Question

I am attempting to understand the c library qsort in the context of pointers to structs.
Here is the existing code that I would like to manipulate:

The structure:

#define MAX_NAME 20
#define NUM_MONTHS 12

typedef struct EMP {
    char name[MAX_NAME+1];
    int monthSales[NUM_MONTHS];
    int total;
} Emp;

The global initialization of the data and its size:

Emp *data;//where all entries are kept
int empSize;

and I have constructed 2 arrays of Emp pointers that I would like refer to the data in different orders:

Emp *nameArray[empSize];//an array of pointers to point to entries alphabetically
Emp *salesArray[empSize]; //an array of pointers to pointing to entries by sales

after they have been identically assigned, I would like to use a qsort to arrange them differently.
the nameArray alphabetically, using the name in the struct and
the salesArray largest to smallest, using the total in the struct

What should the compare methods and the qsort arguments look like?

Thanks

Answer 1

You just need to define two different comparison functions. Each comparison function should take in two pointers to void (in this case, you would cast them to Emp ** types) and then return a negative integer, zero, or a positive integer if the first entry is less than, equal to, or greater than the second one, respectively.

For the total-based sort, you can simply subtract the second total from the first. If the first total is less than the second, this results in a negative number, and the opposite is true when the first total is greater than the second. When they are equal, zero is returned.

int compareByTotal(const void *first, const void *second)
{
    int firstTotal = (*(Emp **)first)->total;
    int secondTotal = (*(Emp **)second)->total;

    return firstTotal - secondTotal;
}

The second one, since it’s a string comparison, can return the value of a strcmp (which obeys the same return value conventions):

int compareByName(const void *first, const void *second)
{
    const char *firstName = (*(Emp **)first)->name;
    const char *secondName = (*(Emp **)second)->name;

    return strcmp(firstName, secondName);
}

Then you can just call qsort passing in those function names:

/* given: */
Emp *nameArray[empSize];//an array of pointers to point to entries alphabetically
Emp *salesArray[empSize]; //an array of pointers to pointing to entries by sales

/* use: */
qsort(nameArray, empSize, sizeof(*nameArray), &compareByName);
qsort(salesArray, empSize, sizeof(*salesArray), &compareByTotal);