Quicksort

// xcode
// paulogp

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define MAX_ELEMENTS 30

void swap_elements (int *x, int *y);
int get_pivot (int the_min, int the_max);
void quicksort (int the_list[], int the_min, int the_max);
void print_list (int the_list[]);

void swap_elements (int *the_a, int *the_b)
{
    int the_temp;

    the_temp = *the_a;
    *the_a = *the_b;
    *the_b = the_temp;
}

int get_pivot (int the_min, int the_max)
{
    // middle
    return ((the_min + the_max) / 2);
}

void quicksort (int the_list[], int the_min, int the_max)
{
    int the_key, i, j, the_pivot;

    if (the_min < the_max)
    {
        the_pivot = get_pivot (the_min, the_max);

        swap_elements (&the_list[the_min], &the_list[the_pivot]);
        the_key = the_list[the_min];

        i = the_min + 1;
        j = the_max;

        while (i <= j)
        {
            while ((i <= the_max) && (the_list[i] <= the_key))
                i++;

            while ((j >= the_min) && (the_list[j] > the_key))
                j--;

            if (i < j)
                swap_elements (&the_list[i], &the_list[j]);
        }

        // swap two elements
        swap_elements (&the_list[the_min], &the_list[j]);

        // recursively sort the lesser list
        quicksort (the_list, the_min, (j - 1));
        quicksort (the_list, (j + 1), the_max);
    }
}

void print_list (int the_list[])
{
    int i = 0;

    while (the_list[i] != '\0')
    {
        printf("%d\t", the_list[i]);
        i += 1;
    }
}

int main (int argc, const char * argv[])
{
    int the_list[MAX_ELEMENTS];

    int i = 0;

    // seed the random sequence generated by rand()
    srand ((unsigned)time(NULL));

    // generate random numbers and fill them to the list
    for (i = 0; i < MAX_ELEMENTS; i++)
    {
        the_list[i] = (rand() % 20) + 1; // random from 1 to 20
    }

    printf("the list before sorting is:\n");
    print_list (the_list);

    // sort the list using quicksort
    quicksort (the_list, 0, MAX_ELEMENTS-1);

    // print the result
    printf ("\nthe list after sorting using quicksort algorithm:\n");
    print_list (the_list);

    return 0;
}