c - When testing my bubblesort algorithm, the console closes after a second

Question

I checked and there are no syntax errors. I am using Code::Blocks for coding in C and this is the first time that I encounter a similar error. When I run the code, the console starts and closes after a second. It doesn't even print what is inside the printf.

#include <stdio.h>

int main(void)
{
    int n;
    int vet[n];
    int t;
    int i;
    int lim;
    int lastswap;
    int swap;

    printf("Array lenght:");
    scanf("%d
", &n);
    for (i = 0; i < n; i++)
    {
        scanf("%d
", &vet[i]);
    }
    lim = 0;
    do
    {
        swap = 0;
        for (i = n - 1; i > lim; i--)
        {
            if (vet[i] < vet[i - 1])
            {
                t = vet[i];
                vet[i] = vet[i - 1];
                vet[i - 1] = t;
                swap = 1;
                lastswap = i;
            }
            lim = lastswap;
        }

    } while (swap != 0);
    for (i = 0; i < n; i++)
    {
        printf("%d
", vet[i]);
    }
}

question from:https://stackoverflow.com/questions/65870222/when-testing-my-bubblesort-algorithm-the-console-closes-after-a-second

Answer 1

Main issue

The value of n is undefined when it is being used to specify the size of array vet. This is an error and a C compiler should stop you from doing this (even if it's configured to C99 or C11).^[1][2]

Dynamic array (on the heap)

The preferred approach to create an array of a size that is determined at runtime is to allocate memory on the heap.^[2] For this, these steps should be taken in the correct order:

• declare n
• input n (also validate input: check scanf result, also if the input has a reasonable size)
• allocate vet (as an int pointer via malloc)

... before you write to the array. BTW: Don't forget to free the memory after having used it.

So something like this could be your basic "framework" (change UPPER_LIMIT to your needs):

#define UPPER_LIMIT 1024 /* or enum { UPPER_LIMIT=1024 }; */

int n;
if (scanf("%d", &n) == 1) {
    if (0 < n && n < UPPER_LIMIT) {
        int* vet = malloc(n*sizeof *vet);
        if (vet) {

            /* work on vet */
        }
        free(vet);
    }
}

Fixed-size array

Another option is to use an array of fixed dimension, then you have to check the n from input to be within the range of the fixed array:

#define UPPER_LIMIT 1024 /* or enum { UPPER_LIMIT=1024 }; */

int n;
int vet[UPPER_LIMIT];
if (scanf("%d", &n) == 1) {
    if (0 < n && n < UPPER_LIMIT) {

        /* work on vet */
    }
}

Secondary issue

To use newlines in the format string of scanf causes issues discussed in Behaviour of scanf when newline is in the format string. So better switch to scanf("%d", &n); and scanf("%d", &vet[i]);. BTW: Don't forget to check the return value of scanf and the input.

Footnotes

^[1] It's not clear why GDB online Debugger accepts this as valid code but this could be the topic of another question.

^[2] C99 supports variable-length arrays (VLA), that is a way to declare arrays with a size that is determined at runtime. For this the size variable must be defined before the array declaration with a value greater than zero. (read more in Array declaration, section Variable-length arrays). C11 incorporates variable-size arrays as a implementation defined feature - that means that standard-compliant C99 code may not be portable. There is also no consensus about the actual benefit of VLAs see for example Difficulty in understanding variable-length arrays in C

Variable-size array C99(+):

#define UPPER_LIMIT 1024 /* or enum { UPPER_LIMIT=1024 }; */

int n;
if (scanf("%d", &n) == 1) {
    if (0 < n && n < UPPER_LIMIT) {
        int vet[n];

        /* work on vet */
    }
}