[Guide for C] Variables in C Language

When we want to store any information(data) on our computer/laptop, we store it in the computer's memory space. Instead of remembering the complex address of that memory space where we have stored our data, our operating system provides us with an option to create folders, name them, so that it becomes easier for us to find it and access it.

Similarly, in C language, when we want to use some data value in our program, we can store it in a memory space and name the memory space so that it becomes easier to access it.

The naming of an address is known as variable. Variable is the name of memory location. Unlike constant, variables are changeable, we can change value of a variable during execution of a program. A programmer can choose a meaningful variable name. Example : average, height, age, total etc.

Datatype of Variable

A variable in C language must be given a type, which defines what type of data the variable will hold.

It can be:
- char: Can hold/store a character in it.
- int: Used to hold an integer.
- float: Used to hold a float value.
- double: Used to hold a double value.
- void

Rules to name a Variable
- Variable name must not start with a digit.
- Variable name can consist of alphabets, digits and special symbols like underscore _.
- Blank or spaces are not allowed in variable name.
- Keywords are not allowed as variable name.
- Upper and lower case names are treated as different, as C is case-sensitive, so it is suggested to keep the variable names in lower case.

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Declaring, Defining and Initializing a variable

Declaration of variables must be done before they are used in the program. Declaration does the following things.
It tells the compiler what the variable name is.
It specifies what type of data the variable will hold.
Until the variable is defined the compiler doesn't have to worry about allocating memory space to the variable.
Declaration is more like informing the compiler that there exist a variable with following datatype which is used in the program.
A variable is declared using the extern keyword, outside the main() function.

extern int a;
extern float b;
extern double c, d;

Defining a variable means the compiler has to now assign a storage to the variable because it will be used in the program. It is not necessary to declare a variable using extern keyword, if you want to use it in your program. You can directly define a variable inside the main() function and use it.

To define a function we must provide the datatype and the variable name. We can even define multiple variables of same datatype in a single line by using comma to separate them.

int a;
float b, c;

Initializing a variable means to provide it with a value. A variable can be initialized and defined in a single statement, like:

int a = 10;

Let's write a program in which we will use some variables.

#include <stdio.h>

// Variable declaration(optional)
extern int a, b;
extern int c;

int main () {

    /* variable definition: */
    int a, b;
 
    /* actual initialization */
    a = 7;
    b = 14;
    /* using addition operator */
    c = a + b;
    /* display the result */
    printf("Sum is : %d \n", c);
    
    return 0;
}

Sum is : 21

You must be thinking how does this printf() works, right? Do not worry, we will learn about it along with other ways to input and output data in C language in the next tutorial.