Mod of negative number is melting my brain
c#, math, modulo
Solution
I always use my own `mod` function, defined as
int mod(int x, int m) {
return (x%m + m)%m;
}
Of course, if you're bothered about having two calls to the modulus operation, you could write it as
int mod(int x, int m) {
int r = x%m;
return r<0 ? r+m : r;
}
or variants thereof.
The reason it works is that "x%m" is always in the range [-m+1, m-1]. So if at all it is negative, adding m to it will put it in the positive range without changing its value modulo m.
Problem
I'm trying to mod an integer to get an array position so that it will loop round. Doing `i % arrayLength` works fine for positive numbers but for negative numbers it all goes wrong. ``` 4 % 3 == 1 3 % 3 == 0 2 % 3 == 2 1 % 3 == 1 0 % 3 == 0 -1 % 3 == -1 -2 % 3 == -2 -3 % 3 == 0 -4 % 3 == -1 ``` so i need an implementation of ``` int GetArrayIndex(int i, int arrayLength) ``` such that ``` GetArrayIndex( 4, 3) == 1 GetArrayIndex( 3, 3) == 0 GetArrayIndex( 2, 3) == 2 GetArrayIndex( 1, 3) == 1 GetArrayIndex( 0, 3) == 0 GetArrayIndex(-1, 3) == 2 GetArrayIndex(-2, 3) == 1 GetArrayIndex(-3, 3) == 0 GetArrayIndex(-4, 3) == 2 ``` I've done this before but for some reason it's melting my brain today :(