Editing Modulo (section)

==Common pitfalls==
When the result of a modulo operation has the sign of the dividend (truncated definition), it can lead to surprising mistakes.

For example, to test if an integer is [[parity (mathematics)|odd]], one might be inclined to test if the remainder by 2 is equal to 1:

<syntaxhighlight lang="cpp">
bool is_odd(int n) {
    return n % 2 == 1;
}
</syntaxhighlight>

But in a language where modulo has the sign of the dividend, that is incorrect, because when {{math|''n''}} (the dividend) is negative and odd, {{math|''n''}} mod 2 returns −1, and the function returns false.

One correct alternative is to test that the remainder is not 0 (because remainder 0 is the same regardless of the signs):

<syntaxhighlight lang="cpp">
bool is_odd(int n) {
    return n % 2 != 0;
}
</syntaxhighlight>

Or with the binary arithmetic:
<syntaxhighlight lang="cpp">
bool is_odd(int n) {
    return n & 1;
}
</syntaxhighlight>