Question

I am a little confused with Enumerator#reject in ruby. Consider the following code:

(1..10).select {|i| i % 3 == 0 || i % 5 == 0 } => [3, 5, 6, 9, 10]

Shouldn’t the following line be equivalent?

(1..10).reject {|i| i % 3 != 0 || i % 5 != 0 } => []

If I just use one condition on the reject method, the result is as expected. but If I include the OR operator the result turns out to be empty. Could somebody clarify this for me.

(1..10).reject {|i| i % 3 != 0} => [3, 6, 9]

Answer 1

You are making a basic logic mistake:

!(A || B) is equivalent to !A && !B and NOT equivalent to !A || !B.

So if you change the || in your second example to a &&, then your second example would give the same result as the first:

(1..10).reject {|i| i % 3 != 0 && i % 5 != 0 } # => [3, 5, 6, 9, 10]