The code doesn't print the position of elements at the far left or right positions. I've looked at it for hours please help me find out whats wrong.

type here
#include<stdio.h>
#define max 50

void binarysearch(int l,int r,int key,int num[])
{
    int mid=(l r)/2;
    if(num[mid]==key)
    {
        printf("Element found at location %d",mid 1);return;
    }
    else if(num[mid]<key)
    {
        return binarysearch(l,mid-1,key,num);
    }
    else
    {
        return binarysearch(mid 1,r,key,num);
    }
    printf("Element not found ");return;
}

int main()
{
    int i,key,size,num[max];
    printf("Enter size of array : ");scanf("%d",&size);
    for(i=0;i<size;i  )
    {
        printf("Enter the element : ");scanf("%d",&num[i]);fflush(stdin);
    }
    printf("Enter key to search : ");scanf("%d",&key);
    binarysearch(0,size-1,key,num);
    return 0;
}

I'm gonna go insane. Send help

CodePudding user response：

For starters you need to check within the function whether r is not less than l. Without this check the function can invoke undefined behavior.

These if statements are incorrect

else if(num[mid]<key)
{
    return binarysearch(l,mid-1,key,num);
}
else
{
    return binarysearch(mid 1,r,key,num);
}

For example if num[mid] is less than key then you need to call the function recursively like

return binarysearch(mid 1, r, key,num);

otherwise like

return binarysearch(1, mid-1,key,num);

Pay attention to that the function should not output any message. It should returns to the caller either the index of the found element or for example -1 if the searched value is not found in the array. It is the caller of the function that will decide whether to output a message.

It would be much better to declare the function at least like

int binarysearch( const int num[], int size, int key );

or like

int * binarysearch( const int num[], size_t size, int key );

For the last declaration the function returns a pointer to the found element or NULL.

Here is a demonstration program that shows the function implemengtation.

#include <stdio.h>

int * binarysearch( const int a[], size_t n, int key )
{
    if (n == 0)
    {
        return NULL;
    }
    else if (a[n / 2] < key)
    {
        return ( int * )binarysearch( a   n / 2   1, n - n / 2 - 1, key );
    }
    else if ( key < a[n / 2] )
    {
        return ( int * )binarysearch( a, n / 2, key );
    }
    else
    {
        return ( int * )a   n / 2;
    }
}

int main( void )
{
    int a[] = { 1, 3, 5 };
    const size_t N = sizeof( a ) / sizeof( *a );

    for (int key = 0; key < a[N - 1]   2;   key)
    {
        int *p = binarysearch( a, N, key );

        if (p != NULL)
        {
            printf( "The value %d is found at position %td.\n", key, p - a );
        }
        else 
        {
            printf( "The value %d is not found.\n", key );
        }
    }
}

The program output is

The value 0 is not found.
The value 1 is found at position 0.
The value 2 is not found.
The value 3 is found at position 1.
The value 4 is not found.
The value 5 is found at position 2.
The value 6 is not found.

Pay also attention to that this call in your program

fflush(stdin);

has undefined behavior.

CodePudding user response：

You've simply wrote the code in opposite places.

The right code would be:

#include<stdio.h>
#define max 50

void binarysearch(int l,int r,int key,int num[])
{
    int mid=(l r)/2;
    if(num[mid]==key)
    {
        printf("Element found at location %d",mid 1);return;
    }
    else if(num[mid]<key)
    {
        // Wrong Code
        // you've done the opposite operation
        //return binarysearch(l,mid-1,key,num);

        // Correct Code
        return binarysearch(mid 1,r,key,num);
    }
    else
    {
        // Wrong Code
        // same problem as before.. you've done opposite operation
        //return binarysearch(mid 1,r,key,num);

        // Correct Code
        return binarysearch(l,mid-1,key,num);
    }
    printf("Element not found ");return;
}

int main()
{
    int i,key,size,num[max];
    printf("Enter size of array : ");scanf("%d",&size);
    for(i=0;i<size;i  )
    {
        printf("Enter the element : ");scanf("%d",&num[i]);fflush(stdin);
    }
    printf("Enter key to search : ");scanf("%d",&key);
    binarysearch(0,size-1,key,num);
    return 0;
}

Hope this would help.

CodePudding user response：

There are two problems:

1- Your arguments about the recursive called functions are wrong.

2- The recursive call is infinite.

You should change your binarysearch function to this:

void binarysearch(int l, int r, int key, int num[])
{
    int mid=(l r)/2;
    if(l > r)
    {
        printf("Element not found");
        return;
    }
    if(num[mid] == key)
    {
        printf("Element found at location %d", mid 1);
        return;
    }
    if(num[mid] < key)
    {
        return binarysearch(mid 1, r, key, num);
    }
    return binarysearch(l, mid-1, key, num);
}