Same input different output in C

Question

I wrote a C program in Clion, using dynamic memory allocation. I noticed sometimes gave different output for the same input. At first I tought there was a problem with Clion (I was using it for the first time), but I also ran the program from the windows command line, and the same bug occured.

main.c

#include "main.h"

int main()
{
    int n;

    scanf("%d", &n);

    int *t = (int*)calloc(n, sizeof(int));

    for (int i = 0; i < n; ++i) {
        scanf("%d", &t[i]);
    }

    int p = peak(t, n);

    printf( p == -1 ? "No peak" : "Peak %d, position %d", t[p], p );

    free(t);

    return 0;
}

main.h

//
// Created by Botond on 2021. 02. 18..
//

#ifndef EXTRA_MAIN_H
#define EXTRA_MAIN_H

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

int peak( int *t, int n)
{
    int p = -1, i = 0;

    while(t[i] <= t[i+1])
        i++;

    p = i;

    while(t[i] >= t[i+1])
        i++;

    if(i == n + 1)
        return p;
    else
        return -1;
}

#endif //EXTRA_MAIN_H

Your peak function has no checks to avoid running off the end of the array. — Nate Eldredge
– Nate Eldredge, Commented Feb 19, 2021 at 22:22
Also you should generally not define functions in .h files. The .h file should only contain the declaration int peak(int *, int); and the code itself should go in a separate .c file which also includes the .h file, and which you compile separately and link together. — Nate Eldredge
– Nate Eldredge, Commented Feb 19, 2021 at 22:25
Regarding .c and .h files, stackoverflow.com/questions/5904530/… might be good reading. — Nate Eldredge
– Nate Eldredge, Commented Feb 19, 2021 at 22:30
Accessing outside the array bounds causes undefined behavior. That's why you can get different results. — Barmar
– Barmar, Commented Feb 19, 2021 at 22:34
see also: c - Do I cast the result of malloc? - Stack Overflow — MikeCAT
– MikeCAT, Commented Feb 19, 2021 at 22:48

Nate Eldredge · Accepted Answer · 2021-02-19 23:03:57Z

2

When the array passed to peak is {5,5,4,3,2,1}:

    while(t[i] <= t[i+1])
        i++;

will continue until i == 1. Then

    while(t[i] >= t[i+1])
        i++;

will still be true when i == 4, so the loop will iterate again and the test becomes t[5] >= t[6]. Since the array only has 6 elements (t[0]..t[5]), the access to t[6] causes undefined behavior. In particular, it could read whatever garbage value happens to be next in memory, so that your program behaves as if the input contained additional garbage values that might be different from one run to the next. The program could also crash or cause other sorts of problems.

You need an extra condition in these loops that checks i against n, so that they don't access t[n] and instead terminate when i+1 >= n.

answered Feb 19, 2021 at 23:03

Nate Eldredge

62.4k7 gold badges78 silver badges121 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Same input different output in C

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related