‘qsort()’ function is used to sort an array of values in ascending order. This function is defined in ‘stdlib.h’ header and is prototyped as below
void qsort(void *array, size_t no_of_elements, size_t size_of_element, int (*compare)(void const *, void const *));
Notice that first argument is array to be sorted, second argument specifies no. of elements in the array, third is size of element (in characters) and last argument is pointer-to-function that compares elements. Note further that ‘compare()’ function takes two arguments of type ‘void const *’, therefore cast the valuse to proper type. ‘compare()’ function returns integer value less than, equal to and greater than if fist value is smaller, equal to or larger than second value. Let’s consider a C program that reads in strings, calls ‘qsort()’ to sort an array of strings.
/* qsort.c -- program calls qsort() to sort an array of strings */ /* in ascending order */ #include <stdio.h> #include <stdlib.h> #include <string.h> #define NOSTRINGS 5 #define SIZE 26 int string_compare(void const *, void const *); int main(void) { int i; char strings[NOSTRINGS][SIZE]; /* read strings from the user */ printf("User, write in %d character strings...\n", NOSTRINGS); for (i= 0; i < NOSTRINGS; i++) fgets(strings[i], SIZE, stdin); /* display array of unsorted strings */ printf("Before Sorting:\n"); for (i = 0; i < NOSTRINGS; i++) fputs(strv style="text-align:justify"> ings[i], stdout); /* calling qsort() to sort the array in ascending order */ qsort(strings, NOSTRINGS, SIZE, string_compare); printf("\n"); /* array of sorted strings */ printf("After Sorting:\n"); for (i = 0; i < NOSTRINGS; i++) printf(strings[i], stdout); return 0; } int string_compare(void const *str1, void const *str2) { return strcmp((char *)str1, (char *)str2); }
Output of the program is as follows
Before Sorting: Linux Kernel Primer Expert C Programming Robert Love Kernel Development Pointers on C After Sorting: Expert C Programming Kernel Development Linux Kernel Primer Pointers on C Robert Love
‘bsearch()’ function is used in a C program to locate a given value in a sorted array using ‘binary search mechanism’. If the array isn’t sorted, result is undefined! If value is found, function returns pointer to the value otherwise returns NULL! A simple C program might further enforce this concept.
/* bsearch.c -- program calls bsearch() to search for a given value */ /* in the sorted array */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <assert.h> #define NOSTRINGS 5 #define SIZE 26 int string_compare(void const *, void const *); int main(void) { int i; char strings[NOSTRINGS][SIZE]; char search_str[SIZE]; char *status; /* read strings from the user */ printf("User, write in %d character strings...\n", NOSTRINGS); for (i= 0; i < NOSTRINGS; i++) fgets(strings[i], SIZE, stdin); printf("\n\n"); /* display array of unsorted strings */ printf("Before Sorting:\n"); for (i = 0; i < NOSTRINGS; i++) fputs(strings[i], stdout); /* sort the array */ qsort(strings, NOSTRINGS, SIZE, string_compare); printf("\n"); /* array of sorted strings */ printf("After Sorting:\n"); for (i = 0; i < NOSTRINGS; i++) printf(strings[i], stdout); printf("\n\n"); /* read the string to be searched for */ printf("User, write in string you wish to search for...\n"); fgets(search_str, SIZE, stdin); printf("\n"); /* calling bsearch() to locate given string in the array */ status = bsearch(search_str, strings, NOSTRINGS, SIZE, string_compare); /* verify if given value is found */ assert(status != NULL); printf("desired string is found!\n"); return 0; } int string_compare(void const *str1, void const *str2) { return strcmp((char *)str1, (char *)str2); }
Output of the program is as follows
User, write in 5 character strings... kernel Development Linux Kernel Primer Robert Love Pointers on C Expert C Programming Before Sorting: kernel Development Linux Kernel Primer Robert Love Pointers on C Expert C Programming After Sorting: Expert C Programming Linux Kernel Primer Pointers on C Robert Love kernel Development User, write in string you wish to search for... Robert Love desired string is found!
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