Thread: Program not outputting to console

I need some help... My code works completely fine in visual studio. It also is able to compile with no errors on the school unix system. However, it does not output anything to the school's terminal...

I think it has to do with my sorting function, as when I comment it out, everything is outputted fine... Help please?

Code:

void sort(int* number, int n){
    ...

    // sort the first half
    for (int i = 0; i < split; i++) {
        for (int j = i + 1; j < split; j++) {
            if (firstHalf[i] > firstHalf[j]) {
                int tmp = firstHalf[i];
                firstHalf[i] = firstHalf[j];
                firstHalf[j] = tmp;
            }
        }
    }

    // sort the second half
    for (int i = 0; i < split; i++) {
        for (int j = i + 1; j < split; j++) {
            if (secHalf[i] > secHalf[j]) {
                int tmp = secHalf[i];
                secHalf[i] = secHalf[j];
                secHalf[j] = tmp;
            }
        }
    }

    // compare and combine
    int i = 0, j = 0, counter = 0;
    while (counter < n && (i <= split && j <= split)) {
        if (i == split) {
            while (j < split) {
                number[counter] = secHalf[j];
                j++;
                counter++;
            }
        }
        else if (j == split) {
            while (i < split) {
                number[counter] = firstHalf[i];
                i++;
                counter++;
            }
        }
        else if (firstHalf[i] < secHalf[j]) {
            // smaller value is from first half
            number[counter] = firstHalf[i];
            i++;
            counter++;
        }
        else if (secHalf[j] < firstHalf[i]) {
            // smaller value is from second half
            number[counter] = secHalf[j];
            j++;
            counter++;
        }
        else if (firstHalf[i] == secHalf[i]) {
            number[counter] = firstHalf[i];
            number[counter + 1] = secHalf[j];
            i++;
            j++;
            counter += 2;
        }
    }

    ...
}

int main(){

    ...

    /*It does not print out "Before sorting: ...". It will print it out if I use fflush(stdout)*/
    printf("Before sorting: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", number[i]);
    }
    
    /*It gets stuck here as it doesn't output the sorted results which is after this call*/
    sort(number, n);

    ...

    return 0;
}

What was the point of editing out various bits of code?
I mean, there are all sorts of ways of messing up declarations and initialisations.

Post the WHOLE thing.

I just thought those parts weren't important...

Anyways, here's the entire sort function. Reminder: it compiled fine in both visual studio and on the school unix environment. Just doesn't output anything in the school env

Code:

void sort(int* number, int n){
    int split = n / 2;
    int* firstHalf = (int*) malloc(split * sizeof(int));
    int* secHalf = (int*) malloc(split * sizeof(int));

    for (int i = 0; i < split; i++) {
          firstHalf[i] = number[i];
    }

    for (int i = 0, j = split; j < n; i++, j++) {
         secHalf[i] = number[j];
    }

    // sort the first half
    for (int i = 0; i < split; i++) {

        for (int j = i + 1; j < split; j++) {
            if (firstHalf[i] > firstHalf[j]) {
                int tmp = firstHalf[i];
                firstHalf[i] = firstHalf[j];
                firstHalf[j] = tmp;
            }
        }
    }
 
    // sort the second half
    for (int i = 0; i < split; i++) {
        for (int j = i + 1; j < split; j++) {
            if (secHalf[i] > secHalf[j]) {
                int tmp = secHalf[i];
                secHalf[i] = secHalf[j];
                secHalf[j] = tmp;
            }
        }
    }
 
    // compare and combine
    int i = 0, j = 0, counter = 0;
    while (counter < n && (i <= split && j <= split)) {
        if (i == split) {
            while (j < split) {
                number[counter] = secHalf[j];
                j++;
                counter++;
            }
        }
        else if (j == split) {
            while (i < split) {
                number[counter] = firstHalf[i];
                i++;
                counter++;
            }
        }
        else if (firstHalf[i] < secHalf[j]) {
            // smaller value is from first half
            number[counter] = firstHalf[i];
            i++;
            counter++;
        }
        else if (secHalf[j] < firstHalf[i]) {
            // smaller value is from second half
            number[counter] = secHalf[j];
            j++;
            counter++;
        }
        else if (firstHalf[i] == secHalf[i]) {
            number[counter] = firstHalf[i];
            number[counter + 1] = secHalf[j];
            i++;
            j++;
            counter += 2;
        }
    }
 
    free(firstHalf);
    free(secHalf);
    firstHalf = NULL;
    secHalf = NULL;

