Using qsort() and bsearch() with strings

Using qsort() and bsearch() with strings - example program in C

By: Manoj Kumar in C Tutorials on 2007-10-03

This program makes use of an array of pointers to strings.You can "sort" the strings by sorting the array of pointers. However, this method requires a modification in the comparison function. This function is passed pointers to the two items in the array that are compared. However, you want the array of pointers sorted based not on the values of the pointers themselves but on the values of the strings they point to.

Because of this, you must use a comparison function that is passed pointers to pointers. Each argument to comp() is a pointer to an array element, and because each element is itself a pointer (to a string), the argument is therefore a pointer to a pointer. Within the function itself, you dereference the pointers so that the return value of comp() depends on the values of the strings pointed to.

The fact that the arguments passed to comp() are pointers to pointers creates another problem. You store the search key in buf[], and you also know that the name of an array (buf in this case) is a pointer to the array. However, you need to pass not buf itself, but a pointer to buf. The problem is that buf is a pointer constant, not a pointer variable. buf itself has no address in memory; it's a symbol that evaluates to the address of the array. Because of this, you can't create a pointer that points to buf by using the address-of operator in front of buf, as in &buf.

What to do? First, create a pointer variable and assign the value of buf to it. In the program, this pointer variable has the name key. Because key is a pointer variable, it has an address, and you can create a pointer that contains that address--in this case, key1. When you finally call bsearch(), the first argument is key1, a pointer to a pointer to the key string. The function bsearch() passes that argument on to comp(), and everything works properly.

1: /* Using qsort() and bsearch() with strings. */
2:
3: #include <stdio.h>
4: #include <stdlib.h>
5: #include <string.h>
6:
7: #define MAX 20
8:
9: int comp(const void *s1, const void *s2);
10:
11: main()
12: {
13:     char *data[MAX], buf[80], *ptr, *key, **key1;
14:     int count;
15:
16:     /* Input a list of words. */
17:
18:     printf("Enter %d words, pressing Enter after each.\n",MAX);
19:
20:     for (count = 0; count < MAX; count++)
21:     {
22:         printf("Word %d: ", count+1);
23:         gets(buf);
24:         data[count] = malloc(strlen(buf)+1);
25:         strcpy(data[count], buf);
26:     }
27:
28:     /* Sort the words (actually, sort the pointers). */
29:
30:     qsort(data, MAX, sizeof(data[0]), comp);
31:
32:     /* Display the sorted words. */
33:
34:     for (count = 0; count < MAX; count++)
35:         printf("\n%d: %s", count+1, data[count]);
36:
37:     /* Get a search key. */
38:
39:     printf("\n\nEnter a search key: ");
40:     gets(buf);
41:
42:     /* Perform the search. First, make key1 a pointer */
43:     /* to the pointer to the search key.*/
44:
45:     key = buf;
46:     key1 = &key;
47:     ptr = bsearch(key1, data, MAX, sizeof(data[0]), comp);
48:
49:     if (ptr != NULL)
50:         printf("%s found.\n", buf);
51:     else
52:         printf("%s not found.\n", buf);
53:     return(0);
54: }
55:
56: int comp(const void *s1, const void *s2)
57: {
58:     return (strcmp(*(char **)s1, *(char **)s2));
59: }
Enter 20 words, pressing Enter after each.
Word 1: apple
Word 2: orange
Word 3: grapefruit
Word 4: peach
Word 5: plum
Word 6: pear
Word 7: cherries
Word 8: banana
Word 9: lime
Word 10: lemon
Word 11: tangerine
Word 12: star
Word 13: watermelon
Word 14: cantaloupe
Word 15: musk melon
Word 16: strawberry
Word 17: blackberry
Word 18: blueberry
Word 19: grape
Word 20: cranberry
1: apple
2: banana
3: blackberry
4: blueberry
5: cantaloupe
6: cherries
7: cranberry
8: grape
9: grapefruit
10: lemon
11: lime
12: musk melon
13: orange
14: peach
15: pear
16: plum
17: star
18: strawberry
19: tangerine
20: watermelon
Enter a search key: orange
orange found.