Thread: Has anyone seen anything like this?

I was practicing writing this code and couldn't figure out why it wasn't sorting the ID's. I compared it to what I had written before and saved to a text file and couldn't find any difference, except for the numbers I was using for the ID's. I tried using the numbers that I had used previously, and it worked. It doesn't make any sense. This is the set of ID's that work: 313, 453, 502, 101, 892, 475, 792, 912, 343, 633. I'm using MinGW btw.

Code:

#include <stdio.h>
#include <stdlib.h>
int main (void) {

    int ctr, tempID, left, right, didSwap;
    float tempBal;

// It won't sort these ID's properly, but when I swap these ID's with the ones above, it sorts those properly.
    int custID[10] = {123, 456, 789, 987, 654, 321, 741, 852, 963, 147}; 

    float custBal[10] = {0.00, 98.74, 102.63, 54.26, 326.89, 0.00, 19.95, 220.63, 56.66, 150.55};
    
    for (left = 0; left < 9; left++) {
        didSwap = 0;
        for (right = left; right < 10; right++) {
            if (custID[right] < custID[left]) {
                tempID = custID[right];
                tempBal = custBal[right];
                custID[right] = custID[left];
                custBal[right] = custBal[left];
                custID[left] = tempID;
                custBal[left] = tempBal;
                didSwap = 1;
            }
        }
        if (didSwap == 0) {
            break;
        }
    }
    for (ctr = 0; ctr < 10; ctr++) {
        printf("%d, ", custID[ctr]);
    }
    return 0;
}

Which sort algorithm were you trying to implement? Looking at your code, you are basically checking to see that for the number in each position, none of the numbers to its right is less than it, otherwise you swap them. Doing it this way, you cannot break when there are no swaps because you still have to check the remaining positions. Why this set of input reveals the bug is that the very first element in the array is the smallest, hence none of the numbers to its right are less than it, thus there are no swaps on the very first iteration.

Perhaps you were thinking of implementing a variant of bubble sort instead, in which case you can break when there are no swaps because no swaps implies that all the elements have been "bubbled" into place. Or, perhaps you were thinking of implementing selection sort, where you find say, the least element in the unsorted portion and do just one swap with the element in the current position on each iteration.