Thread: Program working fine until I make one change..plz help

Hello everyone,

This is a program which asks the user 10 times to give a rating to a television show on a scale of 1 to 10(inclusive). Am posting the before and after code. There is no problem with the before code.

Before code

Code:

#include <stdio.h>

int main ()

{
    int ask, i, ratingcounter[i], response, display ;

    for (i = 1; i <= 10; i++)
    ratingcounter[i] = 0 ;

    printf ("Rate the television show on a scale of 1 to 10\n\n") ;

    for (ask = 1; ask <= 10; ask++)
    {
        scanf ("%i", &response) ;

        printf ("\n") ;

        if (response < 1 || response > 10)
        printf ("Bad response\n\n") ;


        else
        ratingcounter[response]++ ;

    }

    printf ("Rating        Number of responses\n") ;
    printf ("------        -------------------\n\n") ;

    for (display = 1; display <= 10; display++)
    printf ("  %i                   %i\n\n", display, ratingcounter[display]) ;


    return 0 ;

}

In the after code, I've added a new variable which increases the bad responses.
I've made the changes that I've made in bold for easier view.

After code

Code:

#include <stdio.h>

int main ()

{
    int ask, i, ratingcounter[i], response, display, br = 0 ;

    for (i = 1; i <= 10; i++)
    ratingcounter[i] = 0 ;

    printf ("Rate the television show on a scale of 1 to 10\n\n") ;

    for (ask = 1; ask <= 10; ask++)
    {
        scanf ("%i", &response) ;

        printf ("\n") ;

        if (response < 1 || response > 10)

        {
            printf ("Bad response\n\n") ;
            br++ ;
        }



        else
        ratingcounter[response]++ ;

    }

    printf ("Rating        Number of responses\n") ;
    printf ("------        -------------------\n\n") ;

    for (display = 1; display <= 10; display++)
    printf ("  %i                   %i\n\n", display, ratingcounter[display]) ;

    printf ("\nTotal bad responses = %i\n\n", br) ;

    return 0 ;

}

I've attached the error that I get after I compile/run the after code.
What is wrong here, please help.

Contrary to your statement that the "before" code has no problem, both your "before" and "after" code exhibit exactly the same fault.

It is in this line (in your "before" code), specifically the bits I have highlighted in red.

Code:

      int ask, i, ratingcounter[i], response, display ;

i is uninitialized, so accessing its value here (as a dimension of ratingcounter) gives undefined behaviour. Changing i later on, or accessing some element of ratingcounter, does not magically cause the number of elements in ratingcounter to change, and accessing any element of the array ratingcounter also gives undefined behaviour.

In both the "before" and "after" code, some random area of memory is being trashed. The only difference is that your "before" code exhibits no symptom but (thanks to the addition of the variable br) memory used by "after" code is laid out differently, and the random area of memory being trashed causes a symptom that the operating system detects, so the operating system terminates your program with prejudice.

Thanks grumpy that solved the problem and you explained the "under the hood" part very well

Solution: ratingcounter[11] instead of ratingcounter[i]