binary search in a sorted 2d array using row / column pointers

July 3, 2023

I’m trying to build a function that uses binary search to find a number in a sorted 2d array and I’m given this function prototype that I can’t change:

int findNum(int array[N][M], int num,unsigned int* row, unsigned int* col);

I tried this:

int findNum(int array[N][M], int num,unsigned int* row, unsigned int* col){
    int low = 0 , mid , high = N*M - 1;

    while (low <= high)
    {
        mid = (low + high)/2;
        row = (mid / N);
        col = (mid % M);

        if (num < *(array + row + col)){
            high = mid - 1;
        }
        else if (num > *(array + row + col)){
            low = mid + 1;
        }
        else{
            return 1;
        }
    }
     
    return 0;
}

Apparently it’s wrong because pointer plus pointer is not allowed (*(array + row + col)). I can’t change the function prototype so I have to use the row / column pointers. I think it’s also wrong how I used row = (mid / N); and col = (mid % M); because on one side they are pointers and the other just integers.

How can I fix this?

>Solution :

row and col look like pointers to integers that should be populated with the position of the value being searched for, given the function finds the value.

Something like:

#include <stdio.h>

#define N 3
#define M 3

int findNum(int array[N][M], int num, unsigned int *row, unsigned int *col)
{
    int low = 0, mid, high = N * M - 1;

    while (low <= high) {
        mid = (low + high) / 2;
        *row = (mid / N);
        *col = (mid % M);

        if (num < array[*row][*col]) {
            high = mid - 1;
        } else if (num > array[*row][*col]) {
            low = mid + 1;
        } else {
            return 1;
        }
    }

    return 0;
}

int main(void)
{
    int data[N][M] = {
        { 1, 2, 3 },
        { 4, 5, 6 },
        { 7, 8, 9 }
    };

    unsigned int r, c;

    if (findNum(data, 4, &r, &c)) {
        printf("R: %u C: %u\n", r, c);
    } else
        puts("Not found.");
}

Output: