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11.2.1 String I/O: gets() and puts()

One of the first operations we may need for strings is the ability to read or write strings from the standard input or to the standard output. For example, if we had a task:

STR0: Read strings until end of file, convert each string to upper case, and print the modified string.

we could easily write an algorithm for the task:

while not EOF, read a string
         convert string to upper case
         print string

To implement this algorithm, we need three functions: read a string from standard input stream, write a string to standard output, and convert a string to upper case. We have shown crude versions of these first two functions in the previous, section; however, the standard library provides these operations as well. Library function, gets(s) reads a string from the standard input into an array pointed to by s; and, puts(s) writes a string pointed to by s to the standard output. The prototypes for these functions, declared in stdio.h, are: .

STRING  gets(STRING  string);
    int puts(STRING  string);

If reading is successful, gets() returns the pointer to the string; otherwise, it returns a NULL pointer, i.e. a pointer whose value is zero. A returned value of NULL usually implies an end of file. When gets() reads a string, it reads the input characters until a newline is read, discards the newline, appends a NULL character to the string, and stores the string where s points. Similarly, puts() outputs the string, s, after stripping the NULL and appending a newline. It returns the last character value output if successful; otherwise it returns EOF. Note, the arguments to these functions and the return value from gets() are character pointers, i.e. equivalent to our STRING data type, and we can consider them as such. The argument of gets() MUST be a string; otherwise, the function attempts to store characters wherever the argument points, which can create a possibly fatal error when the program executes.

We will write and use a function, ucstr(), which converts a string to upper case. The whole program is simple: it reads a string, converts it to upper case, and prints it; and is shown in Figure 11.2

In the driver, the loop expression reads a line into s; if successful, the returned value is a non-zero pointer, s, and the loop is executed. In the loop body, the string, s, is converted to upper case, and printed. The function, ucstr(), converts a string to upper case by traversing the string and converting each character to upper case using library routine, toupper(), which returns the upper case version of its argument if it is a lower case letter; otherwise it returns the argument unchanged.

Sample Session:

• ***String to Upper Case***
• Type strings, EOF to terminate
• Hello
• HELLO
• Pad 19A
• PAD 19A
• good morning
• GOOD MORNING
• '136D

while (*gets(s))
    puts(s);

printf("H(elp, Q(uit, D(isplay\n");
while (gets(s)) {
    switch (toupper(*s)) {

         case 'H': help();
                   break;

         case 'Q': exit(0);

         case 'D': display();
                   break;

         default: ;
    }

    printf("H(elp, Q(uit, D(isplay\n");
}