module Enumerable(T)

Overview

The Enumerable mixin provides collection classes with several traversal, searching, filtering and querying methods.

Including types must provide an #each method, which yields successive members of the collection.

For example:

class Three
  include Enumerable(Int32)

  def each(&)
    yield 1
    yield 2
    yield 3
  end
end

three = Three.new
three.to_a                # => [1, 2, 3]
three.select &.odd?       # => [1, 3]
three.all? { |x| x < 10 } # => true

Note that most search and filter methods traverse an Enumerable eagerly, producing an Array as the result. For a lazy alternative refer to the Iterator and Iterable modules.

Direct including types

Defined in:

enumerable.cr
set.cr

Class Method Summary

Instance Method Summary

Class Method Detail

def self.element_type(x) #

Returns a value with the same type as an element of x, even if x is not an Enumerable.

Used by splat expansion inside array literals. For example, this code

[1, *{2, 3.5}, 4]

will end up calling typeof(1, ::Enumerable.element_type({2, 3.5}), 4).

NOTE there should never be a need to call this method outside the standard library.


[View source]

Instance Method Detail

def accumulate(initial : U) : Array(U) forall U #

Returns an array containing initial and its prefix sums with the elements in this collection.

Expects U to respond to the #+ method.

[1, 2, 3, 4, 5].accumulate(6) # => [6, 7, 9, 12, 16, 21]

[View source]
def accumulate : Array(T) #

Returns an array of the prefix sums of the elements in this collection. The first element of the returned array is same as the first element of self.

Expects all element types to respond to the #+ method.

[1, 2, 3, 4, 5, 6].accumulate # => [1, 3, 6, 10, 15, 21]

[View source]
def accumulate(initial : U, &block : U, T -> U) : Array(U) forall U #

Returns an array containing initial and the successive values of applying a binary operation, specified by the given block, to this collection's elements.

Similar to #accumulate(&block : T, T -> T), except the initial value is provided by an argument and needs not have the same type as the elements in the collection. This initial value is always present in the returned array.

[1, 3, 5, 7].accumulate(9) { |x, y| x * y } # => [9, 9, 27, 135, 945]

[View source]
def accumulate(&block : T, T -> T) : Array(T) #

Returns an array containing the successive values of applying a binary operation, specified by the given block, to this collection's elements.

For each element in the collection the block is passed an accumulator value and the element. The result becomes the new value for the accumulator and is also appended to the returned array. The initial value for the accumulator is the first element in the collection.

[2, 3, 4, 5].accumulate { |x, y| x * y } # => [2, 6, 24, 120]

[View source]
def all?(& : T -> ) : Bool #

Returns true if the passed block is truthy for all elements of the collection.

["ant", "bear", "cat"].all? { |word| word.size >= 3 } # => true
["ant", "bear", "cat"].all? { |word| word.size >= 4 } # => false

[View source]
def all?(pattern) : Bool #

Returns true if pattern === element for all elements in this enumerable.

[2, 3, 4].all?(1..5)        # => true
[2, 3, 4].all?(Int32)       # => true
[2, "a", 3].all?(String)    # => false
%w[foo bar baz].all?(/o|a/) # => true

[View source]
def all? : Bool #

Returns true if all of the elements of the collection are truthy.

[nil, true, 99].all? # => false
[15].all?            # => true

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def any?(& : T -> ) : Bool #

Returns true if the passed block is truthy for at least one element of the collection.

["ant", "bear", "cat"].any? { |word| word.size >= 4 } # => true
["ant", "bear", "cat"].any? { |word| word.size > 4 }  # => false

[View source]
def any?(pattern) : Bool #

Returns true if pattern === element for at least one element in this enumerable.

[2, 3, 4].any?(1..3)      # => true
[2, 3, 4].any?(5..10)     # => false
[2, "a", 3].any?(String)  # => true
%w[foo bar baz].any?(/a/) # => true

[View source]
def any? : Bool #

Returns true if at least one of the collection's members is truthy.

[nil, true, 99].any? # => true
[nil, false].any?    # => false
([] of Int32).any?   # => false
  • #present? does not consider truthiness of elements.
  • #any?(&) and #any(pattern) allow custom conditions.

NOTE #any? usually has the same semantics as #present?. They only differ if the element type can be falsey (i.e. T <= Nil || T <= Pointer || T <= Bool). It's typically advised to prefer #present? unless these specific truthiness semantics are required.


