How to implement qsort() to work for an array of structures?

Question

I have to solve several C problems and most of them involve having to use qsort() somewhere but no matter how much I take help from the net I can't get it to work. Take this code for example:

#include <stdio.h>
#include <string.h>
struct date
{
    int day;
    int month;
    int year;
};struct date d[5]={
    {12,12,2012},
    {23,02,2014},
    {31,01,2222},
    {32,21,2011},
    {12,01,1990}
    };

int compare(const void * a, const void * b)
{
    struct date *orderA = (date *)a;
  struct date *orderB = (date *)b;

  return (  orderA->year -orderB->year  );
}
int main()
{
    int i;
    qsort(d,5,sizeof(date),compare);

    for(i=0;i<5;i++)
    printf("%d %d %d\n",d[i].day,d[i].month,d[i].year);
    return 0;

}

I get errors that date is undeclared even though it is already. And I can't understand compare functions at all and have to copy them from the net. Help me please. My teacher at college is a total imbecile.

And use struct date* orderA = (struct date*)a; in the compare function. — Riley
– Riley, Commented Sep 16, 2016 at 17:04
The compare function is pretty simple. Return something less than 0 if the first argument comes before the second in the final array. Return 0 if both arguments are "equal". Return something greater than 0 if the first argument comes after the second in the final array. Let me know if I can help clear that up any more. — Riley
– Riley, Commented Sep 16, 2016 at 17:24
In C++, you can use the simple name date after defining struct date, but in C you must add typedef struct date date; to be able to use the unqualified type name date. — Jonathan Leffler
– Jonathan Leffler, Commented Sep 17, 2016 at 5:33

dbush · Accepted Answer · 2016-09-16 17:11:33Z

1

date is not a type. struct date is. You need to use the struct keyword when referencing a structure type.

Also, if you define the pointers in your comparison function as const the cast isn't needed.

const struct date *orderA = a;
const struct date *orderB = b;

answered Sep 16, 2016 at 17:11

dbush

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1 Comment

dbush Over a year ago

@overloood glad I could help. Feel free to accept this answer if you found it usefull.

cdlane · Accepted Answer · 2016-09-17 00:03:52Z

Minor issues with your code: you need to include stdlib.h (for qsort()) but you're not using string.h in your example; you use date as a struct and a type but don't define it that -- you can define it as both at the same time via typedef; you can expand your comparision beyond years if you want.

A rework of your code addressing the above issues:

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

typedef struct date {
    int day;
    int month;
    int year;
} Date;

int compare(const void *a, const void *b) {
    const Date *date_A = a;
    const Date *date_B = b;

    return date_A->year == date_B->year ? 
        date_A->month == date_B->month ?
            date_A->day - date_B->day
                : date_A->month - date_B->month
                    : date_A->year - date_B->year;
}

int main() {

    Date dates[] = {
        {12, 12, 2012},
        {23, 02, 2014},
        {31, 01, 2222},
        {32, 21, 2011},
        {12, 01, 1990}
    };

    size_t count = sizeof(dates) / sizeof(Date);

    qsort(dates, count, sizeof(Date), compare);

    for (int i = 0; i < count; i++) {
        Date *date = &dates[i];
        printf("%d %d %d\n", date->day, date->month, date->year);
    }

    return 0;
}

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