INPUT AND OUTPUT

INPUT AND OUTPUT

When we say Input, it means to feed some data into a program. An input can be
given in the form of a file or from the command line. C programming provides a
set of built-in functions to read the given input and feed it to the program as per
requirement.

When we say Output, it means to display some data on screen, printer, or in
any file. C programming provides a set of built-in functions to output the data on
the computer screen as well as to save it in text or binary files.

The Standard Files

C programming treats all the devices as files. So devices such as the display are
addressed in the same way as files and the following three files are
automatically opened when a program executes to provide access to the
keyboard and screen.

Standard File                                File Pointer                                    Device Standard input                                 stdin                                                   Keyboard Standard output                               stdout                                                 Screen Standard error                                  stderr                                                 Your screen

The file pointers are the means to access the file for reading and writing
purpose. This section explains how to read values from the screen and how to
print the result on the screen.

The getchar() and putchar() Functions

The int getchar(void) function reads the next available character from the
screen and returns it as an integer. This function reads only single character at a
time. You can use this method in the loop in case you want to read more than
one character from the screen.

The int putchar(int c) function puts the passed character on the screen and
returns the same character. This function puts only single character at a time.
You can use this method in the loop in case you want to display more than one
character on the screen. Check the following example:

#include <stdio.h> int main( ) { int c; printf( "Enter a value :"); c = getchar( ); printf( "\nYou entered: "); putchar( c ); return 0; }

When the above code is compiled and executed, it waits for you to input some
text. When you enter a text and press enter, then the program proceeds and
reads only a single character and displays it as follows:

$./a.out Enter a value : this is test You entered: t

The gets() and puts()Functions

The char *gets(char *s) function reads a line from stdin into the buffer
pointed to by s until either a terminating newline or EOF (End of File).
The int puts(const char *s) function writes the string ‘s’ and ‘a’ trailing
newline to stdout.

When the above code is compiled and executed, it waits for you to input some
text. When you enter a text and press enter, then the program proceeds and
reads the complete line till end, and displays it as follows:

$./a.out Enter a value : this is test You entered: This is test

The scanf() and printf()Functions

The int scanf(const char *format, ...) function reads the input from the
standard input stream stdin and scans that input according to the
format provided.
The int printf(const char *format, ...) function writes the output to the
standard output stream stdout and produces the output according to the format
provided.
The format can be a simple constant string, but you can specify %s, %d, %c,
%f, etc., to print or read strings, integer, character, or float, respectively. There
are many other formatting options available which can be used based on
requirements. Let us now proceed with a simple example to understand the
concepts better:

#include <stdio.h> int main( ) { char str[100]; int i; printf( "Enter a value :"); scanf("%s %d", str, &i); printf( "\nYou entered: %s %d ", str, i); return 0; }

When the above code is compiled and executed, it waits for you to input some
text. When you enter a text and press enter, then program proceeds and reads
the input and displays it as follows:

$./a.out Enter a value : seven 7 You entered: seven 7

Here, it should be noted that scanf() expects input in the same format as you
provided %s and %d, which means you have to provide valid inputs like "string
integer". If you provide "string string" or "integer integer", then it will be
assumed as wrong input. Secondly, while reading a string, scanf() stops reading
as soon as it encounters a space, so "this is test" are three strings for scanf().