Nil can't be coerced into Fixnum

Question

I have the following function:

def valid_credit_card?(number)
    digits = number.scan(/./).map(&:to_i)
    check = digits.pop

    sum = digits.reverse.each_slice(2).map do |x, y|
        [(x * 2).divmod(10), y]
    end.flatten.inject(:+)

    (10 - sum % 10) == check
end

But for some reason I keep getting the following error message: nil can't be coerced into Fixnum

And for some reason I can't figure out why the error is being thrown. Any ideas why this might be happening?

pje · Answer 1 · 2012-10-31T20:02:25.070

2

Your method fails when digits has an odd number of elements. In that case, when you call each_slice(2) on the last iteration, x will be the last element of digits and y will be nil. So when you get to the inject(:+) phase, the last element of the array is nil, and your coercion error happens when the interpreter hits something like 2 + nil.

Why not add an initial check on the number of digits inputted? Something like:

return false unless digits.length == 16

edited Oct 31 '12 at 20:02

answered Oct 31 '12 at 19:54

pje

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ah, that makes sense! How would I go about fixing this? – dennismonsewicz Oct 31 '12 at 19:58
The problem with returning false if the digits don't equal 16 means that not all credit cards will pass the Luhn Algorithm (which is what the function above is supposed to do) – dennismonsewicz Nov 01 '12 at 13:58

Nil can't be coerced into Fixnum

1 Answers1