| Module | Enumerable |
| In: |
enumerator.c
enum.c lib/set.rb |
The Enumerable mixin provides collection classes with several traversal and searching methods, and with the ability to sort. The class must provide a method each, which yields successive members of the collection. If Enumerable#max, min, or sort is used, the objects in the collection must also implement a meaningful <=> operator, as these methods rely on an ordering between members of the collection.
Passes each element of the collection to the given block. The method returns true if the block never returns false or nil. If the block is not given, Ruby adds an implicit block of {|obj| obj} (that is all? will return true only if none of the collection members are false or nil.)
%w{ant bear cat}.all? {|word| word.length >= 3} #=> true
%w{ant bear cat}.all? {|word| word.length >= 4} #=> false
[ nil, true, 99 ].all? #=> false
Passes each element of the collection to the given block. The method returns true if the block ever returns a value other than false or nil. If the block is not given, Ruby adds an implicit block of {|obj| obj} (that is any? will return true if at least one of the collection members is not false or nil.
%w{ant bear cat}.any? {|word| word.length >= 3} #=> true
%w{ant bear cat}.any? {|word| word.length >= 4} #=> true
[ nil, true, 99 ].any? #=> true
Returns a new array with the results of running block once for every element in enum.
(1..4).collect {|i| i*i } #=> [1, 4, 9, 16]
(1..4).collect { "cat" } #=> ["cat", "cat", "cat", "cat"]
Returns the number of items in enum for which equals to item. If a block is given, counts the number of elements yielding a true value.
ary = [1, 2, 4, 2]
ary.count(2) # => 2
ary.count{|x|x%2==0} # => 3
Calls block for each element of enum repeatedly forever. Returns nil if and only if the collection is empty. Enumerable#cycle saves elements in an internal array so changes to enum after the first pass have no effect.
a = ["a", "b", "c"]
a.cycle {|x| puts x } # print, a, b, c, a, b, c,.. forever.
Passes each entry in enum to block. Returns the first for which block is not false. If no object matches, calls ifnone and returns its result when it is specified, or returns nil
(1..10).detect {|i| i % 5 == 0 and i % 7 == 0 } #=> nil
(1..100).detect {|i| i % 5 == 0 and i % 7 == 0 } #=> 35
Drops first n elements from enum, and returns rest elements in an array.
a = [1, 2, 3, 4, 5, 0] a.drop(3) # => [4, 5, 0]
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.
a = [1, 2, 3, 4, 5, 0]
a.drop_while {|i| i < 3 } # => [3, 4, 5, 0]
Iterates the given block for each array of consecutive <n> elements.
e.g.:
(1..10).each_cons(3) {|a| p a}
# outputs below
[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
[4, 5, 6]
[5, 6, 7]
[6, 7, 8]
[7, 8, 9]
[8, 9, 10]
Iterates the given block for each slice of <n> elements.
e.g.:
(1..10).each_slice(3) {|a| p a}
# outputs below
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
[10]
Calls block with two arguments, the item and its index, for each item in enum.
hash = Hash.new
%w(cat dog wombat).each_with_index {|item, index|
hash[item] = index
}
hash #=> {"cat"=>0, "wombat"=>2, "dog"=>1}
Returns an array containing the items in enum.
(1..7).to_a #=> [1, 2, 3, 4, 5, 6, 7]
{ 'a'=>1, 'b'=>2, 'c'=>3 }.to_a #=> [["a", 1], ["b", 2], ["c", 3]]
Passes each entry in enum to block. Returns the first for which block is not false. If no object matches, calls ifnone and returns its result when it is specified, or returns nil
(1..10).detect {|i| i % 5 == 0 and i % 7 == 0 } #=> nil
(1..100).detect {|i| i % 5 == 0 and i % 7 == 0 } #=> 35
Returns an array containing all elements of enum for which block is not false (see also Enumerable#reject).
(1..10).find_all {|i| i % 3 == 0 } #=> [3, 6, 9]
Passes each entry in enum to block. Returns the index for the first for which block is not false. If no object matches, returns nil
(1..10).find_index {|i| i % 5 == 0 and i % 7 == 0 } #=> nil
(1..100).find_index {|i| i % 5 == 0 and i % 7 == 0 } #=> 34
Returns an array of every element in enum for which Pattern === element. If the optional block is supplied, each matching element is passed to it, and the block‘s result is stored in the output array.
(1..100).grep 38..44 #=> [38, 39, 40, 41, 42, 43, 44]
c = IO.constants
c.grep(/SEEK/) #=> ["SEEK_END", "SEEK_SET", "SEEK_CUR"]
res = c.grep(/SEEK/) {|v| IO.const_get(v) }
res #=> [2, 0, 1]
Returns a hash, which keys are evaluated result from the block, and values are arrays of elements in enum corresponding to the key.
(1..6).group_by {|i| i%3} #=> {0=>[3, 6], 1=>[1, 4], 2=>[2, 5]}
Returns true if any member of enum equals obj. Equality is tested using ==.
IO.constants.include? "SEEK_SET" #=> true IO.constants.include? "SEEK_NO_FURTHER" #=> false
enum.reduce(initial, sym) => obj
enum.reduce(sym) => obj
enum.reduce(initial) {| memo, obj | block } => obj
enum.reduce {| memo, obj | block } => obj
Combines all elements of enum by applying a binary operation, specified by a block or a symbol that names a method or operator.
If you specify a block, then for each element in enum<i> the block is passed an accumulator value (<i>memo) and the element. If you specify a symbol instead, then each element in the collection will be passed to the named method of memo. In either case, the result becomes the new value for memo. At the end of the iteration, the final value of memo is the return value fo the method.
If you do not explicitly specify an initial value for memo, then uses the first element of collection is used as the initial value of memo.
Examples:
# Sum some numbers
(5..10).reduce(:+) #=> 45
# Same using a block and inject
(5..10).inject {|sum, n| sum + n } #=> 45
# Multiply some numbers
(5..10).reduce(1, :*) #=> 151200
# Same using a block
(5..10).inject(1) {|product, n| product * n } #=> 151200
# find the longest word
longest = %w{ cat sheep bear }.inject do |memo,word|
memo.length > word.length ? memo : word
end
longest #=> "sheep"
Returns a new array with the results of running block once for every element in enum.
(1..4).collect {|i| i*i } #=> [1, 4, 9, 16]
(1..4).collect { "cat" } #=> ["cat", "cat", "cat", "cat"]
Returns the object in enum with the maximum value. The first form assumes all objects implement Comparable; the second uses the block to return a <=> b.
a = %w(albatross dog horse)
a.max #=> "horse"
a.max {|a,b| a.length <=> b.length } #=> "albatross"
Returns the object in enum that gives the maximum value from the given block.
a = %w(albatross dog horse)
a.max_by {|x| x.length } #=> "albatross"
Returns true if any member of enum equals obj. Equality is tested using ==.
IO.constants.include? "SEEK_SET" #=> true IO.constants.include? "SEEK_NO_FURTHER" #=> false
Returns the object in enum with the minimum value. The first form assumes all objects implement Comparable; the second uses the block to return a <=> b.
a = %w(albatross dog horse)
a.min #=> "albatross"
a.min {|a,b| a.length <=> b.length } #=> "dog"
Returns the object in enum that gives the minimum value from the given block.
a = %w(albatross dog horse)
a.min_by {|x| x.length } #=> "dog"
Returns two elements array which contains the minimum and the maximum value in the enumerable. The first form assumes all objects implement Comparable; the second uses the block to return a <=> b.
a = %w(albatross dog horse)
a.minmax #=> ["albatross", "horse"]
a.minmax {|a,b| a.length <=> b.length } #=> ["dog", "albatross"]
Returns two elements array array containing the objects in enum that gives the minimum and maximum values respectively from the given block.
a = %w(albatross dog horse)
a.minmax_by {|x| x.length } #=> ["dog", "albatross"]
Passes each element of the collection to the given block. The method returns true if the block never returns true for all elements. If the block is not given, none? will return true only if none of the collection members is true.
%w{ant bear cat}.none? {|word| word.length == 5} #=> true
%w{ant bear cat}.none? {|word| word.length >= 4} #=> false
[].none? #=> true
[nil].none? #=> true
[nil,false].none? #=> true
Passes each element of the collection to the given block. The method returns true if the block returns true exactly once. If the block is not given, one? will return true only if exactly one of the collection members is true.
%w{ant bear cat}.one? {|word| word.length == 4} #=> true
%w{ant bear cat}.one? {|word| word.length > 4} #=> false
%w{ant bear cat}.one? {|word| word.length < 4} #=> false
[ nil, true, 99 ].one? #=> false
[ nil, true, false ].one? #=> true
Returns two arrays, the first containing the elements of enum for which the block evaluates to true, the second containing the rest.
(1..6).partition {|i| (i&1).zero?} #=> [[2, 4, 6], [1, 3, 5]]
enum.reduce(initial, sym) => obj
enum.reduce(sym) => obj
enum.reduce(initial) {| memo, obj | block } => obj
enum.reduce {| memo, obj | block } => obj
Combines all elements of enum by applying a binary operation, specified by a block or a symbol that names a method or operator.
