Accumulate a set of a set. For example, in an ORM design where `Group has_many User` we might have something equivalent to the following.
Group = Struct.new(:users) User = Struct.new(:name, :friends) user1 = User.new('John', []) user2 = User.new('Jane', ['Jill']) user3 = User.new('Joe' , ['Jack', 'Jim']) group1 = Group.new([user1, user2]) group2 = Group.new([user2, user3]) groups = [group1, group2]
Now we can accumulate the users of all groups.
groups.accumulate.users #=> [user1, user2, user3]
You may pass an argument to perform chains, e.g. the following returns the names of users from all groups.
groups.accumulate(2).users.name #=> ['John','Jane','Joe']
Or we can gather all the friends of all users in groups.
groups.accumulate(2).users.friends #=> ['Jill','Jack','Jim']
This is more convenient then the equivalent.
groups.accumulate.users.accumulate.friends #=> ['Jill','Jack','Jim']
CREDIT: George Moshchovitis, Daniel Emirikol
Same as accumulate, but does not apply uniq to final result.
groups.accumulate_all(2).users.friends #=> ['Jill', 'Jill','Jack','Jim']
Similar to group_by but returns an array of the groups. Returned elements are sorted by block.
%w{this is a test}.cluster_by {|x| x[0]} #=> [ ['a'], ['is'], ['this', 'test'] ]
CREDIT: Erik Veenstra
Count the number of items in an enumerable equal (==) to the given object(s).
e = [ 'a', 1, 'a' ] e.count(1) #=> 1 e.count('a') #=> 2
The method can count more than one item by supplying additional arguments.
e.count('a', 1) #=> 3
A block may also be used to select countable entries.
e.count{ |x| String === x } #=> 2
Note that Hash#count only considers values.
e = { 'a' => 2, 'x' => 2, 'b' => 1 } e.count(1) #=> 1 e.count(2) #=> 2
NOTE: This is one of the few method overrides in Facets.
CREDIT: Trans
Without a block: wrap the Enumerable object in such a way that map, select and similar operations are performed "horizontally" across a series of blocks, instead of building an array of results at each step. This reduces memory usage, allows partial results to be provided early, and permits working with infinite series.
a = (1..1_000_000_000).defer.select{ |i| i % 2 == 0 }. map{ |i| i + 100 }. take(10).to_a
With a block: the block acts as an arbitrary filter on the data. Unlike map, it can choose to drop elements from the result, and/or add additional ones. The first object passed to the block is the receiver of the output.
(1..1_000_000_000). defer { |out,i| out << i if i % 2 == 0 }. # like select defer { |out,i| out << i + 100 }. # like map take(10).to_a
Use a method like to_a or to_h at the end of the chain when you want an Array or Hash built with the results, or each{…} if you just want to output each result and discard it.
Iterate through slices. If slice steps is not given, the arity of the block is used.
x = [] [1,2,3,4].each_by{ |a,b| x << [a,b] } x #=> [ [1,2], [3,4] ] x = [] [1,2,3,4,5,6].each_by(3){ |a| x << a } x #=> [ [1,2,3], [4,5,6] ]
This is just like each_slice, except that it will check the arity of the block. If each_slice ever suppots this this method can be deprecated.
CREDIT: Trans
A variation of inject that saves one from having to return the aggregate/memo argument.
Say we want to count characters in a string. Using the each_with_object method we have:
"string".each_with_object(Hash.new(0)) do |c, h| h[c] += 1 end
versus using inject which would be:
"string".inject(Hash.new(0)) do |h, c| h[c] +=1 h end
Notice that the order of the block parameters is reversed.
This method used be called injecting and had the same parameter order as inject, but Ruby 1.9 has adopted this method, so we support it instead.
Returns an elemental object. This allows you to map a method on to every element.
r = [1,2,3].every + 3 r #=> [4,5,6]
Returns an elementwise Functor designed to make R-like elementwise operations possible. This is very much like the every method, but it treats array argument specially.
([1,2].ewise + 3) #=> [4,5]
Vector to vector
([1,2].ewise + [4,5]) #=> [5,7]
Special thanks to Martin DeMello for helping to develop this.
Expand all elements of an Enumerable object.
[0, 2..3, 5..7].expand #=> [0,[2, 3],[5,6,7]]
CREDIT: Trans
The block acts as an arbitrary filter on the data. Unlike map, it can choose to drop elements from the result and/or add additional elements. The first object passed to the block is the receiver of the output.
x = (1..10000) x = x.filter{ |out,i| out << i if i % 2 == 0 } # like select x = x.filter{ |out,i| out << i + 100 } # like map x = x.take(3) x #=> [102, 104, 106]
This is very similar to each_with_object, but filter handles argument better by reversing their order and using the splat operator. (This was also once known as injecting.)
CREDIT: David Black, Louis J Scoras
Yield each element to the block and return the result of the block when that result evaluates as true, terminating early like detect and find.
obj1 = Object.new obj2 = Object.new def obj1.foo?; false; end def obj2.foo?; true ; end def obj1.foo ; "foo1"; end def obj2.foo ; "foo2"; end [obj1, obj2].find_yield{ |obj| obj.foo if obj.foo? } #=> "foo2"
Another example.
[1,2,3,4,5].find_yield{ |i| j = i+1; j if j % 4 == 0 } #=> 4
If the block is never true, return the object given in the first parameter, or nil if none specified.
[1,2,3].find_yield{ |_| false } #=> nil [false].find_yield(1){ |_| false } #=> 1
Generates a hash mapping each unique symbol in the array to the absolute frequency it appears.
