What is the difference between `Range#include?` and `Range#cover?`?

ruby range
sawa picture sawa · Feb 6, 2014 · Viewed 7.8k times · Source

Edit Fixed following toro2k's comment.

Range#include? and Range#cover? seem to be different as seen in the source code 1, 2, and they are different in efficiency.

t = Time.now
500000.times do
    ("a".."z").include?("g")
end
puts Time.now - t    # => 0.504382493

t = Time.now
500000.times do
    ("a".."z").cover?("g")
end
puts Time.now - t    # => 0.454867868

Looking at the source code, Range#include? seems to be more complex than Range#cover?. Why can't Range#include? be simply an alias of Range#cover? What is their difference?

Answer

Neil Slater picture Neil Slater · Feb 6, 2014

The two methods are designed to do two slightly different things on purpose. Internally they are implemented very differently too. You can take a look at the sources in the documentation and see that .include? is doing a lot more than .cover?

The .cover? method is related to the Comparable module, and checks whether an item would fit between the end points in a sorted list. It will return true even if the item is not in the set implied by the Range.

The .include? method is related to the Enumerable module, and checks whether an item is actually in the complete set implied by the Range. There is some finessing with numerics - Integer ranges are counted as including all the implied Float values (I'm not sure why).

These examples might help:

('a'..'z').cover?('yellow')
# => true

('a'..'z').include?('yellow')
# => false

('yellaa'..'yellzz').include?('yellow')
 => true

Additionally, if you try

('aaaaaa'..'zzzzzz').include?('yellow')

you should notice it takes a much longer time than

('aaaaaa'..'zzzzzz').cover?('yellow')

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