If you specify a block, then for each element in enum<i> the block is passed an accumulator value (<i>memo) and the element. If you specify a symbol instead, then each element in the collection will be passed to the named method of memo. In either case, the result becomes the new value for memo. At the end of the iteration, the final value of memo is the return value fo the method.
If you do not explicitly specify an initial value for memo, then uses the first element of collection is used as the initial value of memo.
Examples:
# Sum some numbers
(5..10).reduce(:+) #=> 45
# Same using a block and inject
(5..10).inject {|sum, n| sum + n } #=> 45
# Multiply some numbers
(5..10).reduce(1, :*) #=> 151200
# Same using a block
(5..10).inject(1) {|product, n| product * n } #=> 151200
# find the longest word
longest = %w{ cat sheep bear }.inject do |memo,word|
memo.length > word.length ? memo : word
end
longest #=> "sheep"
Returns an array for all elements of enum for which block is false (see also Enumerable#find_all).
(1..10).reject {|i| i % 3 == 0 } #=> [1, 2, 4, 5, 7, 8, 10]
Returns an array containing all elements of enum for which block is not false (see also Enumerable#reject).
(1..10).find_all {|i| i % 3 == 0 } #=> [3, 6, 9]
Returns an array containing the items in enum sorted, either according to their own <=> method, or by using the results of the supplied block. The block should return -1, 0, or +1 depending on the comparison between a and b. As of Ruby 1.8, the method Enumerable#sort_by implements a built-in Schwartzian Transform, useful when key computation or comparison is expensive..
%w(rhea kea flea).sort #=> ["flea", "kea", "rhea"]
(1..10).sort {|a,b| b <=> a} #=> [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
Sorts enum using a set of keys generated by mapping the values in enum through the given block.
%w{ apple pear fig }.sort_by {|word| word.length}
#=> ["fig", "pear", "apple"]
The current implementation of sort_by generates an array of tuples containing the original collection element and the mapped value. This makes sort_by fairly expensive when the keysets are simple
require 'benchmark'
include Benchmark
a = (1..100000).map {rand(100000)}
bm(10) do |b|
b.report("Sort") { a.sort }
b.report("Sort by") { a.sort_by {|a| a} }
end
produces:
user system total real Sort 0.180000 0.000000 0.180000 ( 0.175469) Sort by 1.980000 0.040000 2.020000 ( 2.013586)
However, consider the case where comparing the keys is a non-trivial operation. The following code sorts some files on modification time using the basic sort method.
files = Dir["*"]
sorted = files.sort {|a,b| File.new(a).mtime <=> File.new(b).mtime}
sorted #=> ["mon", "tues", "wed", "thurs"]
This sort is inefficient: it generates two new File objects during every comparison. A slightly better technique is to use the Kernel#test method to generate the modification times directly.
files = Dir["*"]
sorted = files.sort { |a,b|
test(?M, a) <=> test(?M, b)
}
sorted #=> ["mon", "tues", "wed", "thurs"]
This still generates many unnecessary Time objects. A more efficient technique is to cache the sort keys (modification times in this case) before the sort. Perl users often call this approach a Schwartzian Transform, after Randal Schwartz. We construct a temporary array, where each element is an array containing our sort key along with the filename. We sort this array, and then extract the filename from the result.
sorted = Dir["*"].collect { |f|
[test(?M, f), f]
}.sort.collect { |f| f[1] }
sorted #=> ["mon", "tues", "wed", "thurs"]
This is exactly what sort_by does internally.
sorted = Dir["*"].sort_by {|f| test(?M, f)}
sorted #=> ["mon", "tues", "wed", "thurs"]
Returns first n elements from enum.
a = [1, 2, 3, 4, 5, 0] a.take(3) # => [1, 2, 3]
Passes elements to the block until the block returns nil or false, then stops iterating and returns an array of all prior elements.
a = [1, 2, 3, 4, 5, 0]
a.take_while {|i| i < 3 } # => [1, 2]
Returns an array containing the items in enum.
(1..7).to_a #=> [1, 2, 3, 4, 5, 6, 7]
{ 'a'=>1, 'b'=>2, 'c'=>3 }.to_a #=> [["a", 1], ["b", 2], ["c", 3]]
Makes a set from the enumerable object with given arguments. Needs to +require "set"+ to use this method.
Takes one element from enum and merges corresponding elements from each args. This generates a sequence of n-element arrays, where n is one more than the count of arguments. The length of the resulting sequence will be enum#size</code. If the size of any argument is less than <code>enum#size, 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]]
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