Ruby single line ternary in function, is returning boolean [duplicate]

Question

This question already has answers here:

Ternary expression with "defined?" returns "expression" instead of value? (3 answers)

Closed 8 years ago.

def fib(n)
  [0,1].include? n ? n : (fib(n-1) + fib(n-2))
end

fib 5 => false

Why is this? Since it works when broken out into standard if then else.

Answer 1

Operator precedence. You're effectively doing this:

[0, 1].include? (n ? n : fib...)

That is, the result of n ? n : (fib(n-1) + fib(n-2)) is found, and that is passed to include?.

Use parenthesis to force the order of evaluation you intend:

[0, 1].include?(n) ? n : (f(n - 1) + fib(n - 2))