ "x", "y", "z" ])   #=> ["x", "y", "z"]
a                              #=> ["x", "y", "z"]

#reverse ⇒ Array

Returns a new array containing self's elements in reverse order.

[ "a", "b", "c" ].reverse   #=> ["c", "b", "a"]
[ 1 ].reverse               #=> [1]

#reverse! ⇒ Array

Reverses self in place.

a = [ "a", "b", "c" ]
a.reverse!       #=> ["c", "b", "a"]
a                #=> ["c", "b", "a"]

#reverse_each {|item| ... } ⇒ Array #reverse_each ⇒ Enumerator

Same as #each, but traverses self in reverse order.

a = [ "a", "b", "c" ]
a.reverse_each {|x| print x, " " }

produces:

c b a

#rindex(obj) ⇒ Integer? #rindex {|item| ... } ⇒ Integer? #rindex ⇒ Enumerator

Returns the index of the last object in self #== to obj.

If a block is given instead of an argument, returns the index of the first object for which the block returns true, starting from the last object.

Returns nil if no match is found.

See also #index.

If neither block nor argument is given, an ::Enumerator is returned instead.

a = [ "a", "b", "b", "b", "c" ]
a.rindex("b")             #=> 3
a.rindex("z")             #=> nil
a.rindex { |x| x == "b" } #=> 3

#rotate(count = 1) ⇒ Array

Returns a new array by rotating self so that the element at #count is the first element of the new array.

If #count is negative then it rotates in the opposite direction, starting from the end of self where -1 is the last element.

a = [ "a", "b", "c", "d" ]
a.rotate         #=> ["b", "c", "d", "a"]
a                #=> ["a", "b", "c", "d"]
a.rotate(2)      #=> ["c", "d", "a", "b"]
a.rotate(-3)     #=> ["b", "c", "d", "a"]

#rotate!(count = 1) ⇒ Array

Rotates self in place so that the element at #count comes first, and returns self.

If #count is negative then it rotates in the opposite direction, starting from the end of the array where -1 is the last element.

a = [ "a", "b", "c", "d" ]
a.rotate!        #=> ["b", "c", "d", "a"]
a                #=> ["b", "c", "d", "a"]
a.rotate!(2)     #=> ["d", "a", "b", "c"]
a.rotate!(-3)    #=> ["a", "b", "c", "d"]

#sample ⇒ Object #sample(random: rng) ⇒ Object #sample(n) ⇒ Array #sample(n, random: rng) ⇒ Array

Choose a random element or n random elements from the array.

The elements are chosen by using random and unique indices into the array in order to ensure that an element doesn't repeat itself unless the array already contained duplicate elements.

If the array is empty the first form returns nil and the second form returns an empty array.

The optional rng argument will be used as the random number generator.

a = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]
a.sample         #=> 7
a.sample(4)      #=> [6, 4, 2, 5]

#select {|item| ... } ⇒ Array #select ⇒ Enumerator

Returns a new array containing all elements of ary for which the given block returns a true value.

If no block is given, an ::Enumerator is returned instead.

[1,2,3,4,5].select { |num|  num.even?  }   #=> [2, 4]

a = %w{ a b c d e f }
a.select { |v| v =~ /[aeiou]/ }  #=> ["a", "e"]

See also Enumerable#select.

#select! {|item| ... } ⇒ Array? #select! ⇒ Enumerator

Invokes the given block passing in successive elements from self, deleting elements for which the block returns a false value.

The array may not be changed instantly every time the block is called.

If changes were made, it will return self, otherwise it returns nil.

See also #keep_if

If no block is given, an ::Enumerator is returned instead.

#shift ⇒ Object? #shift(n) ⇒ Array

Removes the first element of self and returns it (shifting all other elements down by one). Returns nil if the array is empty.

If a number n is given, returns an array of the first n elements (or less) just like array.slice!(0, n) does. With ary containing only the remainder elements, not including what was shifted to new_ary. See also #unshift for the opposite effect.

args = [ "-m", "-q", "filename" ]
args.shift     #=> "-m"
args           #=> ["-q", "filename"]

args = [ "-m", "-q", "filename" ]
args.shift(2)  #=> ["-m", "-q"]
args           #=> ["filename"]

#shuffle ⇒ Array #shuffle(random: rng) ⇒ Array

Returns a new array with elements of self shuffled.

a = [ 1, 2, 3 ]           #=> [1, 2, 3]
a.shuffle                 #=> [2, 3, 1]
a                         #=> [1, 2, 3]

The optional rng argument will be used as the random number generator.

a.shuffle(random: Random.new(1))  #=> [1, 3, 2]

#shuffle! ⇒ Array #shuffle!(random: rng) ⇒ Array

Shuffles elements in self in place.

a = [ 1, 2, 3 ]           #=> [1, 2, 3]
a.shuffle!                #=> [2, 3, 1]
a                         #=> [2, 3, 1]

The optional rng argument will be used as the random number generator.

a.shuffle!(random: Random.new(1))  #=> [1, 3, 2]

#length ⇒ Integer #size ⇒ Integer

Alias for #length.

#[](index) ⇒ Object? #[](start, length) ⇒ Array? #[](range) ⇒ Array? #slice(index) ⇒ Object? #slice(start, length) ⇒ Array? #slice(range) ⇒ Array?

Alias for #[].

#slice!(index) ⇒ Object? #slice!(start, length) ⇒ Array? #slice!(range) ⇒ Array?

Deletes the element(s) given by an #index (optionally up to #length elements) or by a range.

Returns the deleted object (or objects), or nil if the #index is out of range.

a = [ "a", "b", "c" ]
a.slice!(1)     #=> "b"
a               #=> ["a", "c"]
a.slice!(-1)    #=> "c"
a               #=> ["a"]
a.slice!(100)   #=> nil
a               #=> ["a"]

#sort ⇒ Array #sort {|a, b| ... } ⇒ Array

Returns a new array created by sorting self.

Comparisons for the sort will be done using the #<=> operator or using an optional code block.

The block must implement a comparison between a and b and return an integer less than 0 when b follows a, 0 when a and b are equivalent, or an integer greater than 0 when a follows b.

See also Enumerable#sort_by.

a = [ "d", "a", "e", "c", "b" ]
a.sort                    #=> ["a", "b", "c", "d", "e"]
a.sort { |x,y| y <=> x }  #=> ["e", "d", "c", "b", "a"]

#sort! ⇒ Array #sort! {|a, b| ... } ⇒ Array

Sorts self in place.

Comparisons for the sort will be done using the #<=> operator or using an optional code block.

The block must implement a comparison between a and b and return an integer less than 0 when b follows a, 0 when a and b are equivalent, or an integer greater than 0 when a follows b.

See also Enumerable#sort_by.

a = [ "d", "a", "e", "c", "b" ]
a.sort!                    #=> ["a", "b", "c", "d", "e"]
a.sort! { |x,y| y <=> x }  #=> ["e", "d", "c", "b", "a"]

#sort_by! {|obj| ... } ⇒ Array #sort_by! ⇒ Enumerator

Sorts self in place using a set of keys generated by mapping the values in self through the given block.

If no block is given, an ::Enumerator is returned instead.

#take(n) ⇒ Array

Returns first n elements from the array.

If a negative number is given, raises an ::ArgumentError.

See also #drop

a = [1, 2, 3, 4, 5, 0]
a.take(3)             #=> [1, 2, 3]

#take_while {|obj| ... } ⇒ Array #take_while ⇒ Enumerator

Passes elements to the block until the block returns nil or false, then stops iterating and returns an array of all prior elements.

If no block is given, an ::Enumerator is returned instead.

See also #drop_while

a = [1, 2, 3, 4, 5, 0]
a.take_while { |i| i < 3 }  #=> [1, 2]

#to_a ⇒ Array

Returns self.

If called on a subclass of Array, converts the receiver to an Array object.

