Reading and Writing Strings
Now let's focus on how to read or write strings with the standard input and output streams—that is, stdin and stdout. In C, there are several functions you can use to deal with string reading or writing. The following subsections introduce some of the functions.
The gets() and puts() Functions
The gets() function can be used to read characters from the standard input stream.
The syntax for the gets() function is#include <stdio.h>
char *gets(char *s);
Here the characters read from the standard input stream are stored in the character array identified by s. The gets() function stops reading, and appends a null character \0 to the array, when a newline or end-of-file (EOF) is encountered. The function returns s if it concludes successfully. Otherwise, a null pointer is returned.
The puts() function can be used to write characters to the standard output stream (that is, stdout).
The syntax for the puts() function is#include <stdio.h>
int puts(const char *s);
Here s refers to the character array that contains a string. The puts() function writes the
string to the stdout. If the function is successful, it returns 0. Otherwise, a nonzero value is returned.
The puts() function appends a newline character to replace the null character at the end of a character array.
Both the gets() and puts() functions require the header file stdio.h. In Listing 13.4, you can see the application of the two functions.
TYPE
Listing 13.4. Using the gets() and puts() functions.1: /* 13L04.c: Using gets() and puts() */
2: #include <stdio.h>
3:
4: main()
5: {
6: char str[80];
7: int i, delt = `a' - `A';
8:
9: printf("Enter a string less than 80 characters:\n");
10: gets( str );
11: i = 0;
12: while (str[i]){
13: if ((str[i] >= `a') && (str[i] <= `z'))
14: str[i] -= delt; /* convert to upper case */
15: ++i;
16: }
17: printf("The entered string is (in uppercase):\n");
18: puts( str );
19: return 0;
20: }
ANALYSIS
After running the executable 13L04.exe, I enter a line of characters from the keyboard and have the characters (all in uppercase) shown on the screen:
OUTPUT
C:\app>13L04
Enter a string less than 80 characters:
This is a test.
The entered string is (in uppercase):
THIS IS A TEST.
C:\app>
The program in Listing 13.4 accepts a string of characters entered from the keyboard (that is, stdin), and then converts all lowercase characters to uppercase ones. Finally, the modified string is put back to the screen.
In line 6, a character array (str) is declared that can hold up to 80 characters. The gets() function in line 10 reads any characters the user enters from the keyboard until the user presses the Enter key, which is interpreted as a newline character. The characters read in by the gets() function are stored into the character array indicated by str. The newline character is not saved into str. Instead, a null character is appended to the array as a terminator.
The while loop in lines 12_15 has a conditional expression, str[i]. The while loop keeps iterating as long as str[i] returns logical TRUE. Within the loop, the value of each character represented by str[i] is evaluated in line 13, to find out whether the character is a lowercase character within the range of a through z. If the character is one of the lowercase characters, it is converted into uppercase by subtracting the value of an int variable, delt, from its current value in line 14. The delt variable is initialized in line 7 by the value of the `a' - `A' expression, which is the difference between a lowercase character and its uppercase counterpart. In other words, by subtracting the difference of `a' and `A' from the lower case integer value, we obtain the uppercase integer value.
Then the puts() function in line 18 outputs the string with all uppercase characters to stdout, which leads to the screen by default. A newline character is appended by the puts() function to replace the null character at the end of the string.
Using %s with the printf() Function
We've used the printf() function in many program examples in this book. As you know, many format specifiers can be used with the printf() function to specify different display formats for numbers of various types.
For instance, you can use the string format specifier, %s, with the printf() function to display a character string saved in an array on the screen. (See the example in Listing 13.3.)
In the next section, the scanf() function is introduced as a way to read values of various data types with different format specifiers, including the format specifier %s.
The scanf() Function
The scanf() function provides another way to read strings from the standard input stream. Moreover, this function can actually be used to read various types of input data. The formats of arguments to the scanf() function are quite similar to those used in the printf() function.
The syntax for the scanf() function is#include <stdio.h>
int scanf(const char *format, …);
Here various format specifiers can be included inside the format string referenced by the char pointer variable format. If the scanf() function concludes successfully, it returns the num-ber of data items read from the stdin. If an error occurs, the scanf() function returns EOF (end-of-file).
Note that using the string format specifier %s causes the scanf() function to read characters until a space, a newline, a tab, a vertical tab, or a form feed is encountered. Characters read by the scanf() function are stored into an array referenced by the corresponding argument. The array should be big enough to store the input characters.
A null character is automatically appended to the array after the reading.
The program in Listing 13.5 shows how to use various format specifiers with the scanf() function.
TYPE
Listing 13.5. Using the scanf() function with various format specifiers.1: /* 13L05.c: Using scanf() */
2: #include <stdio.h>
3:
4: main()
5: {
6: char str[80];
7: int x, y;
8: float z;
9:
10: printf("Enter two integers separated by a space:\n");
11: scanf("%d %d", &x, &y);
12: printf("Enter a floating-point number:\n");
13: scanf("%f", &z);
14: printf("Enter a string:\n");
15: scanf("%s", str);
16: printf("Here are what you've entered:\n");
17: printf("%d %d\n%f\n%s\n", x, y, z, str);
18: return 0;
19: }
OUTPUT
The following output is a sample displayed on the screen after I run the executable 13L05.exe and enter data (which appears in bold) from my keyboard:
ANALYSIS
C:\app>13L05
Enter two integers separated by a space:
10 12345
Enter a floating-point number:
1.234567
Enter a string:
Test
Here are what you've entered:
10 12345
1.234567
Test
C:\app>
In Listing 13.5, there are one char array (str), two int variables (x and y), and a float variable (z) declared in lines 6_8.
Then, the scanf() function in line 11 reads in two integers entered by the user and saves them into the memory locations reserved for the integer variables x and y. The statement in line 13 reads and stores a floating-point number into z. Note that the format specifiers, %d and %f, are used to specify proper formats for entered numbers in lines 11 and 13.
Line 15 reads a series of characters entered by the user with the scanf() function by using the format specifier %s, and then saves the characters, plus a null character as the terminator, into the array pointed to by str.
To prove that the scanf() function reads all the numbers and characters entered by the user, the printf() function in line 17 displays the contents saved in x, y, z, and str on the screen. Sure enough, the result shows that the scanf() does a good job.
One thing you need to be aware of is that the scanf() function doesn't actually start reading the input until the Enter key is pressed. Data entered from the keyboard is placed in an input buffer. When the Enter key is pressed, the scanf() function looks for its input in the buffer. You'll learn more about buffered input and output in Hour 21, "Disk File Input and Output: Part I."
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