CS157

CS157: Intro to C, Part II

Spring 2018

Qsort

See this page as a slide show

Using qsort

CS157 Qsort

Honesty

The Idiot Savant

qsort

#include <stdlib.h>

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

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 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;
    }

(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

Modified: 2017-04-24T13:25

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