Thread: qsort and structures

If I have a structure with two different datatypes as fields of the structure, ie:

Code:

typedef struct {
int value;
char *name;
} item;

Now I have an array of these items and I want to sort them using qsort first by value, then by name. (ie, if there are two elements that have the same value, I would then sort those two elements by their name)

How would I approach this using qsort? Thanks.

Hi!

start by writing a function that does the comparison:

Code:

int mycomp(const void *p1, const void *p2)
{
   const item *item1=p1, *item2=p2;
   int result;

   /* set result to: */
   /*  0 - if item1 and item2 are equal */
   /* <0 - if item1 should come before item2 */
   /* >0 - if item1 should come after item2 */

   return result;   
}

hope this helps

alex

Is there a way to use that function on two different fields (with same data type, ie: int)? I don't want to rewrite a function that looks almost exactly the same for each field that I have.

> I think you can't use qsort() easily in this case.
Of course you can, dead easy

*borrows Alex's code*

Code:

int mycomp(const void *p1, const void *p2) {
   const item *item1=p1, *item2=p2;
   if ( item1->value < item2->value ) {
      return -1;
   } else
   if ( item1->value > item2->value ) {
      return +1;
   } else {
      return strcmp( item1->name, item2->name );
   }
}

// blah
item array[10];
qsort( array, 10, sizeof(item), mycomp );

theweirdo: no, you can't write one function to handle multiple fields, at least there is no neat way to do it. If you really want you can use a hack: a global variable that gives the offset of the field relative to the start of the struct.

vVv: I think using qsort is easier than your solution. Also: mystrcmp("test", "testing") = 0.

alex