Converting "c99" loop to regular stuff [closed]

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Story: I tried to convert a c99 script to regular gcc.

Problem: The output is empty.

Expected output: 3,2,1

length is the number of elements in the array.

Update: the script is designed to sort the elements of the array in a descending order.

The code:

#include <stdio.h>

int main() {

    int arr[] = { 1,2,3 };
    int temp = 0;
    int length = sizeof(arr) / sizeof(arr[0]);
    int i = 0;
    int j = i + 1;

    for (i < length; i++;) {
        for (j < length; j++;) {
            if (arr[i] < arr[j]) {
                temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
    }

    int y = 0;

    for (y < length; y++;) {
        printf("%d ", arr[y]);
    }

    return 0;
}

What does "c99 to regular stuff" mean? I don't see anything here that is even C99-specific. — jamesdlin
– jamesdlin, Commented Sep 22, 2019 at 6:09
Your for-loop syntax is wrong. You're using a condition where the initializer is expected and incrementing where the condition is expected. — jamesdlin
– jamesdlin, Commented Sep 22, 2019 at 6:12
More accurately, regarding your for-loop usage, the syntax is ok; the logic is wrong. This will compile, but will not do what you probably expect. At least three warnings about no-effect code should come from this. If that doesn't happen, you need to turn up your warnings and treat them as errors. — WhozCraig
– WhozCraig, Commented Sep 22, 2019 at 6:15
You loop for (y < length; y++;) is weird — you have a condition where you should have an initialization (or nothing — so the test does nothing at all) and the y++ is the tested condition and it fails on the first iteration because y is zero or false on the first iteration and it is a post-increment. All your code is legitimate under C90, let alone C99 or C11 or C18 (unless the automatic array initialization was not supported in C90 — I'd need to research that, and I'm too lazy to do so because it is almost 20 years irrelevant). — Jonathan Leffler
– Jonathan Leffler, Commented Sep 22, 2019 at 6:16
@JonathanLeffler Well, you're right about constants, but referring to arr[0] from within arr's initializer has fun issues: stackoverflow.com/a/52309196/1848654 — melpomene
– melpomene, Commented Sep 22, 2019 at 6:41

Michaël Roy · Accepted Answer · 2019-09-22 07:08:28Z

1

Your syntax for for loops is the issue.

Here is the correct way to write your loops.

int i, j;
for (i = 0; i < length; ++i)         // for (initialisation; test condition; operation)
{
    for (j = i + 1; j < length; ++j) // note that j is initialized with i + 1 on each iteration of 
                                     // the outer loop.  That's what makes the bubble sort work.
    {
         /* test and swap if needed */
    }
}

for (i = 0; i < length; ++i)  // note that i is reset to zero, so we can scan the array from 
                              // a known position (the top) to bottom.
{
    /* printout */
}

edited Sep 22, 2019 at 7:08

answered Sep 22, 2019 at 6:36

Michaël Roy

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Comments

Ian Newson · Accepted Answer · 2019-09-22 06:41:14Z

0

Your semicolon is in the wrong place, move it to the far left just inside the parentheses.

Loop syntax is:

for (intializer; break condition; iterator)

answered Sep 22, 2019 at 6:41

Ian Newson

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