One of the problems newcomers to Ruby experience is that there are often quite a few ways to do same thing. For instance - you can obtain the number of items in Enumerable objects (instances of classes using the Enumerable mixin, which would often be collections like Array , Hash , Set , etc) by either using Enumerable#count or the methods length and its alias size that such classes often provide.

arr = [ 1 , 2 , 3 ] arr . length # => 3 arr . size # => 3 arr . count # => 3 h = { a: 1 , b: 2 } h . length # => 2 h . size # => 2 h . count # => 2 str = 'name' str . length # => 4 str . size # => 4 # str.count won't work as String does not include Enumerable

Which one should you use? Let me help with this choice.

length is a method that’s not part of Enumerable - it’s part of a concrete class (like String or Array ) and it’s usually running in O(1) (constant) time. That’s as fast as it gets, which means that using it is probably a good idea.

Whether you should use length or size is mostly a matter of personal preference. Personally I use size for collections (hashes, arrays, etc) and length for strings, since for me objects like hashes and stacks don’t have a length, but a size (defined in terms of the elements they contain). Conversely, it’s perfectly normal to assume that some text has some length. Anyways, in the end you’re invoking the same method, so the semantic distinction is not important.

Enumerable#count , on the other hand, is a totally different beast. It’s usually meant to be used with a block or an argument and will return the number of matches in an Enumerable :

arr = [ 1 , 1 , 2 , 3 , 5 , 6 , 8 ] arr . count ( & :even? ) # => 3 arr . count ( 1 ) # => 2

You can, however, invoke it without any arguments and it will return the size of the enumerable on which it was invoked:

arr . count # => 7

There’s a performance implication with this, though - to calculate the size of the enumerable the count method will traverse it, which is not particularly fast (especially for huge collections). Some classes (like Array ) implement an optimized version of count in terms of length , but many don’t.

The takeaway for you is that you should avoid using the count method if you can use length or size .

A note to Rails developers - ActiveRecord::Relation ’s length , size and count methods have a totally different meaning, but that’s irrelevant to our current discussion.

That’s all for now, folks! As usual I’m looking forward to hearing your thoughts here and on Twitter!