[:a,:a,:b,:c,:c,:c].frequency #=> {:a=>2,:b=>1,:c=>3}
CREDIT: Brian Schröder
Like `map`/`collect`, but generates a Hash. The block is expected to return two values: the key and the value for the new hash.
numbers = (1..3) squares = numbers.graph{ |n| [n, n*n] } # { 1=>1, 2=>4, 3=>9 } sq_roots = numbers.graph{ |n| [n*n, n] } # { 1=>1, 4=>2, 9=>3 }
CREDIT: Andrew Dudzik (adudzik), Trans
Enumerable#group_by is used to group items in a collection by something they have in common. The common factor is the key in the resulting hash, the array of like elements is the value.
(1..5).group_by { |n| n % 3 } #=> { 0 => [3], 1 => [1, 4], 2 => [2,5] }
Applied to an array.
["I had", 1, "dollar and", 50, "cents"].group_by { |e| e.class } #=> { String => ["I had","dollar and","cents"], Fixnum => [1,50] }
Applied to a hash:
{:a=>1, :b=>2, :c=>1}.group_by{ |k,v| v } #=> { 1=>[[:c,1], [:a,1]], 2=>[[:b,2]] }
CREDIT: Erik Veenstra
Like group_by, but maps the second value returned from the block.
a = [1,2,3,4,5] a.map_by{ |e| [e % 2, e + 1] } #=> { 0=>[3,5], 1=>[2,4,6] }
Works well with a hash too.
h = {"A"=>1, "B"=>1, "C"=>1, "D"=>2, "E"=>2} h.map_by{ |k,v| [v, k.downcase] } #=> {1=>["a", "b", "c"], 2=>["d", "e"]}
If a second value is not returned, map_by acts like group_by.
h = {"A"=>1, "B"=>1, "C"=>1, "D"=>2, "E"=>2} h.map_by{ |k,v| v } #=> {1=>[["A",1], ["B",1], ["C",1]], 2=>[["D",2], ["E",2]]}
Send a message to each element and collect the result.
[1,2,3].map_send(:+, 3) #=> [4,5,6]
CREDIT: Sean O‘Halpin
Same as collect but with an iteration counter.
a = [1,2,3].collect_with_index { |e,i| e*i } a #=> [0,2,6]
CREDIT: Gavin Sinclair
Modulate. Divide an array into groups by modulo of the index.
[2,4,6,8].modulate(2) #=> [[2,6],[4,8]]
CREDIT: Trans
NOTE: Would the better name for this be ‘collate’?
Enumerable#none? is the logical opposite of the builtin method Enumerable#any?. It returns true if and only if none of the elements in the collection satisfy the predicate.
If no predicate is provided, Enumerable#none? returns true if and only if none of the elements have a true value (i.e. not nil or false).
[].none? # => true [nil].none? # => true [5,8,9].none? # => false (1...10).none? { |n| n < 0 } # => true (1...10).none? { |n| n > 0 } # => false
CREDIT: Gavin Sinclair
Returns an array of elements for the elements that occur n times. Or according to the results of a given block.
a = [1,1,2,3,3,4,5,5] a.occur(1).sort #=> [2,4] a.occur(2).sort #=> [1,3,5] a.occur(3).sort #=> [] a.occur(1..1).sort #=> [2,4] a.occur(2..3).sort #=> [1,3,5] a.occur { |n| n == 1 }.sort #=> [2,4] a.occur { |n| n > 1 }.sort #=> [1,3,5]
Enumerable#one? returns true if and only if exactly one element in the collection satisfies the given predicate.
If no predicate is provided, Enumerable#one? returns true if and only if exactly one element has a true value (i.e. not nil or false).
[].one? # => false [nil].one? # => false [5].one? # => true [5,8,9].one? # => false (1...10).one? { |n| n == 5 } # => true (1...10).one? { |n| n < 5 } # => false
CREDIT: Gavin Sinclair
Per element meta-functor.
([1,2,3].per(:map) + 3) #=> [4,5,6] ([1,2,3].per(:select) > 1) #=> [2,3]
Using fluid notation.
([1,2,3].per.map + 3) #=> [4,5,6] ([1,2,3].per.select > 1) #=> [2,3]
A versitle compaction method. Like map but used to filter out multiple items in a single step.
Without trash arguments nil is assumed.
[1, nil, 2].purge #=> [1,2]
If trash arguments are given, each argument is compared for a match using #==.
(1..6).purge(3,4) #=> [1,2,5,6]
If a block is given, the yield is used in the matching condition instead of the element itsef.
(1..6).purge(0){ |n| n % 2 } #=> [1,3,5]
NOTE: This could just as well be an override of the core compact method, but to avoid potential issues associated with overriding core methods we use the alternate name purge.
CREDIT: Trans
Returns a recursive functor, that allows enumerable methods to iterate through enumerable sub-elements. By default it only recurses over elements of the same type.
Uses #+ to sum the enumerated elements.
[1,2,3].sum #=> 6 [3,3,3].sum #=> 9
Like Enumerable#map_send but each iteration is processed via a separate thread.
CREDIT: Sean O‘Halpin
Convert an Enumerable object into a hash by first turning it into an array.
CREDIT: Trans
Like uniq, but determines uniqueness based on a given block.
(-5..5).to_a.uniq_by {|i| i*i } #=> [-5, -4, -3, -2, -1, 0]
Recursively iterate over all Enumerable elements, or subset given :type=>[type1, type2, …].
[1, 2, 8..9].visit{ |x| x.succ } # => [2, 3, [9, 10]]