Why is this Binary search program giving an error?

Question

I wrote the following program for conducting Binary search on an array when the elements are entered in ascending order.

#include<stdio.h>
#include<conio.h>

void main()
{
    int key,high,low,mid,n,i,a[100];

    clrscr();

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

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

    printf("Enter the key element:");
    scanf("%d",&key);

    low=0;
    mid=n-1;
    mid=(low+high)/2;
    while(low <= high && key != a[mid])
    {
        mid=(low+high)/2;
        if (key > a[mid])
            low=mid+1;
        else
            high=mid-1;
    }

    if (a[mid] == key)
        printf("\nKey element is present at position %d",mid+1);
    else
        printf("\nElement not present.");
    getch();
}

When I ran the compiled program in Dev C++, I got a 'Memory could not be read' error. How can this be corrected?

Sergey Kalinichenko · Accepted Answer · 2011-12-07 16:20:48Z

4

You are assigning mid=n-1; instead of high=n-1;.

answered Dec 7, 2011 at 16:20

Sergey Kalinichenko

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Comments

Luchian Grigore · Accepted Answer · 2011-12-07 16:19:46Z

1

You failed to initialize high, so you're using some garbage in mid=(low+high)/2;

You probably want to initialize it along low:

low=0;
high=/*whatever*/;

answered Dec 7, 2011 at 16:19

Luchian Grigore

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Comments

unwind · Accepted Answer · 2011-12-07 16:21:42Z

0

This:

 mid=n-1;
 mid=(low+high)/2;

should probably be:

 high = n-1;
 mid = (low+high)/2;

Notice how you mistakenly write to mid twice, instead of initializing high. Also notice that a proper binary search is quite hard to get right, there are nasty overflow conditions for large arrays.

answered Dec 7, 2011 at 16:21

unwind

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Comments

Sled · Accepted Answer · 2011-12-07 16:56:00Z

0

Typo, just replace mid=n-1; with high=n-1;.

edited Dec 7, 2011 at 16:56

Sled

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answered Dec 7, 2011 at 16:30

hago

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Comments

user3541302 · Accepted Answer · 2014-04-22 18:44:53Z

0

The problem lies on the statement:

mid=n-1;

This should be changed to:

high=n-1;

Because you are just including high variable which is not initialized. This also would result in you using garbage in the rest of your code. Mid=n-1 would lead to a logical error.

Always make sure that you initialize the variables.

answered Apr 22, 2014 at 18:44

user3541302

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