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

// Forward declarations
static int _compare_strings(const void* a_s1, const void* a_s2);
void _print_strings(char* strings[], size_t num_strings);


int main(int argc, char* argv[]) {
    // Create array of sample strings to be sorted
    char* sample_strings[] = { "banana", "cherry", "apple" };
    size_t num_strings = sizeof(sample_strings)/sizeof(sample_strings[0]); // == 3

    // Print the list, unsorted
    printf("Before sort\n");
    _print_strings(sample_strings, num_strings);

    // SORT
    qsort(sample_strings, num_strings, sizeof(sample_strings[0]), _compare_strings);

    // Print the list, sorted
    printf("After sort\n");
    _print_strings(sample_strings, num_strings);

    return EXIT_SUCCESS;
}


int _compare_strings(const void* a1, const void* a2) {
    // Step 1:  Convert void* arguments (address of anything) to a type
    //          that is relevant to what we are sorting.  Since we are
    //          sorting strings (char*), we need to convert the arguments
    //          to char**.
    //
    //          Remember that the arguments are the address of the thing we are
    //          sorting.  If we were sorting integers we would convert the
    //          void* arguments to int*.
    char** a_s1 = (char**)a1;
    char** a_s2 = (char**)a2;

    // Step 2:  Dereference the addresses of strings (char**) to get the strings
    //          we are trying to compare (char*).
    char* s1 = *a_s1;
    char* s2 = *a_s2;

    // Step 3:  Do the comparison.
    int result = strcmp(s1, s2);

    // If result <  0, it means s1 <  s2
    // If result == 0, it means s1 == s2
    // If result >  0, it means s1 >  s2

    return result;

    //// Equivalent to...
    //return strcmp(*((const char**)a1), *((const char**)a2));

    // Note: const is just tells the compiler that you do not intend to
    // modify it.  That way, the compiler can warn you if you do.
}


void _print_strings(char* strings[], size_t num_strings) {
    // Print all strings
    for(int i=0; i < num_strings; i++) {
        // Print one string
        printf("  strings[%d] == \"%s\"\n", i, strings[i]);
    }
}

This content is protected and may not be shared, uploaded, or distributed.    © 2019 Alexander J. Quinn.