Input and Output in C
Declaration and Initialization of Variables:
Any variable in C
programming language must be declared before using it. Variables are declared
using the following rules-
Integer type variables are
declared using the “int” keyword before the variable name.
More than one variable can be declared in a single statement. Syntax for
declaring integer type variables is given below-
int <list_of_variable_names>;
For example:
int n;
int basic, hra;
Real or floating type
variables are declared using the keyword “float” before the variable name.
Syntax for declaring floating type variables is given below-
float <list_of_variable_names>;
For example:
float a;
float rate, gross_salary, weight;
Character type variables are
declared using the keyword “char” before the variable name.
Syntax for declaring character type variables is given below-
char <list_of_variable_names>;
For example:
char c;
char x, y, symbol;
Variables can be initialized
or given values at the time of declaration.
For example:
int n=12, a=5;
float x=1.5, y=3.7;
char a=’t’, b=’#’;
In C programming language
variables should be declared in the beginning of any function.
For example:
int main()
{
int n1, n2=25;
float length=45.8;
statement;
…;
…;
}
Input and Output:
There should be a way for
interacting with the outside world. There are so many readymade library
functions for input and output in C. Two of them are explained here. Others
will be explained, whenever and wherever they are used.
For outputting messages,
values of variables, results of expressions, etc. function printf()
is used. The format of this function is given below-
printf(“<format_string”,
<list_of_variables>);
In the format string, “%d” is used for printing integer
values, “%f” for floating point
values, and “%c” for character
values. These are called format specifiers. You can give any other characters
too in the format string. These characters will be printed as they are. The
number and order of format specifiers should be same as that of the variables
to be printed. To use the function printf(), include the instruction
#include<stdio.h> at the beginning of the program.
This instruction is preprocessor
directive and will be explained later. A few examples of this function are
given below-
printf(“Number of students : %d”, num);
printf(“%d %f”, num, weight);
printf(“%c”, a);
printf(“Basic salary is %d\nGross salary
is %f”, basic, gross);
Please note that in the last
statement, “\n” has been used in the
format string. This is the newline
character, and it takes the cursor to the new line, wherever it is used.
That’s why; output of this statement will be shown in two lines like this-
Basic
salary is 5000
Gross
salary is 8500.50
If you want a program to
receive input from keyboard during execution, here is a library function,
function scanf(). It is the counterpart of
function printf(). The format of this function
is given below-
scanf(“<format_string>”,
&variable1, &variable2,…);
The explanation of format
string is same as in the function printf(). You should give the
format specifiers in the same number and order you want to receive the values
of variables from the keyboard. The ampersand (&) before the variable name is the “address of” operator. It gives the location number or address of
variable in the memory. When we say &variable_name,
it tells to the function scanf() at which memory
location, value, supplied by the user through the keyboard, should be stored.
For example:
scanf(“%d %d %d”, &n1, &n2,
&n3);
scanf(“%f”, &weight);
scanf(“%c”, &ch);
If you want to receive more
than one value at a time from the keyboard using only one scanf()
statement, supply these values using either <space>, or <tab>,
or <enter> keys during program
execution.
Now, whatever you have
learnt till now, apply in a C program. Please type this program in your editor.
/* Program to add three numbers */
#include<stdio.h>
#include<conio.h>
int main()
{
int n1, n2, n3, sum;
clrscr(); /* clears the screen */
printf("Enter 3 numbers: ");
scanf("%d %d %d", &n1, &n2, &n3);
sum=n1+n2+n3;
printf("Sum of 3 numbers is %d", sum);
getch(); /* gets a character from
keyboard, but does not
echo it */
return 0;
}
Compile and execute this
program. Output of this program would be like this-
Enter
3 numbers: 12
25
33
Sum
of 3 numbers is 70
In this program, whatever
written between /* */ is comment
entry, and will not be executed.
/* Program to add three numbers */
Next two lines are
preprocessor directives, which you will learn in later posts. These lines
simply asks to compiler to add two files (stdio.h
and conio.h) in the program. File “stdio.h”
is needed for printf() and scanf()
functions, and file “conio.h”, for clrscr() and getch() functions.
#include<stdio.h>
#include<conio.h>
In this program, there is
only one function, called main(). A C program can contain
more than one function. Function main() is necessary for
execution and execution starts from function main().
Return type of this function is “int”,
which means, it returns an integer value to the operating system. You will
learn more about return type of a function in the later posts.
int main()
{
…;
…;
…;
return 0;
}
The first thing we do in a
function is to declare all the necessary variables. In this program, we have
declared four integer type variables.
int n1, n2, n3, sum;
Next statement clears the
screen and positions the cursor at the top-left corner of the screen. We do
this, because it may be that the screen is showing something from the previous
program. Function clrscr() is a library function.
clrscr();
Next statement prompts the
user to enter three numbers.
printf("Enter 3 numbers: ");
Now, function scanf() waits for the numbers to be entered. You can enter three
numbers separating them either by pressing <space>,
or <tab>, or <enter> key.
scanf("%d %d %d", &n1,
&n2, &n3);
The next statement adds the
numbers and assigns the result in the variable “sum”.
sum=n1+n2+n3;
“+” is the arithmetic operator for addition. “=” is the assignment operator. It is different from “equal to” operator in mathematics. It
assigns the value of right-hand-side expression to the left-hand-side variable.
There are 45 operators in the C
computer language. You will learn more about operators in the later posts.
Next statement prints the
result on the screen.
printf("Sum of 3 numbers is
%d", sum);
Now, the function getch() waits a key-press. If we don’t use this function,
output screen would not show up. That’s why, to hold the screen we have used
this function.
getch();
Last statement returns the
value ‘0’ to the operating system. ‘0’ indicates success.
return 0;
Now, this simple program is
well explained. I hope you have a good foundation at your disposal. I recommend
you to read all my posts from the very beginning.
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