在程序中得到错误的输出

Question

I recently wrote a program, which gives to a wrong output and I dont have the slightest idea why.

This program checks the following: given some 'k' (value), and two arrays A and B, checking if there are some ' x ' which belongs to array A and ' y ' which belongs to B , so that xk=y .

Here is my code:

        #define _CRT_SECURE_NO_WARNINGS
        #include<stdio.h>
        #include<stdlib.h>

        int* buildArray(int size);
        int partition(int arr[], int low, int high);
        void quickSort(int *arr, int low, int high);
        int findValuesForDifference(int* A, int n, int* B, int m, int k);

        void main()
        {
            int n, m, k;
            int *A, *B;
            printf("please enter a number for the size of array A : ");
            scanf("%d", &n);
            printf("\nenter %d numbers for array A: ", n);
            A = buildArray(n);
            printf("please enter a number for the size of array B : ");
            scanf("%d", &m);
            printf("\nenter %d numbers for array A: ", m);
            B = buildArray(m);
            printf("\nplease enter a number for k: ");
            scanf("%d", &k);
            if (findValuesForDifference(A, n, B, m, k))
                printf("\nthere are some x which belongs to A and y which belongs to B such that x-y=k\n");
            else
                printf("\nthere are not any x which belongs to A and y which belongs to B such that x-y=k\n");

            free(B);
            free(A);
        }

        int findValuesForDifference(int* A, int n, int* B, int m, int k)
        {
            int low = 0, high = n - 1, middle, i;

            quickSort(A, low, high);

    /*using binary search sorted Array A, for each element of array B*/
            for (i = 0; i < m; i++)
            {
                while (low <= high)
                {
                    middle = (low + high) / 2;
                    if (k + B[i] == A[middle])
                        return 1;
                    else if (k + B[i] < A[middle])
                        high = middle - 1;
                    else
                        low = middle + 1;
                }
            }
            return 0;
        }

            int partition(int arr[], int low, int high)
            {
                int pivot = arr[high], i = (low - 1), j;

                for (j = low; j <= high - 1; j++)
                {

                    if (arr[j] <= pivot)
                    {
                        i++;    
                        swap(&arr[i], &arr[j]);
                    }
                }
                swap(&arr[i + 1], &arr[high]);
                return (i + 1);
            }

            void quickSort(int* arr, int low, int high)
            {
                int pivot;

                if (low < high)
                {
                    pivot = partition(arr, low, high);

                    quickSort(arr, low, pivot - 1);
                    quickSort(arr, pivot + 1, high);
                }
            }

           int* buildArray(int size)

           { int i; 
             int* arr = (int*)malloc(size * sizeof(int));       
             if (!arr) 
                 { printf("ERROR! Not enough memory!\n");  
                   exit(1); 
                 } 
             for (i = 0; i < size; i++) 
                  scanf("%d", &arr[i]); return arr; 
            }

For array A with size n=4 and the elements : 14 2 12 2 ,and array B with the size m=6 and the elements : 25 11 2 25 17 8 , and k=3 , I get the following wrong output
there are not any x which belongs to A and y which belongs to B such that xy=k ,

while the expected output is there are some x which belongs to A and y which belongs to B such that xy=k , because - for instance, there are 14 which belongs to A and 11 which belongs to B so that 14-11=3.

Answer 1

In your findValuesForDifference() function, you only set the values of low and high once, when defining these variables.

You need to reset their values at every iteration of the main loop, otherwise your binary search will only work once:

int findValuesForDifference(int *A, int n, int *B, int m, int k)
{
    int low, high, middle, i;

    quickSort(A, low, high);

    /* using binary search sorted Array A, for each element of array B */
    for (i = 0; i < m; i++) {
        low = 0;
        high = n - 1
        while (low <= high) {
            middle = (low + high) / 2;
            if (k + B[i] == A[middle])
                return 1;
            else if (k + B[i] < A[middle])
                high = middle - 1;
            else
                low = middle + 1;
        }
    }
    return 0;
}