Variable and Constant in C Programming

In here we will learn about Variable and Constant in C Programming. What is variable ? What is Constant ? What are the Variable Declaration rules ? The ranges of variables Why we use constants and types of constant

What is variable?

Variables: A variable is nothing but a name of a memory address that our program can manupulate . You can consider as an empty pot that you can put something or some value . In C programming, it has different data types that we have learnt previous chapter . In a variable which type value you want to place and what is ranges it depends on the data type.In C programming there is some rules to declare and use of the variables.


Variable Declaration:

  Data_Type Variable_name;//must give a space between data type and variable name.

Rules for Naming variables:

  1. Variable names must begin with a letter of the alphabet or an underscore( _ ) but for beginner programmer I will suggest you to start with alphabet.

  2. After the first initial letter you can use other letters, numbers etc but you can not have space in a variable name

  3. You can not use same name for different variables.

  4. If you are declare more than one variable of same type must be separate with ","(comma) and must give semicolon(;) end of the line.

  5. You can not use the keywords(reserved words)that we shown on previous chapter as a variable name .

  6. We can not duplicate variable name in the same function.

Let's see an example
  
  #include<stdio.h>
  int main(){
    int a; // declare a single variable
    int c, d; //declare more than one variable of same data type.
    float  ad, salary; //declare float type variable
    double n; //declare double type variable
    char g; // declare a Charater type variable
    return 0;
  }

Ranges of Data type

DATA TYPE RANGES
short int -32,768 to 32,767
unsigned short int 0 to 65,535
int -2,147,483,648 to 2,147,483,647
long int -2,147,483,648 to 2,147,483,647
long long int -263 to 263-1
char 1 Charater
float -2,147,483,648 to 2,147,483,647
double -263 to 263-1

There is two types of variables-

  1. Local variables:If you decalare variable in between a function like main()or any other function that is call local variables . You can not access the Local variables outside the function where it is declared.

  2. Global variables:If declare the variable outside of the function that is call global variable.You can access the Global variables from any where of your program.

For an example:

  #include<stdio.h>
  int  a,b,c; //global variables. 
  int main() {
    int x,y,z; //local variable
    return 0;
  }

Constants: The Constants is a fixed value that you can not change using any of operators in your program.It is used to get rid of writing same value over and over again. like value of PI 3.14159265358979323846264338327950288 ....... if you need to write same value for several times is very much annoying so instead of that you can declare it as a constant. There are two different ways to declare constant and these are -

1.Using #define.
2.Using const key before declare variables.
See the following example: 


  #include<stdio.h>
  #define PI =  3.14159265358979323 //using define
  int main() {
   const int a = 3.14159265358979323; // using const. Must use ; end of the line 
   return 0;
  }