Sams Teach Yourself C in 21 Days

Manipulating Strings 489

17

abcdefghijklmnopqrstuvw

abcdefghijklmnopqrstuvwx

abcdefghijklmnopqrstuvwxy

abcdefghijklmnopqrstuvwxyz

The ASCII codes for the letters b–z are 98–122. This program uses these ASCII

codes in its demonstration of strcat(). The forloop on lines 17–22 assigns

these values in turn to str2[0]. Because str2[1]is already the null character (line 15),

the effect is to assign the strings “b”,“c”, and so on to str2. Each of these strings is

concatenated with str1(line 20), and then str1is displayed on-screen (line 21).

Using the strncat()Function ......................................................................

The library function strncat()also performs string concatenation, but it lets you specify

how many characters of the source string are appended to the end of the destination

string. The prototype is

char *strncat(char *str1, const char *str2, size_t n);

Ifstr2contains more than ncharacters, the first ncharacters are appended to the end of

str1. If str2contains fewer than ncharacters, all of str2is appended to the end of

str1. In either case, a terminating null character is added at the end of the resulting

string. You must allocate enough space for str1to hold the resulting string. The function

returns a pointer to str1. Listing 17.6 uses strncat()to produce the same output as

Listing 17.5.

LISTING17.6 strncat.c. Using the strncat()function to concatenate strings

1: /* The strncat() function. */

2:

3: #include <stdio.h>

4: #include <string.h>

5:

6: char str2[] = “abcdefghijklmnopqrstuvwxyz”;

7:

8: int main( void )

9: {

10: char str1[27];

11: int n;

12:

13: for (n=1; n< 27; n++)

14: {

15: strcpy(str1, “”);

16: strncat(str1, str2, n);

17: puts(str1);

18: }

19: return 0;

20: }

ANALYSIS

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