Wrapping Array Indices

Objective

Keep an array index value within the correct range while incrementing and decrementing.

(My) naive approach

int count = 10;
int idx = 0;

// increment
idx = idx == count-1 ? 0 : idx+1;

// decrement
idx = idx == 0 ? count-1 : idx-1;

This works well enough, but there is a better solution.

Better approach

Use modulo¹

int count = 10;
int idx = 0;

// increment
++idx;
idx = (idx % count + count) % count;

// decrement
--idx;
idx = (idx % count + count) % count;

This approach is more robust (works for increments greater than 1), and doesn’t require branching.