Binary search with pointers in C

Question

I have been asked to write a binary search program with some specific conditions. I have to pass the lower bound, upper bound, pointer to the array, and the search element. the program I have written gave me warnings. I am not able to correct the error in the program. Please point me where I am wrong.

#include <stdio.h>
int BinarySearch(int , int , int *, int );

int main()
{
    int n, i, a[20], h, l, x, r=0;
    int *p;

    printf("Enter the number of elements:\n");
    scanf("%d",&n);

    printf("Enter the elements:\n");
    for( i=0 ; i<n ; i++)
    {
        scanf("%d",&a[i]);
    }

    p = &a[0];
    printf("Enter the element to be searched:\n");
    scanf("%d", &x);

    l = 0;
    h = n-1;

    r = BinarySearch(l, h, p, x);

    if(r == 1)
    printf("The element %d is found in position %d", x, i);
    else
    printf("The element %d is not present in the array", x);

    return 0;
}

int BinarySearch(int l, int h, int *p, int x)
{
    int mid, a[20], f =0;
    *p = a[0];
    mid = ( l + h )/2;

    while( l <= h )
    {
        if( a[mid] == x )
        {
            f=1;
            break;
        }
        else if( a[mid] > x )
        {
            h = mid-1;
        }
        else if( a[mid] < x )
        {
            l = mid+1;
        }
    }
    if(f == 1)
    {return 1;}
    else
    {return -1;}
}

I get this warning when I compile this.

main.c|38|warning: 'a[0]' is used uninitialized in this function [-Wuninitialized]|

when I remove the line *p = a[0]; , then I get the following error messages:

main.c|43|warning: 'a[mid]' may be used uninitialized in this function [-Wmaybe-uninitialized]|
main.c|48|warning: 'a[mid]' may be used uninitialized in this function [-Wmaybe-uninitialized]|
main.c|52|warning: 'a[mid]' may be used uninitialized in this function [-Wmaybe-uninitialized]|

when I run the program, the program takes the value of the search element and terminates after some time. the control is not transferred to the binary search function.

Answer 1

Let's think how your int BinarySearch(int l, int h, int *p, int x) should work:

1. l,h are the bounds
2. p is pointer to data (the data is a[20] declared in main: int n, i, a[20], h, l, x, r=0; )
3. x is the value to check against

If p is the pointer to your data ,there is no need (and it's actually wrong, because it will not search the data you want) in declaring a new a[20] inside the function, nor in *p = a[0]; , since p already points to the right address... Remove those, and use if( p[mid] == x ) instead of if( a[mid] == x )

Answer 2

The original array is defined inside main() and a separate array is defined in BinarySearch(). The pointer p is assigned to point to array defined in main.

p = &a[0];

The pointer p is passed in BinarySearch() and then the assignment

*p = a[0]

is infact changing the pointer contents to first element of new array defined in BinarySearch(). The new array is uninitialized and hence the appropriate compiler warnings.

Answer 3

Both previous answers seem valid, but the quick correction to the problem would be to revise:

int mid, a[20], f =0;
*p = a[0];

to

int mid, f =0;

Then update all references to a in function BinarySearch to p.

There is no need to allocate an entire array a when you are simply reading data from the array pointed to by p.