#to_ary ⇒ Array

Returns self.

#to_h ⇒ Hash

Returns the result of interpreting ary as an array of [key, value] pairs.

[[:foo, :bar], [1, 2]].to_h
  # => {:foo => :bar, 1 => 2}

#inspect ⇒ String #to_s ⇒ String
Also known as: #inspect

Creates a string representation of self.

[ "a", "b", "c" ].to_s     #=> "[\"a\", \"b\", \"c\"]"

#transpose ⇒ Array

Assumes that self is an array of arrays and transposes the rows and columns.

a = [[1,2], [3,4], [5,6]]
a.transpose   #=> [[1, 3, 5], [2, 4, 6]]

If the length of the subarrays don't match, an ::IndexError is raised.

#uniq ⇒ Array #uniq {|item| ... } ⇒ Array

Returns a new array by removing duplicate values in self.

If a block is given, it will use the return value of the block for comparison.

It compares values using their #hash and #eql? methods for efficiency.

self is traversed in order, and the first occurrence is kept.

a = [ "a", "a", "b", "b", "c" ]
a.uniq   # => ["a", "b", "c"]

b = [["student","sam"], ["student","george"], ["teacher","matz"]]
b.uniq { |s| s.first } # => [["student", "sam"], ["teacher", "matz"]]

#uniq! ⇒ Array? #uniq! {|item| ... } ⇒ Array?

Removes duplicate elements from self.

If a block is given, it will use the return value of the block for comparison.

It compares values using their #hash and #eql? methods for efficiency.

self is traversed in order, and the first occurrence is kept.

Returns nil if no changes are made (that is, no duplicates are found).

a = [ "a", "a", "b", "b", "c" ]
a.uniq!   # => ["a", "b", "c"]

b = [ "a", "b", "c" ]
b.uniq!   # => nil

c = [["student","sam"], ["student","george"], ["teacher","matz"]]
c.uniq! { |s| s.first } # => [["student", "sam"], ["teacher", "matz"]]

#unshift(obj, ...) ⇒ Array

Prepends objects to the front of self, moving other elements upwards. See also #shift for the opposite effect.

a = [ "b", "c", "d" ]
a.unshift("a")   #=> ["a", "b", "c", "d"]
a.unshift(1, 2)  #=> [ 1, 2, "a", "b", "c", "d"]

#values_at(selector, ...) ⇒ Array

Returns an array containing the elements in self corresponding to the given selector(s).

The selectors may be either integer indices or ranges.

See also #select.

a = %w{ a b c d e f }
a.values_at(1, 3, 5)          # => ["b", "d", "f"]
a.values_at(1, 3, 5, 7)       # => ["b", "d", "f", nil]
a.values_at(-1, -2, -2, -7)   # => ["f", "e", "e", nil]
a.values_at(4..6, 3...6)      # => ["e", "f", nil, "d", "e", "f"]

#zip(arg, ...) ⇒ Array #zip(arg, ...) {|arr| ... } ⇒ nil

Converts any arguments to arrays, then merges elements of self with corresponding elements from each argument.

This generates a sequence of ary.size n-element arrays, where n is one more than the count of arguments.

If the size of any argument is less than the size of the initial array, nil values are supplied.

If a block is given, it is invoked for each output array, otherwise an array of arrays is returned.

a = [ 4, 5, 6 ]
b = [ 7, 8, 9 ]
[1, 2, 3].zip(a, b)   #=> [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
[1, 2].zip(a, b)      #=> [[1, 4, 7], [2, 5, 8]]
a.zip([1, 2], [8])    #=> [[4, 1, 8], [5, 2, nil], [6, nil, nil]]

#|(other_ary) ⇒ Array

Set Union — Returns a new array by joining ary with other_ary, excluding any duplicates and preserving the order from the original array.

It compares elements using their #hash and #eql? methods for efficiency.

[ "a", "b", "c" ] | [ "c", "d", "a" ]    #=> [ "a", "b", "c", "d" ]

See also #uniq.