TIL Parity check with %

November 28, 2024 · One min read

Builder | Materializing ideas into reality.

The result of modulo operator (%) varies between programming languages especially when one of the operand is negative.

For example, here's what Python outputs:

>>> 29 % 2
1
>>> -29 % 2
1
>>> 29 % (-2)
-1

And here's the output from Java

jshell> 29 % 2
$1 ==> 1
jshell> -29 % 2
$2 ==> -1
jshell> 29 % (-2)
$3 ==> 1

Both give different results. Hence, to check if an integer is odd, it's better to define the function as

bool is_odd(int n) {
    return n % 2 != 0;
}

than

bool is_odd(int n) {
    // !!! incorrect for n < 0 !!!
    return n % 2 == 1; 
}