Leigh Halliday

Implementing Ruby's Collect Method

published Jan 25, 2015

Ruby's Collect method

Ruby's collect method is part of the Enumerable mixin; a mixin which provides very useful and powerful methods for collection objects. collect is a method I use all the time and recently wrote about in my article Working with enumerables: Four powerful collection methods.

collect allows you to transform each element of your collection object (Array, Hash, or your own object) into something else, ending up with an Array.

Here's a simple example of taking an Array of integers and multiplying each of them by 2.

[1, 2, 3].collect{ |num| num * 2 }
# [2, 4, 6]

Re-implementing the Collect method

I wanted to create my own collect method to see how it is implemented. It turns out that it is done quite easily, in about 5 lines of code. I should say that the one use-case I don't cover is when no block is passed to the method, in which case an Enumerator is usually returned.

We'll add our new kollect (so the names don't conflict) to the Enumerable module. Looping through our collection, we'll apply the block to each element, adding the returned result to a new Array that is being created.

module Enumerable
def kollect
new_ary = []
self.each do |elem|
new_ary << yield(elem)

If we compare this to Ruby's source code, it essentially does the same thing, but is a lot simpler because we are working with Ruby itself, and not C.

rb_ary_collect(VALUE ary)
long i;
VALUE collect;

RETURN_SIZED_ENUMERATOR(ary, 0, 0, ary_enum_length);
collect = rb_ary_new2(RARRAY_LEN(ary));
for (i = 0; i < RARRAY_LEN(ary); i++) {
rb_ary_push(collect, rb_yield(RARRAY_AREF(ary, i)));
return collect;

Using our new Collect method

Now all that is left is to see if the kollect method actually works! I wanted to try it using both the normal block syntax, but also the short-hand &:method_name syntax, to ensure both work.

names = ["leigh", "marian"]

puts names.kollect(&:upcase).inspect
puts names.kollect{ |name| name.upcase }.inspect

Great! In both cases it takes the names in the Array and converts them to upper-case.