Enumerating Enumerable: Enumerable#count

by Joey deVilla on July 2, 2008

Welcome to the fourth installment of Enumerating Enumerable, a series of articles in which I challenge myself to do a better job of documenting Ruby’s Enumerable module than RubyDoc.org does. In this article, I’ll cover Enumerable#count, one of the new methods added to Enumerable in Ruby 1.9.

In case you missed the earlier installments, they’re listed (and linked) below:

  1. all?
  2. any?
  3. collect / map

Enumerable#count Quick Summary

Graphic representation of the Enumberable#count method in Ruby

In the simplest possible terms How many items in the collection meet the given criteria?
Ruby version 1.9 only
Expects Either:

  • An argument to be matched against the items in the collection
  • A block containing an expression to test the items in the collection
Returns The number of items in the collection that meet the given criteria.
RubyDoc.org’s entry Enumerable#count

Enumerable#count and Arrays

When used on an array and an argument is provided, count returns the number of times the value of the argument appears in the array:

When used on an array and a block is provided, count returns the number of items in the array for which the block returns true:

RubyDoc.org says that when count is used on an array without an argument or a block, it simply returns the number of items in the array (which is what the length/size methods do). However, when I’ve tried it in irb and ruby, I got results like this:

Enumerable#count and Hashes

As with arrays, when used on a hash and an argument is provided, count returns the number of times the value of the argument appears in the hash. The difference is that for the comparison, each key/value pair is treated as a two-element array, with the key being element 0 and the value being element 1.

count is not useful when used with a hash and an argument. It will only ever return two values:

  • 1 if the argument is a two-element array and there is an item in the hash whose key matches element [0] of the array and whose value matches element [1] of the array.
  • 0 for all other cases.

When used with a hash and a block, count is more useful. count passes each key/value pair in the hash to the block, which you can “catch” as either:

  1. A two-element array, with the key as element 0 and its corresponding value as element 1, or
  2. Two separate items, with the key as the first item and its corresponding value as the second item.

Each key/value pair is passed to the block and count returns the number of items in the hash for which the block returns true.

(You should probably skip The Love Guru completely, or at least until it gets aired on TV for free.)

Previous post:

Next post: