Sams Teach Yourself C in 21 Days

Working with Characters and Strings 239

10

11: /* Display instructions. */

12:

13: puts(“Enter text a line at a time, then press Enter.”);

14: puts(“Enter a blank line when done.”);

15:

16: /* Loop as long as input is not a blank line. */

17:

18: while ( *(ptr = gets(input)) != NULL)

19: printf(“You entered %s\n”, input);

20:

21: puts(“Thank you and good-bye\n”);

22:

23: return 0;

24: }

Enter text a line at a time, then press Enter.

Enter a blank line when done.

First string

You entered First string

Two

You entered Two

Bradley L. Jones

You entered Bradley L. Jones

Thank you and good-bye

Now you can see how the program works. If you enter a blank line (that is, if

you simply press Enter) in response to line 18, the string (which contains 0 char-

acters) is still stored with a null character at the end. Because the string has a length of 0,

the null character is stored in the first position. This is the position pointed to by the

return value of gets(), so if you test that position and find a null character, you know

that a blank line was entered.

Listing 10.6 performs this test in the whilestatement in line 18. This statement is a bit

complicated, so look carefully at the details in order. Figure 10.1 illustrates the compo-

nents of this statement.

LISTING10.6 continued

INPUT/

OUTPUT

ANALYSIS

Because it is not always possible to know how many characters gets()will

read, and because gets()will continue to store characters past the end of

the buffer, it should be used with caution.

Caution

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