Is "find_all" and "select" the same thing?

Question

These two statements give me the same results:

[1,2,3,4].find_all { |x| x.even? }  [1,2,3,4].select{ |x| x.even? } 

Is find_all just an alias? Is there a reason to use one over another?

Answer 1

#find_all and #select are very similar; the difference is very subtle. In most of the cases, they are equivalent. It depends on the class implementing it.

Enumerable#find_all and Enumerable#select run on the same code.

The same happens for Array and Range, as they use Enumerable implementation.

In the case of Hash, #select is redefined to return a Hash instead of an Array, but #find_all is inherited from Enumerable

a = [1, 2, 3, 4, 5, 6] h = {a: 1, b: 2, c: 3, d: 4, e: 5, f: 6}  a.select{|x| x.even?}       # => [2, 4, 6] a.find_all{|x| x.even?}     # => [2, 4, 6]  h.select{|k,v| v.even?}     # => {:b=>2, :d=>4, :f=>6} h.find_all{|k,v| v.even?}   # => [[:b, 2], [:d, 4], [:f, 6]] 

Answer 2

Enumerable#find_all Returns an array containing all elements of enum for which the given block returns a true value, which is not the case for select. Enumerable#select returns the Array, if the receiver on which you are calling #select method, don't have it's own #select method. Otherwise on which receiver you are calling #select method, it will return similar type of receiver, after processing the block condition.

Like Hash#select Returns a new hash consisting of entries for which the block returns true and Array#select Returns a new array containing all elements of ary for which the given block returns a true value. Now Range#select will give you return back an Array, as Range class don't have its own instance method called #select. Rather being an Enumerable,it will call Enumerable#select.

rng = (1..4) ary = [1,2] hsh = {:a => 1}  rng.method(:select) # => #<Method: Range(Enumerable)#select> hsh.method(:select) # => #<Method: Hash#select> ary.method(:select) # => #<Method: Array#select> 

Here is a full demonstration with example in-favor of my explanation above :

hsh = {:a => 2, :b => 3 } ary = [1,2,3] rng = (1..3)  # Look find_all always gives Array. hsh.find_all{ true } # => [[:a, 2], [:b, 3]] ary.find_all{ true } # => [1, 2, 3] rng.find_all{ true } # => [1, 2, 3]  # Look select not giving Array always, explanation of why so is written # above in my answer. hsh.select{ true } # => {:a=>2, :b=>3} ary.select{ true } # => [1, 2, 3] rng.select{ true } # => [1, 2, 3]