Math expression that doesn't work with a parenthesis

Question

I'm tryng to convert a Fahrenheit temperature in correspective Celsius so I wrote:

double f = 90.0;
double c = (90.0-32)*(5/9); // 0.0

But this don't works, better fortune if I don't use the second set of parenthesis:

double c = (90.0-32)*5/9; // 32.2222

Why this (at least for me) strange behavior?

Answer 1

Don't use integers in division since 2/5 = 0 but 2.0/5.0 = 0.4

Answer 2

(90.0-32)*5/9 is evaluated as ((90.0-32)*5)/9, note that the division happens between a double and an int (which is then implicitly cast to double).

In (90.0-32)*(5/9) there is 5/9, which is zero.