[View source]
def chunks(&block : T -> U) forall U #

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

Consecutive elements which return the same block value are chunked together.

For example, consecutive even numbers and odd numbers can be chunked as follows.

ary = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5].chunks { |n| n.even? }
ary # => [{false, [3, 1]}, {true, [4]}, {false, [1, 5, 9]}, {true, [2, 6]}, {false, [5, 3, 5]}]

The following key values have special meaning:

See also: Iterator#chunk.


[View source]
def compact_map(& : T -> _) #

Returns an Array with the results of running the block against each element of the collection, removing nil values.

["Alice", "Bob"].map { |name| name.match(/^A./) }         # => [Regex::MatchData("Al"), nil]
["Alice", "Bob"].compact_map { |name| name.match(/^A./) } # => [Regex::MatchData("Al")]

[View source]
def count(& : T -> ) : Int32 #

Returns the number of elements in the collection for which the passed block is truthy.

[1, 2, 3, 4].count { |i| i % 2 == 0 } # => 2

[View source]
def count(item) : Int32 #

Returns the number of times that the passed item is present in the collection.

[1, 2, 3, 4].count(3) # => 1

[View source]
def cycle(n, & : T -> ) : Nil #

Calls the given block for each element in this enumerable n times.


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def cycle(& : T -> ) : Nil #

Calls the given block for each element in this enumerable forever.


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abstract def each(& : T -> ) #

Must yield this collection's elements to the block.


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def each_cons(count : Int, reuse = false, &) #

Iterates over the collection yielding chunks of size count, but advancing one by one.

[1, 2, 3, 4, 5].each_cons(2) do |cons|
  puts cons
end

Prints:

[1, 2]
[2, 3]
[3, 4]
[4, 5]

By default, a new array is created and yielded for each consecutive slice of elements.

  • If reuse is given, the array can be reused
  • If reuse is true, the method will create a new array and reuse it.
  • If reuse is an instance of Array, Deque or a similar collection type (implementing #<<, #shift and #size) it will be used.
  • If reuse is falsey, the array will not be reused.

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

Chunks of two items can be iterated using #each_cons_pair, an optimized implementation for the special case of count == 2 which avoids heap allocations.


[View source]
def each_cons_pair(& : T, T -> ) : Nil #

Iterates over the collection yielding pairs of adjacent items, but advancing one by one.

[1, 2, 3, 4, 5].each_cons_pair do |a, b|
  puts "#{a}, #{b}"
end

Prints:

1, 2
2, 3
3, 4
4, 5

Chunks of more than two items can be iterated using #each_cons. This method is just an optimized implementation for the special case of count == 2 to avoid heap allocations.


[View source]
def each_slice(count : Int, reuse = false, &) #

Iterates over the collection in slices of size count, and runs the block for each of those.

[1, 2, 3, 4, 5].each_slice(2) do |slice|
  puts slice
end

Prints:

[1, 2]
[3, 4]
[5]

Note that the last one can be smaller.

By default, a new array is created and yielded for each slice.

  • If reuse is given, the array can be reused
  • If reuse is an Array, this array will be reused
  • If reuse is truthy, the method will create a new array and reuse it.

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.


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def each_step(n : Int, *, offset : Int = 0, & : T -> ) : Nil #

Iterates over the collection, yielding every nth element, starting with the first.

%w[Alice Bob Charlie David].each_step(2) do |user|
  puts "User: #{user}"
end

Prints:

User: Alice
User: Charlie

Accepts an optional offset parameter

%w[Alice Bob Charlie David].each_step(2, offset: 1) do |user|
  puts "User: #{user}"
end

Which would print:

User: Bob
User: David

[View source]
def each_with_index(offset = 0, &) #

Iterates over the collection, yielding both the elements and their index.

["Alice", "Bob"].each_with_index do |user, i|
  puts "User ##{i}: #{user}"
end

Prints:

User # 0: Alice
User # 1: Bob

Accepts an optional offset parameter, which tells it to start counting from there. So, a more human friendly version of the previous snippet would be:

["Alice", "Bob"].each_with_index(1) do |user, i|
  puts "User ##{i}: #{user}"
end

Which would print:

User # 1: Alice
User # 2: Bob

[View source]
def each_with_object(obj : U, & : T, U -> ) : U forall U #

Iterates over the collection, passing each element and the initial object obj. Returns that object.

hash = ["Alice", "Bob"].each_with_object({} of String => Int32) do |user, sizes|
  sizes[user] = user.size
end
hash # => {"Alice" => 5, "Bob" => 3}

[View source]
def empty? : Bool #

Returns true if self does not contain any element.

