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Friday, 23 December 2011

Ruby List Comprehensions

I'm busy learning Ruby and I'm discovering that I really like it. I've used Python for some time now so naturally it's the differences between these two paradigmatically similar languages that I notice the most.

A feature of Python that I think is really cool is the list comprehension; a way of specifying a list literal using the definition of the list, rather than it's enumeration, as the code below illustrates:


# literal as an enumeration
a = [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

# literal as a comprehension
b = [ i for i in range( 10 ) ]

This is a really neat feature that can create a sequence from an arbitrary definition, such as the characters in a string, the odd numbers in a sequence etc. You get the idea.

In Ruby, doing this ...

a = [ 1..100 ]

... doesn't give you an array of one hundred elements, but an array containing a single Range object of 1 to 100.

The fact that list comprehensions seem to be missing from Ruby was initially slightly saddening, until I discovered that Ruby's class definitions, like its strings, are mutable. This is what's really got me excited about this language - you want list comprehensions? Add them.

The duck typing offered by dynamic languages along with Ruby's class mutability makes this sort of thing really easy to do, as demonstrated below.

#saved as 'list_comp.rb'

class Array
  def from_e!( enum )
    enum.each { |x| self << x }
    return self
  def self.from_e( enum )
    return [].from_e! enum

This is cool! It lets you create lists in a really intuitive way using definitions, not enumerations - and the best part is that I've added this to the actual, built-in Array class itself - that's the power of Ruby. I love it.

Here are some lists being created from scratch using the class method:

irb(main):001:0> require './list_comp.rb'
=> true
irb(main):002:0> Array.from_e 1..10
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
irb(main):003:0> Array.from_e 'How much wood would a woodchuck chuck?'.each_char
=> ["H", "o", "w", " ", "m", "u", "c", "h", " ", "w", "o", "o", "d", " ", "w", "
o", "u", "l", "d", " ", "a", " ", "w", "o", "o", "d", "c", "h", "u", "c", "k", "
 ", "c", "h", "u", "c", "k", "?"]
irb(main):004:0> Array.from_e 0.step 50, 5
=> [0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50]
irb(main):005:0> Array.from_e 'one two 3 four 55'.scan /\d+/
=> ["3", "55"]

And here is the instance method in action, modifying an existing array:

irb(main):006:0> a=[0]
=> [0]
irb(main):007:0> a.from_e! 1.step 10,1
=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Note the ! on the end of the instance version of from_e - I've noticed and really appreciate the convention of marking mutating (non-const, if you like) instance methods using the exclamation mark and I intend to stick to it. Also, I've tried to follow convention by calling the method from_e - to match the to_s, to_i, etc. pattern.

Now it could be that Ruby already supports this sort of thing and I just haven't found it yet but either way I'm really impressed - it seems you can make this language whatever you want it to be. I'm looking forward to learning more - this is only day 3!


  1. (1..10).to_a
    'How much wood would a woodchuck chuck?'.each_char.to_a
    0.step(50, 5).to_a
    'one two 3 four 55'.scan(/\d+/).to_a

    PS. Ruby is dreadful.

  2. Nice one Steve, I thought there might have been a standard way of doing that.