Bubble Function sorting in C on an array of integers

Question

I have coded out a Bubble Function that is supposed to Bubble sort an array of user input integers, but for some reason, my array is only working with arrays with size 6... otherwise the array outputs a zero for the largest number. Please run this code and help me identify the problem.

        #include <stdio.h>
        //prototype
        void bubble(int array[], int size);



        int main(void) 
        {
            int n,i,j,size,temp,counter = 0;
            //FOR SOME REASON, ONLY INPUT SIZE 6 WORKS PERFECTLY.
            int k;
            printf("How many numbers to sort?\n");
            scanf("%d",&size);
            int array[size];
            for(i=0;i<size;i++)
            {
                printf("Enter number %d\n",i+1);
                scanf("%d",&k);
                array[i] = k;
            }
            printf("Array before sorting:\n");

            for(i=0;i<size;i++)
            {
                printf("%d  ",array[i]);
            }

            bubble(array,size);

        return 0;
        }

        // use this if you want to test print an array
        // for(i=0;i<size;i++)
            // {
            //     printf("%d",array[i]);
            // }
        void bubble(int array[], int size)
        {
            int i, j, temp;
            for(j=0;j<size;j++)
            {  
                printf("\nIteration# %d\n",j+1);
                for(i=0;i<size;i++)
                {
                    if(array[i] > array[i+1])
                    {
                        temp = array[i];
                        array[i] = array[i+1];
                        array[i+1] = temp;
                    }
                    printf("%4d",array[i]);

                }

            }
        }

        // void select(int array[], int size)
        // {
        //     int i, j, temp;
        //     min = array[0];
        //     for(j=0;j<size;j++)
        //     {
        //         if(array[j] < min)
        //         {
        //             array[j] = temp;
        //             min = array[j];



        //         }


        //     }

        // }

Answer 1

Your inner-loop top-end conditional break is size, but within the loop you reference array[i+1], which means you're referring to array[size]. Since C arrays are zero-base indexed, the only allowable indexing is from 0...(size-1). Your code breaches that by one item repeatedly.

Changing the top-end of the inner loop to size-1 will work in your case. but there is arguably a better alternative that alleviates you from remembering the minus-1 in the first place. It involves modifying size as you sort to control the top-end of your inner loop directly. It also eliminates one local variable that you no longer need).

    void bubble(int array[], int size)
    {
        while (size-- > 0)
        {  
            for(int i=0; i<size; ++i)
            {
                if(array[i] > array[i+1])
                {
                    int temp = array[i];
                    array[i] = array[i+1];
                    array[i+1] = temp;
                }
            }
        }
    }

Often called a "goes-down-to" expression (because it looks like a long arrow pointing at a limiting value), the outer loop has been changed to become while (size-- > 0). This takes the current value of size to a temporary, decrements size, and compares the temporary against > 0 (more or less). The result is size now reflects the top limit of your inner loop that you want. Each enumeration of the outer loop will shrink the next inner loop pass by one. The point of bubble sort is that, once an element has been "bubbled up" to its proper position, you need not visit that element ever again, something your code is not taking advantage of.

---

Bonus Optimization

Finally, you can optimize this further and give your bubblesort the one and only redeeming quality the algorithm can offer: O(n) in best case where the sequence is already sorted. You do this by doing "swap-detection". If you ever pass over the inner loop without making a single swap, it makes no sense to perform anymore sorting. The sequence is sorted and you're done. It's a near-freebie addition to the original algorithm above, and looks like this:

void bubble(int array[], int size)
{
    int swapped = 1;
    while (swapped && size-- > 0)
    {  
        swapped = 0;
        for(int i=0; i<size; ++i)
        {
            if(array[i] > array[i+1])
            {
                int temp = array[i];
                array[i] = array[i+1];
                array[i+1] = temp;
                swapped = 1;
            }
        }
    }
}

Given an already sorted sequence of a ten, a hundred, or a hundred-thousand elements, this will finish after only one pass. Worth noting: even one element out of position on one extreme end will make this optimization irrelevant. I.e. if any element that belongs near the beginning is originally near the end, it will take up to size iterations to bring it home, and with that the optimization becomes moot. In short, this sequence

1 3 4 5... 9998 9999 2

will completely foil the optimization above. There are techniques to combat this as well including a two pass inner loop enumeration, where you ascend to bubble up larger values, then reverse direction to descend, bubbling down smaller values. but at this point you're better off using a finer algorithm like quicksort or heapsort. The discussion of that, and indeed the latter half of this post, is beyond the scope of your question.

Answer 2

i<size in combination with i+1 will go past the bounds of the array.

You should replace this:

for(i=0;i<size;i++)

with this:

for(i=0;i<size-1;i++)