Comparing Strings:

Strings are compared to find out whether they are equivalent or unequal. Whenever they are unequal, one string is "more than" or "less than" the other. Determination of "greater" and "less" are made up with the ASCII codes of characters. In case of letters, this is equal to alphabetical order, with the one apparently strange exception which all uppercase letters are "less than" the lower-case letters. This is true since the uppercase letters contain ASCII codes 65 through 90 for A through Z, whereas lower-case a through z are symbolized by 97 through 122. Therefore, "ZEBRA" would be considered to be less than "apple" by such C functions.

The ANSI C library comprises of functions for two types of string comparisons: comparing two whole strings, and comparing a certain number of characters in the two strings.

Comparing Two Entire Strings:

The function strcmp() compares the two strings character by character. Its prototype is

int strcmp(char *str1, char *str2);

The arguments str1 and str2 are the pointers to strings being compared. The functions return values are given in table shown below. Following Listing expalins strcmp().

The values returned by strcmp().

Using strcmp() to compare strings.
 /* The strcmp() function. */
 #include <stdio.h>
 #include <string.h>
  main()
  {
      char str1[80], str2[80];
      int x;
     while (1)
     {
         /* Input two strings. */
         printf("\n\nInput the first string, a blank to exit: ");
         gets(str1);
         if ( strlen(str1) == 0 )
             break;
         printf("\nInput the second string: ");
         gets(str2);
         /* Compare them and display the result. */
         x = strcmp(str1, str2);
 
         printf("\nstrcmp(%s,%s) returns %d", str1, str2, x);
     }
     return(0);
 }

Output:

Input the first string, a blank to exit: First string
Input the second string: Second string
strcmp(First string,Second string) returns -1
Input the first string, a blank to exit: test string
Input the second string: test string
strcmp(test string,test string) returns 0
Input the first string, a blank to exit: zebra
Input the second string: aardvark
strcmp(zebra,aardvark) returns 1
Input the first string, a blank to exit:

Comparing Partial Strings:

The library function strncmp() compares a particular number of characters of one string to the other string. Its prototype is,

int strncmp(char *str1, char *str2, size_t n);

The function strncmp() compares n characters of str2 to str1. Comparison proceeds until n characters have been compared or end of the str1 has been reached. The technique of comparison and return values similar as for strcmp(). The comparison is case-sensitive.

=> Comparing parts of strings with the strncmp():
 
 /* The strncmp() function. */
 #include <stdio.h>
 #include[Sigma]>=tring.h>
 
  char str1[] = "The first string.";
  char str2[] = "The second string.";
  main()
 {
     size_t n, x;
     puts(str1);
     puts(str2);
     while (1)
     {
         puts("\n\nEnter number of characters to compare, 0 to exit.");
         scanf("%d", &n);
         if (n <= 0)
             break;
         x = strncmp(str1, str2, n);
         printf("\nComparing %d characters, strncmp() returns %d.", n, x);
     }
     return(0);
 }

Output:

The first string.
The second string.
Enter number of characters to compare, 0 to exit.
3
Comparing 3 characters, strncmp() returns .©]
Enter number of characters to compare, 0 to exit.
6
Comparing 6 characters, strncmp() returns -1.
Enter number of characters to compare, 0 to exit.
0

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