([] of Int32).empty? # => true
([1]).empty?         # => false
[nil, false].empty?  # => false

[View source]
def find(if_none = nil, & : T -> ) #

Returns the first element in the collection for which the passed block is truthy.

Accepts an optional parameter if_none, to set what gets returned if no element is found (defaults to nil).

[1, 2, 3, 4].find { |i| i > 2 }     # => 3
[1, 2, 3, 4].find { |i| i > 8 }     # => nil
[1, 2, 3, 4].find(-1) { |i| i > 8 } # => -1

[View source]
def find!(& : T -> ) : T #

Returns the first element in the collection for which the passed block is truthy. Raises Enumerable::NotFoundError if there is no element for which the block is truthy.

[1, 2, 3, 4].find! { |i| i > 2 } # => 3
[1, 2, 3, 4].find! { |i| i > 8 } # => raises Enumerable::NotFoundError

[View source]
def first(&) #

Returns the first element in the collection, If the collection is empty, calls the block and returns its value.

([1, 2, 3]).first { 4 }   # => 1
([] of Int32).first { 4 } # => 4

[View source]
def first(count : Int) : Array(T) #

Returns an Array with the first count elements in the collection.

If count is bigger than the number of elements in the collection, returns as many as possible. This include the case of calling it over an empty collection, in which case it returns an empty array.


[View source]
def first : T #

Returns the first element in the collection. Raises Enumerable::EmptyError if the collection is empty.

([1, 2, 3]).first   # => 1
([] of Int32).first # raises Enumerable::EmptyError

[View source]
def first? : T | Nil #

Returns the first element in the collection. When the collection is empty, returns nil.

([1, 2, 3]).first?   # => 1
([] of Int32).first? # => nil

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def flat_map(& : T -> _) #

Returns a new array with the concatenated results of running the block once for every element in the collection. Only Array and Iterator results are concatenated; every other value is directly appended to the new array.

array = ["Alice", "Bob"].flat_map do |user|
  user.chars
end
array # => ['A', 'l', 'i', 'c', 'e', 'B', 'o', 'b']

[View source]
def group_by(& : T -> U) forall U #

Returns a Hash whose keys are each different value that the passed block returned when run for each element in the collection, and which values are an Array of the elements for which the block returned that value.

["Alice", "Bob", "Ary"].group_by { |name| name.size } # => {5 => ["Alice"], 3 => ["Bob", "Ary"]}

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def in_groups_of(size : Int, filled_up_with : U = nil) forall U #

Returns an Array with chunks in the given size, eventually filled up with given value or nil.

[1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]
[1, 2, 3].in_groups_of(2)    # => [[1, 2], [3, nil]]

[View source]
def in_groups_of(size : Int, filled_up_with : U = nil, reuse = false, &) forall U #

Yields a block with the chunks in the given size.

[1, 2, 4].in_groups_of(2, 0) { |e| p e.sum }
# => 3
# => 4

By default, a new array is created and yielded for each group.

  • If reuse is given, the array can be reused
  • If reuse is an Array, this array will be reused
  • If reuse is truthy, the method will create a new array and reuse it.

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.


[View source]
def in_slices_of(size : Int) : Array(Array(T)) #

Returns an Array with chunks in the given size. Last chunk can be smaller depending on the number of remaining items.

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

[View source]
def includes?(obj) : Bool #

Returns true if the collection contains obj, false otherwise.

[1, 2, 3].includes?(2) # => true
[1, 2, 3].includes?(5) # => false

[View source]
def index(& : T -> ) : Int32 | Nil #

Returns the index of the first element for which the passed block is truthy.

["Alice", "Bob"].index { |name| name.size < 4 } # => 1 (Bob's index)

Returns nil if the block is not truthy for any element.


[View source]
def index(obj) : Int32 | Nil #

Returns the first index of obj in the collection.

["Alice", "Bob"].index("Alice") # => 0

Returns nil if obj is not in the collection.


[View source]
def index!(& : T -> ) : Int32 #

Returns the index of the first element for which the passed block is truthy.

["Alice", "Bob"].index! { |name| name.size < 4 } # => 1 (Bob's index)

Raises Enumerable::NotFoundError if there is no element for which the block is truthy.


