what's different between each and collect method in Ruby

Question

From this code I don't know the difference between the two methods, collect and each.

a = ["L","Z","J"].collect{|x| puts x.succ} #=> M AA K 
print  a.class  #=> Array

b = ["L","Z","J"].each{|x| puts x.succ} #=> M AA K
print  b.class #=> Array

Solution 1

Array#each takes an array and applies the given block over all items. It doesn't affect the array or creates a new object. It is just a way of looping over items. Also it returns self.

  arr=[1,2,3,4]
  arr.each {|x| puts x*2}

Prints 2,4,6,8 and returns [1,2,3,4] no matter what

Array#collect is same as Array#map and it applies the given block of code on all the items and returns the new array. simply put 'Projects each element of a sequence into a new form'

  arr.collect {|x| x*2}

Returns [2,4,6,8]

And In your code

 a = ["L","Z","J"].collect{|x| puts x.succ} #=> M AA K 

a is an Array but it is actually an array of Nil's [nil,nil,nil] because puts x.succ returns nil (even though it prints M AA K).

And

 b = ["L","Z","J"].each{|x| puts x.succ} #=> M AA K

also is an Array. But its value is ["L","Z","J"], because it returns self.

Solution 2

Array#each just takes each element and puts it into the block, then returns the original array. Array#collect takes each element and puts it into a new array that gets returned:

[1, 2, 3].each { |x| x + 1 }    #=> [1, 2, 3]
[1, 2, 3].collect { |x| x + 1 } #=> [2, 3, 4]

Solution 3

each is for when you want to iterate over an array, and do whatever you want in each iteration. In most (imperative) languages, this is the "one size fits all" hammer that programmers reach for when you need to process a list.

For more functional languages, you only do this sort of generic iteration if you can't do it any other way. Most of the time, either map or reduce will be more appropriate (collect and inject in ruby)

collect is for when you want to turn one array into another array

inject is for when you want to turn an array into a single value

Solution 4

Here are the two source code snippets, according to the docs...

VALUE
rb_ary_each(VALUE ary)
{
    long i;

    RETURN_ENUMERATOR(ary, 0, 0);
    for (i=0; i<RARRAY_LEN(ary); i++) {
        rb_yield(RARRAY_PTR(ary)[i]);
    }
    return ary;
}

# .... .... .... .... .... .... .... .... .... .... .... ....

static VALUE
rb_ary_collect(VALUE ary)
{
    long i;
    VALUE collect;

    RETURN_ENUMERATOR(ary, 0, 0);
    collect = rb_ary_new2(RARRAY_LEN(ary));
    for (i = 0; i < RARRAY_LEN(ary); i++) {
        rb_ary_push(collect, rb_yield(RARRAY_PTR(ary)[i]));
    }
    return collect;
}

rb_yield() returns the value returned by the block (see also this blog post on metaprogramming).

So each just yields and returns the original array, while collect creates a new array and pushes the results of the block into it; then it returns this new array.

Source snippets: each, collect

Solution 5

The difference is what it returns. In your example above a == [nil,nil,nil] (the value of puts x.succ) while b == ["L", "Z", "J"] (the original array)

From the ruby-doc, collect does the following:

Invokes block once for each element of self. Creates a new array containing the values returned by the block.

Each always returns the original array. Makes sense?

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