2.7 INPUT AND OUTPUT STATEMENTS

 

     Programming in C    

INPUT / OUTPUT IN C

BASIC SCREEN AND KEYBOARD

C provides several input and output functions that give different levels of I/O capability. These functions are, in most cases, implemented as library routines that internally call lower-level input/output functions.

The input and output functions in C are built around the concept of a set of standard data streams that connect each executing program to the basic input/output devices. These standard data streams or files are opened automatically by the operating system and are available to every C program without explicitly opening or closing them.

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Standard Data Streams in C

The standard data streams available in C are:

• stdin → connected to the keyboard

• stdout → connected to the screen

• stderr → connected to the screen (used for error messages)

Additional Data Streams (MS-DOS Based Systems Only)

The following streams are available only on MS-DOS based computers and not on UNIX or other multi-user operating systems:

• stdaux → connected to the first serial communication port

• stdprn → connected to the first parallel printer port

A number of functions and macros exist to provide support for streams of various kinds. The <stdio.h> header file contains the necessary function declarations, macros, and type definitions for input and output operations.

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Classification of Input and Output Functions

The input/output functions in C fall into two categories:

1. Non-formatted input and output functions

2. Formatted input and output functions

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Note

• The input and output functions in C are implemented through standard data streams.

• Input/output functions are classified into non-formatted and formatted functions.

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1. NON-FORMATTED INPUT AND OUTPUT

Non-formatted input and output operations are carried out using standard input-output library functions. These functions handle one character at a time.

• Input functions do not require pressing <Enter> immediately after typing a character.

• Output functions display a single character on the console.

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1.1 SINGLE CHARACTER INPUT AND OUTPUT

Character-oriented input and output are handled using the following functions:

int getchar(void);    // character input
int putchar(int c);  // character output

• getchar() reads a single character from the keyboard.

• putchar() writes a single character to the display screen.

Two additional functions, gets() and puts(), are used for string input and output, which are discussed in later chapters.

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1.2 SINGLE CHARACTER INPUT – getchar()

The getchar() function reads an unsigned character from the input stream stdin and converts it into an int.

• On successful input, the character is returned.

• On end-of-file, the function returns EOF.

• On error, the error indicator is set.

• Characters are read sequentially on successive calls.

General Form

char_variable = getchar();

Although input appears to be of character type, it is actually stored as an integer. Hence, the variable receiving the value must be of type int.

Important Observation

• getchar() must distinguish between a valid character and EOF.

• Since char cannot store EOF, the variable must be declared as int.

• The character is stored in the lower-order byte of the integer variable.

Also, getchar() does not return the character to the program until the Enter key (\n) is pressed.

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1.3 SINGLE CHARACTER OUTPUT – putchar()

The putchar() function writes a single character to the stdout stream.

• On success, it returns the character printed.

• On error, it returns EOF.

• There is no end-of-file condition for output streams.

General Form

putchar(char_variable);

The character stored in the lower-order byte of the variable is displayed on the screen.

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1.4 ADDITIONAL SINGLE CHARACTER INPUT / OUTPUT FUNCTIONS

The following functions are available only in Turbo C and require the header file <conio.h>:

• getch()
  Reads a character without echoing it on the screen.

• getche()
  Reads a character with echo on the screen.

• putch()
  Writes a character directly to the screen.

All these functions handle data in ASCII format.

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Examples Using getchar() and putchar()

1. Display a given character
   Assigns a character to an integer variable and displays it using putchar().

2. Display a keyed-in character
   Reads a character using getchar() and prints it using putchar().

3. Display the next ASCII character
   Increments the ASCII value and prints the next character.

4. Display keyed-in character and its next ASCII value
   Demonstrates ASCII arithmetic.

5. Demonstration of increment and decrement operators
   Shows pre-increment, post-increment, and post-decrement behavior.

6. Nested getchar() and putchar()
   Reads and prints a character in a single statement.

7. Convert lowercase letters to uppercase
   Uses ASCII range checking and conditional operator.

8. Use of getch() and getche()
   Demonstrates input with and without echo.

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2. FORMATTED INPUT AND OUTPUT FUNCTIONS

When input and output are required in a specific format, the standard library functions scanf() and printf() are used.

• scanf() → formatted input

• printf() → formatted output

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2.1 OUTPUT FUNCTION – printf()

The printf() function prints data in a specified format using a control string (format string).

General Form

printf("control_string", variable1, variable2, ...);

• The control string specifies data types and format specifiers.

• Each % conversion code corresponds to a variable in the argument list.

• Ordinary characters are printed as they are.

Important Points

• Format specifiers indicate type conversion and display format.

• Using an incorrect specifier causes incorrect output.

• Precision, width, flags, and size modifiers provide output formatting control.

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Format String Components

A printf format string consists of:

1. Ordinary characters

2. Conversion specifier field (%)

3. Control codes (escape sequences)

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Conclusion

The input and output statements in C provide flexible mechanisms to interact with the user through the keyboard and screen. By using standard data streams, non-formatted, and formatted functions, C allows precise control over data input, output, and presentation, making it a powerful system-level programming language.

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📖 Reference

Pradip Dey & Manas Ghosh,
Computer Fundamentals and Programming in C,
Second Edition, Oxford University Press, 2018.

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