[View source]
def index!(obj) : Int32 #

Returns the first index of obj in the collection.

["Alice", "Bob"].index!("Alice") # => 0

Raises Enumerable::NotFoundError if obj is not in the collection.


[View source]
def index_by(& : T -> U) : Hash(U, T) forall U #

Converts an Enumerable to a Hash by using the value returned by the block as the hash key. Be aware, if two elements return the same value as a key one will override the other. If you want to keep all values, then you should probably use #group_by instead.

["Anna", "Ary", "Alice"].index_by { |e| e.size }
# => {4 => "Anna", 3 => "Ary", 5 => "Alice"}
["Anna", "Ary", "Alice", "Bob"].index_by { |e| e.size }
# => {4 => "Anna", 3 => "Bob", 5 => "Alice"}

[View source]
def join(io : IO, separator = "") : Nil #

Prints to io all the elements in the collection, separated by separator.

[1, 2, 3, 4, 5].join(STDOUT, ", ")

Prints:

1, 2, 3, 4, 5

[View source]
def join(separator, io : IO) : Nil #

Prints to io all the elements in the collection, separated by separator.

[1, 2, 3, 4, 5].join(STDOUT, ", ")

Prints:

1, 2, 3, 4, 5

DEPRECATED Use #join(io : IO, separator = "") instead


[View source]
def join(separator = "") : String #

Returns a String created by concatenating the elements in the collection, separated by separator (defaults to none).

[1, 2, 3, 4, 5].join(", ") # => "1, 2, 3, 4, 5"

[View source]
def join(io : IO, separator = "", & : T, IO -> ) #

Prints to io the concatenation of the elements, with the possibility of controlling how the printing is done via a block.

[1, 2, 3, 4, 5].join(STDOUT, ", ") { |i, io| io << "(#{i})" }

Prints:

(1), (2), (3), (4), (5)

[View source]
def join(separator, io : IO, &) #

Prints to io the concatenation of the elements, with the possibility of controlling how the printing is done via a block.

[1, 2, 3, 4, 5].join(STDOUT, ", ") { |i, io| io << "(#{i})" }

Prints:

(1), (2), (3), (4), (5)

DEPRECATED Use #join(io : IO, separator = "", & : T, IO ->) instead


[View source]
def join(separator = "", & : T -> ) #

Returns a String created by concatenating the results of passing the elements in the collection to the passed block, separated by separator (defaults to none).

[1, 2, 3, 4, 5].join(", ") { |i| -i } # => "-1, -2, -3, -4, -5"

[View source]
def map(& : T -> U) : Array(U) forall U #

Returns an Array with the results of running the block against each element of the collection.

[1, 2, 3].map { |i| i * 10 } # => [10, 20, 30]

[View source]
def map_with_index(offset = 0, & : T, Int32 -> U) : Array(U) forall U #

Like #map, but the block gets passed both the element and its index.

["Alice", "Bob"].map_with_index { |name, i| "User ##{i}: #{name}" }
# => ["User #0: Alice", "User #1: Bob"]

Accepts an optional offset parameter, which tells it to start counting from there.


[View source]
def max(count : Int) : Array(T) #

Returns an array of the maximum count elements, sorted descending.

It compares using <=> so it will work for any type that supports that method.

[7, 5, 2, 4, 9].max(3)                 # => [9, 7, 5]
%w[Eve Alice Bob Mallory Carol].max(2) # => ["Mallory", "Eve"]

Returns all elements sorted descending if count is greater than the number of elements in the source.

Raises Enumerable::ArgumentError if count is negative or if any elements are not comparable.


[View source]
def max : T #

Returns the element with the maximum value in the collection.

It compares using > so it will work for any type that supports that method.

[1, 2, 3].max        # => 3
["Alice", "Bob"].max # => "Bob"

Raises Enumerable::EmptyError if the collection is empty.


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def max? : T | Nil #

Like #max but returns nil if the collection is empty.


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def max_by(& : T -> U) : T forall U #

Returns the element for which the passed block returns with the maximum value.

It compares using > so the block must return a type that supports that method

["Alice", "Bob"].max_by { |name| name.size } # => "Alice"

Raises Enumerable::EmptyError if the collection is empty.


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def max_by?(& : T -> U) : T | Nil forall U #

Like #max_by but returns nil if the collection is empty.


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def max_of(& : T -> U) : U forall U #

Like #max_by but instead of the element, returns the value returned by the block.

