CODEWITHSUNDEEP
arrayscoutput
Mike
Mike

Reputation: 31

Same input different output in C

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

different output for the same input from command line

Upvotes: 0

Views: 540

Answers (1)

Nate Eldredge
Nate Eldredge

Reputation: 57922

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.

Upvotes: 2

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