Question

I’m running through the problems on Project Euler to teach myself Ruby programming. I know there is a built-in function to do this, but I’m avoiding the built-in functions to help me learn.

So I have to write a method to determine if a number is a prime. The first method works, but the second doesn’t. Can anyone explain why?

 def is_prime n
  for d in 2..(n - 1)
   if (n % d) == 0
    return false
   end
  end

  true
 end

 def is_prime2 n
  foundDivider = false
   for d in 2..(n - 1)
    foundDivider = ((n % d) == 0) or foundDivider
   end
  not foundDivider
 end

Answer 1

It’s because = is of higher precedence than or. See Ruby’s operator precedence table below (highest to lowest precedence):

[ ] [ ]=
**
! ~ + -
* / %
+ -
>> <<
&
^ |
<= < > >=
<=> == === != =~ !~
&&
||
.. ...
? :
= %= { /= -= += |= &= >>= <<= *= &&= ||= **=
defined?
not
or and
if unless while until
begin/end

The problematic line is being parsed as…

(foundDivider = ((n % d) == 0)) or foundDivider

…which is certainly not what you mean. There are two possible solutions:

Force the precedence to be what you really mean…

foundDivider = (((n % d) == 0) or foundDivider)

…or use the || operator instead, which has higher precedence than =:

foundDivider = ((n % d) == 0) || foundDivider