TheAlgorithms-C/searching/interpolation_search.c

#include<stdio.h>

/* By comparison, binary search always chooses the middle of the remaining
 * search space, discarding one half or the other, depending on the comparison
 * between the key found at the estimated position and the key sought. The remaining
 * search space is reduced to the part before or after the estimated position.
 * The linear search uses equality only as it compares elements one-by-one from the start, ignoring any sorting.
 * On average the interpolation search makes about log(log(n)) comparisons (if the elements
 * are uniformly distributed), where n is the number of elements to be searched. In the worst case
 * (for instance where the numerical values of the keys increase exponentially) it can make up to O(n) comparisons.
 * In interpolation-sequential search, interpolation is used to find an item near the one being searched for,
 * then linear search is used to find the exact item. */

int interpolationSearch(int arr[], int n, int key)
{
    int low = 0, high = n - 1;
    while (low <= high && key >= arr[low] && key <= arr[high]) {
        /* Calculate the nearest posible position of key */
        int pos = low + ((key - arr[low]) * (high - low)) / (arr[high] - arr[low]);
        if (key > arr[pos])
            low = pos + 1;
        else if (key < arr[pos])
            high = pos - 1;
        else /* Found */
            return pos;
    }
    /* Not found */
    return -1;
}


int main()
{
    int x;
    int arr[] =  {10, 12, 13, 16, 18, 19, 20, 21, 22, 23,
                  24, 33, 35, 42, 47};
    int n = sizeof(arr)/sizeof(arr[0]);

    printf("Array: ");
    for(int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    printf("\nEnter the number to be searched: ");
    scanf("%d",&x); /* Element to be searched */

    int index = interpolationSearch(arr, n, x);

    /* If element was found */
    if (index != -1)
        printf("Element found at position: %d\n", index);
    else
        printf("Element not found.\n");
    return 0;
}