["Alice", "Bob"].max_of { |name| name.size } # => 5 (Alice's size)

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def max_of?(& : T -> U) : U | Nil forall U #

Like #max_of but returns nil if the collection is empty.


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def min(count : Int) : Array(T) #

Returns an array of the minimum count elements, sorted ascending.

It compares using <=> so it will work for any type that supports that method.

[7, 5, 2, 4, 9].min(3)                 # => [2, 4, 5]
%w[Eve Alice Bob Mallory Carol].min(2) # => ["Alice", "Bob"]

Returns all elements sorted ascending if count is greater than the number of elements in the source.

Raises Enumerable::ArgumentError if count is negative or if any elements are not comparable.


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def min : T #

Returns the element with the minimum value in the collection.

It compares using < so it will work for any type that supports that method.

[1, 2, 3].min        # => 1
["Alice", "Bob"].min # => "Alice"

Raises Enumerable::EmptyError if the collection is empty.


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def min? : T | Nil #

Like #min but returns nil if the collection is empty.


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def min_by(& : T -> U) : T forall U #

Returns the element for which the passed block returns with the minimum value.

It compares using < so the block must return a type that supports that method

["Alice", "Bob"].min_by { |name| name.size } # => "Bob"

Raises Enumerable::EmptyError if the collection is empty.


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def min_by?(& : T -> U) : T | Nil forall U #

Like #min_by but returns nil if the collection is empty.


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def min_of(& : T -> U) : U forall U #

Like #min_by but instead of the element, returns the value returned by the block.

["Alice", "Bob"].min_of { |name| name.size } # => 3 (Bob's size)

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def min_of?(& : T -> U) : U | Nil forall U #

Like #min_of but returns nil if the collection is empty.


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def minmax : Tuple(T, T) #

Returns a Tuple with both the minimum and maximum value.

[1, 2, 3].minmax # => {1, 3}

Raises Enumerable::EmptyError if the collection is empty.


[View source]
def minmax? : Tuple(T | Nil, T | Nil) #

Like #minmax but returns {nil, nil} if the collection is empty.


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def minmax_by(& : T -> U) : Tuple(T, T) forall U #

Returns a Tuple with both the minimum and maximum values according to the passed block.

["Alice", "Bob", "Carl"].minmax_by { |name| name.size } # => {"Bob", "Alice"}

Raises Enumerable::EmptyError if the collection is empty.


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def minmax_by?(& : T -> U) : Tuple(T, T) | Tuple(Nil, Nil) forall U #

Like #minmax_by but returns {nil, nil} if the collection is empty.


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def minmax_of(& : T -> U) : Tuple(U, U) forall U #

Returns a Tuple with both the minimum and maximum value the block returns when passed the elements in the collection.

["Alice", "Bob", "Carl"].minmax_of { |name| name.size } # => {3, 5}

Raises Enumerable::EmptyError if the collection is empty.


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def minmax_of?(& : T -> U) : Tuple(U, U) | Tuple(Nil, Nil) forall U #

Like #minmax_of but returns {nil, nil} if the collection is empty.


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def none?(& : T -> ) : Bool #

Returns true if the passed block is truthy for none of the elements of the collection.

[1, 2, 3].none? { |i| i > 5 } # => true

It's the opposite of #all?.


[View source]
def none?(pattern) : Bool #

Returns true if pattern === element for no element in this enumerable.

[2, 3, 4].none?(5..7)      # => true
[2, "a", 3].none?(String)  # => false
%w[foo bar baz].none?(/e/) # => true

[View source]
def none? : Bool #

Returns true if all of the elements of the collection are falsey.

[nil, false].none?       # => true
[nil, false, true].none? # => false

It's the opposite of #all?.


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def one?(& : T -> ) : Bool #

Returns true if the passed block is truthy for exactly one of the elements of the collection.

[1, 2, 3].one? { |i| i > 2 } # => true
[1, 2, 3].one? { |i| i > 1 } # => false

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def one?(pattern) : Bool #

Returns true if pattern === element for just one element in this enumerable.

[1, 10, 100].one?(7..14)   # => true
[2, "a", 3].one?(Int32)    # => false
%w[foo bar baz].one?(/oo/) # => true

[View source]
def one? : Bool #

Returns true if only one element in this enumerable is truthy.

