9.4 Searching and Sorting Example

Here is an example showing the use of qsort and bsearch with an array of structures. The objects in the array are sorted by comparing their name fields with the strcmp function. Then, we can look up individual objects based on their names.

     #include <stdlib.h>
     #include <stdio.h>
     #include <string.h>
     
     /* Define an array of critters to sort. */
     
     struct critter
       {
         const char *name;
         const char *species;
       };
     
     struct critter muppets[] =
       {
         {"Kermit", "frog"},
         {"Piggy", "pig"},
         {"Gonzo", "whatever"},
         {"Fozzie", "bear"},
         {"Sam", "eagle"},
         {"Robin", "frog"},
         {"Animal", "animal"},
         {"Camilla", "chicken"},
         {"Sweetums", "monster"},
         {"Dr. Strangepork", "pig"},
         {"Link Hogthrob", "pig"},
         {"Zoot", "human"},
         {"Dr. Bunsen Honeydew", "human"},
         {"Beaker", "human"},
         {"Swedish Chef", "human"}
       };
     
     int count = sizeof (muppets) / sizeof (struct critter);
     
     
     
     /* This is the comparison function used for sorting and searching. */
     
     int
     critter_cmp (const struct critter *c1, const struct critter *c2)
     {
       return strcmp (c1->name, c2->name);
     }
     
     
     /* Print information about a critter. */
     
     void
     print_critter (const struct critter *c)
     {
       printf ("%s, the %s\n", c->name, c->species);
     }
     
     
     /* Do the lookup into the sorted array. */
     
     void
     find_critter (const char *name)
     {
       struct critter target, *result;
       target.name = name;
       result = bsearch (&target, muppets, count, sizeof (struct critter),
                         critter_cmp);
       if (result)
         print_critter (result);
       else
         printf ("Couldn't find %s.\n", name);
     }
     
     /* Main program. */
     
     int
     main (void)
     {
       int i;
     
       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");
     
       qsort (muppets, count, sizeof (struct critter), critter_cmp);
     
       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");
     
       find_critter ("Kermit");
       find_critter ("Gonzo");
       find_critter ("Janice");
     
       return 0;
     }

The output from this program looks like:

     Kermit, the frog
     Piggy, the pig
     Gonzo, the whatever
     Fozzie, the bear
     Sam, the eagle
     Robin, the frog
     Animal, the animal
     Camilla, the chicken
     Sweetums, the monster
     Dr. Strangepork, the pig
     Link Hogthrob, the pig
     Zoot, the human
     Dr. Bunsen Honeydew, the human
     Beaker, the human
     Swedish Chef, the human
     
     Animal, the animal
     Beaker, the human
     Camilla, the chicken
     Dr. Bunsen Honeydew, the human
     Dr. Strangepork, the pig
     Fozzie, the bear
     Gonzo, the whatever
     Kermit, the frog
     Link Hogthrob, the pig
     Piggy, the pig
     Robin, the frog
     Sam, the eagle
     Swedish Chef, the human
     Sweetums, the monster
     Zoot, the human
     
     Kermit, the frog
     Gonzo, the whatever
     Couldn't find Janice.