Question

I am implementing the standard bubble sort algorithm, and I had a question on pointers.

float *SortValues(float *p, size_t n)
{
    float temp;
    float didSwap;
    float *current;
    float *last = &p[n - 1];
    float *start = p;

    do 
    {
        for (didSwap = 0, current = p; current < last; current++) {
            if (current[0] > current[1]) {
                temp = current[0];
                            current[0] = current[1];
                current[1] = temp;
                didSwap = 1;
            }
        }
        --last;
    }

    while (didSwap);
    return start;
}

I get confused a lot of times using other pointers to point to the start and/or end of the pointer passed in. In something like the code above, I set current to point to p, and start to point to p. The current changes throughout the loop. Since current and p point to the same thing, how does p, and therefore start end up changing to pointing to the same thing as current?

Answer 1

Where start is pointing doesn’t change. What start is pointing at does.

Imagine you have five cups and you put a ball in the green one. Then you tell me to replace the ball in the green cup with a dollar bill. Next time you look in the green cup, it will contain a dollar, not the ball you put there.

The last time through the while loop, when last == start, only the initialization of the for loop is executed so that current == p when the while loop exits.