Thread: Why does parenthesis around a fraction (5/9) break my program? Beginner!

Originally Posted by Cat

To elaborate, it's because of order of operations, and the fact that, when an expression has mixed integers and floating-point operands, the integers are promoted into floats to do the math.

Code:

(fDegrees-32)*5/9;

In this case, fDegrees is a float, so 32 is converted to 32.0 and the result is a float. Next, the float is multiplied by 5, so 5 is promoted to a float, too. Finally, 9 is promoted to a float as floating-point division is done.

Code:

(fDegrees-32)*(5/9);

Here fDegrees-32 is still a float, but the parentheses mean that 5/9 is evaluated before the multiplication. Since 5 and 9 are integer literals, it does integer division, and 5/9 = 0 in integer division.

The simplest way to fix this is to use floating-point literals such as (5.0f/9.0f)

You also likely want atof() not atoi() as you're trying to parse floating-point values, not integers.

I was going to point out he was using the wrong formula and that he should also be using atof but you beat me to it. For the OP though, here are some other formulas you can use as well.

C = (F - 32) * 5 / 9 (this is the one you're trying to use)
C = (F - 32) / (9 / 5) (notice how this one requires the parenthesis around the 9/5. The division of 9/5 must get divided into whatever F-32 is. We don't want F-32 getting divided by just 9).
C = (F - 32) / 1.8