[1, false, false].one? # => true
[1, false, 3].one?     # => false
[1].one?               # => true
[false].one?           # => false

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def partition(& : T -> ) : Tuple(Array(T), Array(T)) #

Returns a Tuple with two arrays. The first one contains the elements in the collection for which the passed block is truthy, and the second one those for which the block is falsey.

[1, 2, 3, 4, 5, 6].partition { |i| i % 2 == 0 } # => {[2, 4, 6], [1, 3, 5]}

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def partition(type : U.class) forall U #

Returns a Tuple with two arrays. The first one contains the elements in the collection that are of the given type, and the second one that are not of the given type.

ints, others = [1, true, nil, 3, false, "string", 'c'].partition(Int32)
ints           # => [1, 3]
others         # => [true, nil, false, "string", 'c']
typeof(ints)   # => Array(Int32)
typeof(others) # => Array(String | Char | Nil)

[View source]
def present? : Bool #

Returns true if self contains at least one element.

([] of Int32).present? # => false
([1]).present?         # => true
[nil, false].present?  # => true
  • #empty? returns the inverse.
  • #any? considers only truthy elements.
  • #any?(&) and #any(pattern) allow custom conditions.

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def product(initial : Number) #

Multiplies initial and all the elements in the collection together. The type of initial will be the type of the product, so use this if (for instance) you need to specify a large enough type to avoid overflow.

Expects all element types to respond to #* method.

[1, 2, 3, 4, 5, 6].product(7) # => 5040

If the collection is empty, returns initial.

([] of Int32).product(7) # => 7

[View source]
def product #

Multiplies all the elements in the collection together.

Expects all element types to respond to #* method.

[1, 2, 3, 4, 5, 6].product # => 720

This method calls .multiplicative_identity on the element type to determine the type of the sum value.

If the collection is empty, returns multiplicative_identity.

([] of Int32).product # => 1

[View source]
def product(initial : Number, & : T -> ) #

Multiplies initial and all results of the passed block for each element in the collection.

["Alice", "Bob"].product(2) { |name| name.size } # => 30 (2 * 5 * 3)

Expects all types returned from the block to respond to #* method.

If the collection is empty, returns 1.

([] of String).product(1) { |name| name.size } # => 1

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def product(& : T -> _) #

Multiplies all results of the passed block for each element in the collection.

["Alice", "Bob"].product { |name| name.size } # => 15 (5 * 3)

Expects all types returned from the block to respond to #* method.

This method calls .multiplicative_identity on the element type to determine the type of the sum value.

If the collection is empty, returns multiplicative_identity.

([] of Int32).product { |x| x + 1 } # => 1

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def reduce(memo, &) #

Just like the other variant, but you can set the initial value of the accumulator.

[1, 2, 3, 4, 5].reduce(10) { |acc, i| acc + i }             # => 25
[1, 2, 3].reduce([] of Int32) { |memo, i| memo.unshift(i) } # => [3, 2, 1]

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def reduce(&) #

Combines all elements in the collection by applying a binary operation, specified by a block, so as to reduce them to a single value.

For each element in the collection the block is passed an accumulator value (memo) and the element. The result becomes the new value for memo. At the end of the iteration, the final value of memo is the return value for the method. The initial value for the accumulator is the first element in the collection. If the collection has only one element, that element is returned.

Raises Enumerable::EmptyError if the collection is empty.

[1, 2, 3, 4, 5].reduce { |acc, i| acc + i } # => 15
[1].reduce { |acc, i| acc + i }             # => 1
([] of Int32).reduce { |acc, i| acc + i }   # raises Enumerable::EmptyError

The block is not required to return a T, in which case the accumulator's type includes whatever the block returns.

# `acc` is an `Int32 | String`
[1, 2, 3, 4, 5].reduce { |acc, i| "#{acc}-#{i}" } # => "1-2-3-4-5"
[1].reduce { |acc, i| "#{acc}-#{i}" }             # => 1

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def reduce?(&) #

Similar to #reduce, but instead of raising when the input is empty, return nil

([] of Int32).reduce? { |acc, i| acc + i } # => nil

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def reject(& : T -> ) #

Returns an Array with all the elements in the collection for which the passed block is falsey.

[1, 2, 3, 4, 5, 6].reject { |i| i % 2 == 0 } # => [1, 3, 5]

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def reject(type : U.class) forall U #

Returns an Array with all the elements in the collection that are not of the given type.

ints = [1, true, 3, false].reject(Bool)
ints         # => [1, 3]
typeof(ints) # => Array(Int32)

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def reject(pattern) : Array(T) #

Returns an Array with all the elements in the collection for which pattern === element is false.

