Friday, 20 April 2012

Modulus for previous array index

In JavaScript, C and C++, x % y will return a negative number if x is negative. This isn't what you want if you're trying to wrap around an array index (for example). Fortunately there's a nice solution. Instead of writing something like this:

  int prev(int x, int N) {
    int prevX = (x - 1) % N;
    if (prevX < 0)
      prevX = N - 1;
    return prevX;
you can simply write:
  int prev(int x, int N) {
    return (x + N - 1) % N;

and let the magic of modular arithmetic do its work. This works for unsigned integers too.

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