How fast can you make linear search?

Question

I\'m looking to optimize this linear search:

static int
linear (const int *arr, int n, int key)
{
        int i = 0;
        while (i < n) {
                      


        

Answer 1

Since you can put known values after the last valid entry, add an extra element n+1 = max to make sure the loop doesn't go past the end of the array without having to test for i < n.

static int
linear (const int *arr, int n, int key)
{
        assert(arr[n] >= key);
        int i = 0;
        while (arr[i] < key) {
                ++i;
        }
        return i;
}

You could also try unrolling the loop, with the same sentinel value:

static int
linear (const int *arr, int n, int key)
{
        assert(arr[n] >= key);
        int i = 0;
        while (true) {
                if (arr [i++] >= key)
                        break;
                if (arr [i++] >= key)
                        break;
                if (arr [i++] >= key)
                        break;
                if (arr [i++] >= key)
                        break;
        }
        return --i;
}