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

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def sample(n : Int, random : Random = Random::DEFAULT) : Array(T) #

Returns an Array of n random elements from self, using the given random number generator. All elements have equal probability of being drawn. Sampling is done without replacement; if n is larger than the size of this collection, the returned Array has the same size as self.

Raises ArgumentError if n is negative.

[1, 2, 3, 4, 5].sample(2)                # => [3, 5]
{1, 2, 3, 4, 5}.sample(2)                # => [3, 4]
{1, 2, 3, 4, 5}.sample(2, Random.new(1)) # => [1, 5]

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def sample(random : Random = Random::DEFAULT) : T #

Returns a random element from self, using the given random number generator. All elements have equal probability of being drawn.

Raises IndexError if self is empty.

a = [1, 2, 3]
a.sample                # => 2
a.sample                # => 1
a.sample(Random.new(1)) # => 3

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def select(& : T -> ) #

Returns an Array with all the elements in the collection for which the passed block is truthy.

[1, 2, 3, 4, 5, 6].select { |i| i % 2 == 0 } # => [2, 4, 6]

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def select(type : U.class) : Array(U) forall U #

Returns an Array with all the elements in the collection that are of the given type.

ints = [1, true, nil, 3, false].select(Int32)
ints         # => [1, 3]
typeof(ints) # => Array(Int32)

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def select(pattern) : Array(T) #

Returns an Array with all the elements in the collection for which pattern === element.

[1, 3, 2, 5, 4, 6].select(3..5) # => [3, 5, 4]
["Alice", "Bob"].select(/^A/)   # => ["Alice"]

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def size : Int32 #

Returns the number of elements in the collection.

[1, 2, 3, 4].size # => 4

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def skip(count : Int) #

Returns an Array with the first count elements removed from the original collection.

If count is bigger than the number of elements in the collection, returns an empty array.

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

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def skip_while(& : T -> ) : Array(T) #

Skips elements up to, but not including, the first element for which the block is falsey, and returns an Array containing the remaining elements.

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

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def sum(initial) #

Adds initial and all the elements in the collection together. The type of initial will be the type of the sum, so use this if (for instance) you need to specify a large enough type to avoid overflow.

Expects all element types to respond to #+ method.

[1, 2, 3, 4, 5, 6].sum(7) # => 28

If the collection is empty, returns initial.

([] of Int32).sum(7) # => 7

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def sum #

Adds all the elements in the collection together.

Expects all element types to respond to #+ method.

[1, 2, 3, 4, 5, 6].sum # => 21

This method calls .additive_identity on the yielded type to determine the type of the sum value.

If the collection is empty, returns additive_identity.

([] of Int32).sum # => 0

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def sum(initial, & : T -> ) #

Adds initial and all results of the passed block for each element in the collection.

["Alice", "Bob"].sum(1) { |name| name.size } # => 9 (1 + 5 + 3)

Expects all types returned from the block to respond to #+ method.

If the collection is empty, returns initial.

([] of String).sum(1) { |name| name.size } # => 1

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def sum(& : T -> ) #

Adds all results of the passed block for each element in the collection.

["Alice", "Bob"].sum { |name| name.size } # => 8 (5 + 3)

Expects all types returned from the block to respond to #+ method.

This method calls .additive_identity on the yielded type to determine the type of the sum value.

If the collection is empty, returns additive_identity.

([] of Int32).sum { |x| x + 1 } # => 0

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def take_while(& : T -> ) : Array(T) #

Passes elements to the block until the block returns a falsey value, then stops iterating and returns an Array of all prior elements.

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

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def tally(hash) #

Tallies the collection. Accepts a hash to count occurrences. The value corresponding to each element must be an integer. The number of occurrences is added to each value in hash, and hash is returned.

hash = {} of Char => Int32
words = ["crystal", "ruby"]
words.each { |word| word.chars.tally(hash) }
hash # => {'c' => 1, 'r' => 2, 'y' => 2, 's' => 1, 't' => 1, 'a' => 1, 'l' => 1, 'u' => 1, 'b' => 1}

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def tally : Hash(T, Int32) #

Tallies the collection. Returns a hash where the keys are the elements and the values are numbers of elements in the collection that correspond to the key.

