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See this page as a slide show

Using qsort

CS157 Qsort

Honesty

• OK, really, programmers very rarely write code to do sorting.
• In the shell, they generally use the sort command.
• In C, they generally use qsort.

The Idiot Savant

• Imagine that you had a semi-brilliant assistant who was very good at sorting.
• However, your assistant doesn’t know alphabetical order.
• He can do everything that’s needed for sorting except actually determine if, say, “Jay” comes before “David”.
• Therefore, he frequently comes to you, asking if this word comes before this other word.
• qsort is like that.

qsort

#include <stdlib.h>

void qsort(void *base, int count, int size, compare);

• base - a pointer to the beginning of the array that needs sorting
• count - how many things are in the array
• size - the size of each thing in the array
• compare - a function that compares two things
  • Really—just two things. It only has to compare a single pair of items.

Calling qsort

    int numbers[] = {11,2,1957,10,14,1956,11,1,1955};
    qsort(numbers, 9, sizeof(int), compare);

or

    struct Who { char name[20]; double gpa; };
    struct Who faculty[] = {
        {"Jack",3.9}, {"Bruce",3.5}, {"Darrell",0.2}, {"Dale",2.9}
    };
    qsort(faculty, 4, sizeof(struct Who), compare_gpa);

The comparison function

    int compare(const void *av, const void *bv) {
        const int *a = av, *b = bv;
        if (*a < *b) return -1;
        if (*a > *b) return 1;
        return 0;
    }

• The function is given two const void * pointers.
• Convert the generic pointers to pointers of the right type.
• const means that we won’t change those values.

The comparison function

    int compare(const void *av, const void *bv) {
        const int *a = av, *b = bv;
        if (*a < *b) return -1;
        if (*a > *b) return 1;
        return 0;
    }

• If the first value should go before the second value, return -1.
• If the first value should go after the second value, return 1.
• If they’re equal, return 0.

(Really, any negative/positive/zero values are good enough.)

The comparison function

    int compare(const void *av, const void *bv) {
        const int *a = av, *b = bv;
        if (*a < *b) return -1;
        return *a > *b;
    }

This does the same thing as the previous example.

A comparison function for strings

    int compare(const void *av, const void *bv) {
        const char *a = av, *b = bv;
        return strcmp(a,b);          // perfect!
    }

The return values for a qsort comparison function are exactly like the return values of strcmp.

Comparing structs

    struct Who { char name[20]; double gpa; };

    int compare_gpa(const void *av, const void *bv) {
        const struct Who *a = av, *b = bv;
        if (a->gpa < b->gpa) return -1;
        if (a->gpa > b->gpa) return 1;
        return 0;
    }

This comparison function compares two structs, comparing them by GPA.

Sorting ints

int compare(const void *av, const void *bv) {
    const int *a = av, *b = bv;
    if (*a < *b) return -1;
    if (*a > *b) return 1;
    return 0;
}

int main() {
    int numbers[] = {11,2,1957,10,14,1956,11,1,1955};
    qsort(numbers, 9, sizeof(int), compare);
    for (int i=0; i<9; i++)
        printf("%d\n", numbers[i]);
}

1
2
10
11
11
14
1955
1956
1957

Sorting structs

struct Who { char name[20]; double gpa; };
int compare_gpa(const void *av, const void *bv) {
    const struct Who *a = av, *b = bv;
    if (a->gpa < b->gpa) return -1;
    if (a->gpa > b->gpa) return 1;
    return 0;
}
int compare_name(const void *av, const void *bv) {
    const struct Who *a = av, *b = bv;
    return -strcmp(a->name,b->name);
}
int compare_gpaname(const void *av, const void *bv) {
    const struct Who *a = av, *b = bv;
    if (a->gpa < b->gpa) return -1;
    if (a->gpa > b->gpa) return 1;
    return -strcmp(a->name,b->name);
}


int main() {
    struct Who faculty[] = {
        {"Jack",3.999}, {"Bruce",3.5}, {"Darrell",3.5}, {"Dale",2.9}
    };
    qsort(faculty, 4, sizeof(struct Who), compare_gpaname);
    for (int i=0; i<4; i++)
        printf("%-8s %.2f\n", faculty[i].name, faculty[i].gpa);
}

Dale     2.90
Darrell  3.50
Bruce    3.50
Jack     4.00

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