}

As for the main function

Code:

int main(){
    int n = 20;

    int* number = (int*) malloc(n * sizeof(int));

    for (int i = 0; i < n; i++) {
        number[i] = (rand() % (20 - 0));
    }

   /*It does not print out "Before sorting: ...". It will print it out if I use fflush(stdout)*/
    printf("Before sorting: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", number[i]);
    }

   /*It gets stuck here as it doesn't output the sorted results which is after this call*/
    sort(number, n);

    printf("\nAfter sorting: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", number[i]);
    }

    printf("\n");

    return 0;
}

A debug session

Code:

(gdb) print *number@20
$3 = {0, 1, 2, 15, 13, 15, 6, 12, 9, 1, 2, 7, 10, 19, 3, 6, 0, 6, 12, 16}
(gdb) print *firstHalf@10
$4 = {1, 3, 6, 6, 9, 12, 13, 15, 15, 17}
(gdb) print *secHalf@10
$5 = {0, 2, 3, 6, 6, 7, 10, 12, 16, 19}
(gdb) print i
$6 = 1
(gdb) print j
$7 = 2
(gdb) n
43              if (i == split) {
(gdb) 
50              else if (j == split) {
(gdb) 
57              else if (firstHalf[i] < secHalf[j]) {
(gdb) 
63              else if (secHalf[j] < firstHalf[i]) {
(gdb) 
69              else if (firstHalf[i] == secHalf[i]) {
(gdb) 
42          while (counter < n && (i <= split && j <= split)) {
(gdb) 
43              if (i == split) {
(gdb) 
50              else if (j == split) {
(gdb) 
57              else if (firstHalf[i] < secHalf[j]) {
(gdb) 
63              else if (secHalf[j] < firstHalf[i]) {
(gdb) 
69              else if (firstHalf[i] == secHalf[i]) {
(gdb) 
42          while (counter < n && (i <= split && j <= split)) {

The code just locks up in an infinite loop.



> My code works completely fine in visual studio
No, it just didn't run into this particular bug.
Beware of declaring early victory in future.

Which condition caused it to lock up in an infinite loop? Is it that while loop?

One problem that you have in debugging is that you use rand() to generate the input. This is fine when your code works, but having a different input each time means that you cannot say, take the input and try out your implementation manually to see where it went wrong.

So, temporarily comment out the part that generates input and instead say, copy the input from Salem's post #5. Run your program with this input using the debugger and see what is the problem. If you don't quite get it, write down the input and sort it "by hand", following the code of your sort function line by line.

EDIT:
There is an alternative in that you could call srand() with a fixed seed, and looking at your code perhaps that is what you are doing (if you don't call srand it will be as if you called srand(1)), but because the implementation of srand and rand varies between standard library implementations, it would be better to use explicitly fixed input so that you test with the same input everywhere.

> Which condition caused it to lock up in an infinite loop? Is it that while loop?
You have 5 conditions.
When it locks up, all 5 conditions evaluate to false, so nothing actually happens to get the program out of that state.

You might try adding a final

Code:

else {
    printf("Huh? how did this happen?\n");
}

FWIW, you have a typo - but I wasn't going to tell you exactly where it is in your code.
That's for you to still stare at for a while.
So you get the full impact of the "D'oh!!!!" moment when you finally realise it.

There are enough clues in my debug log to help you figure it out.
The rest is up to you.

I think I found that typo Salem. It's in the last "else if" right? Gahh, it helped having a constant set of integers. As the ones that were generated in the ide worked but not the ones generated on the school environment.

Thanks for the tip you two!