["a", "b", "c", "b"].tally # => {"a"=>1, "b"=>2, "c"=>1}

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def tally_by(hash, &) #

Tallies the collection. Accepts a hash to count occurrences. The value corresponding to each element must be an integer. Returns hash where the keys are the elements and the values are numbers of elements in the collection that correspond to the key after transformation by the given block.

hash = {} of Char => Int32
words = ["Crystal", "Ruby"]
words.each { |word| word.chars.tally_by(hash, &.downcase) }
hash # => {'c' => 1, 'r' => 2, 'y' => 2, 's' => 1, 't' => 1, 'a' => 1, 'l' => 1, 'u' => 1, 'b' => 1}

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def tally_by(&block : T -> U) : Hash(U, Int32) forall U #

Tallies the collection. Returns a hash where the keys are the elements and the values are numbers of elements in the collection that correspond to the key after transformation by the given block.

["a", "A", "b", "B"].tally_by(&.downcase) # => {"a" => 2, "b" => 2}

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def to_a : Array(T) #

Returns an Array with all the elements in the collection.

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

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def to_a(& : T -> U) : Array(U) forall U #

Returns an Array with the results of running block against each element of the collection.

(1..5).to_a { |i| i * 2 } # => [2, 4, 6, 8, 10]

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def to_h #

Creates a Hash out of an Enumerable where each element is a 2 element structure (for instance a Tuple or an Array).

[[:a, :b], [:c, :d]].to_h        # => {:a => :b, :c => :d}
Tuple.new({:a, 1}, {:c, 2}).to_h # => {:a => 1, :c => 2}

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def to_h(& : T -> Tuple(K, V)) forall K, V #

Creates a Hash out of Tuple pairs (key, value) returned from the block.

(1..3).to_h { |i| {i, i ** 2} } # => {1 => 1, 2 => 4, 3 => 9}

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def to_set : Set(T) #

Returns a new Set with each unique element in the enumerable.


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def to_set(&block : T -> U) : Set(U) forall U #

Returns a new Set with the unique results of running block against each element of the enumerable.


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def zip(*others : Indexable | Iterable | Iterator, &) #

Yields elements of self and others in tandem to the given block.

Raises an IndexError if any of others doesn't have as many elements as self. See #zip? for a version that yields nil instead of raising.

a = [1, 2, 3]
b = ["a", "b", "c"]

a.zip(b) { |x, y| puts "#{x} -- #{y}" }

The above produces:

1 -- a
2 -- b
3 -- c

An example with multiple arguments:

(1..3).zip(4..6, 7..9) do |x, y, z|
  puts "#{x} -- #{y} -- #{z}"
end

The above produces:

1 -- 4 -- 7
2 -- 5 -- 8
3 -- 6 -- 9

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def zip(*others : Indexable | Iterable | Iterator) #

Returns an Array of tuples populated with the elements of self and others traversed in tandem.

Raises an IndexError if any of others doesn't have as many elements as self. See #zip? for a version that yields nil instead of raising.

a = [1, 2, 3]
b = ["a", "b", "c"]

a.zip(b) # => [{1, "a"}, {2, "b"}, {3, "c"}]

An example with multiple arguments:

a = [1, 2, 3]
b = (4..6)
c = 8.downto(3)

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

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def zip?(*others : Indexable | Iterable | Iterator, &) #

Yields elements of self and others in tandem to the given block.

All of the elements in self will be yielded: if others don't have that many elements they will be returned as nil.

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

a.zip?(b) { |x, y| puts "#{x.inspect} -- #{y.inspect}" }

The above produces:

1 -- "a"
2 -- "b"
3 -- nil

An example with multiple arguments:

(1..3).zip?(4..5, 7..8) do |x, y, z|
  puts "#{x.inspect} -- #{y.inspect} -- #{z.inspect}"
end

The above produces:

1 -- 4 -- 7
2 -- 5 -- 8
3 -- nil -- nil

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def zip?(*others : Indexable | Iterable | Iterator) #

Returns an Array of tuples populated with the elements of self and others traversed in tandem.

All elements in self are returned in the Array. If matching elements in others are missing (because they don't have that many elements) nil is returned inside that tuple index.

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

a.zip?(b) # => [{1, "a"}, {2, "b"}, {3, nil}]

An example with multiple arguments:

a = [1, 2, 3]
b = (4..5)
c = 8.downto(7)

a.zip?(b, c) # => [{1, 4, 8}, {2, 5, 7}, {3, nil, nil}]

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