Question

I’m new to Ruby, and I’m having a strange problem with the inject method.

When I do:

(1..10).inject(0) {|count,x| count + 1}

the result is 10, as expected. But when I do

(1..10).inject(0) {|count,x| count + 1 if (x%2 == 0)}

I get an error:

NoMethodError: undefined method `+' for nil:NilClass
    from (irb):43
    from (irb):43:in `inject'
    from (irb):43:in `each'
    from (irb):43:in `inject'
    from (irb):43

I don’t really understand why (presumably) count is nil in the second example, but not the first. In any case, how would I count evens from 1 to 10 using inject?

Answer 1

The expression count + 1 if (x%2 == 0) returns nil when the condition isn’t true, which count gets set to because that’s the nature of the inject method.

You could fix it by returning count + 1 when it’s an even number and just count when it’s not:

(1..10).inject(0) { |count,x| x % 2 == 0 ? count + 1 : count }

A completely different solution is to use select to select the even numbers and use the Array#length method to count them.

(1..10).select { |x| x % 2 == 0 }.length