1. Variables in C

• C variable is a named location in a memory where a program can manipulate the data. This location is used to hold the value of the variable.
• A variable is a data name that may be used to store a data value.
• It remains unchanged during the execution of program.
• To indicate storage area, each variable should be given a unique name.
• Variable names are case sensitive. Like num, Num, NUM these all three are different variable names.

Some examples of such names are –

Average, Height, Total, Counter_1, Class_10

Rules for Constructing Variable Name:

1. 1. Characters Allowed:
   • Underscore(_)
   • Capital Letters ( A – Z )
   • Small Letters ( a – z )
   • Digits ( 0 – 9 )
2. Blanks & Commas are not allowed.
3. No Special Symbols other than underscore(_) are allowed.
4. First Character should be alphabet or Underscore.
5. Variable name should not be reserved word.

Some valid variable’s name in C –

number, number1, _number, NUM_1, num_, num_of_students

Some invalid variable’s name in C –

2. Local Variable in C

• A local variable in C is declared inside a function.
• It is visible only inside their function, only statements inside function can access that local variable.
• Local variables is declared when control enters a function and destroyed, when control exits from function.

Program to understand the concept of Local variable in C

#include <stdio.h>

int add(int x, int y)
{
  int sum = x + y;  /* Variable sum is local variable of Add function  and can only be accessed from within add()*/ 
  return sum;
}

int main()
{
    /* a and b are local variables of main function so that main function can access a and b,add() cannot access Variable a and b*/

    int a = 5, b = 10; 
    printf(“Sum = %d”, add(a, b));
    return 0;
}

OUTPUT

Sum = 15

3. Global Variable in C

• A variable that is declared outside of all functions.
• It holds their values between function calls and throughout the program execution.

Program to understand the concept of Global Variable in C

#include<stdio.h>

void demo();       //Function Declaration

int a = 10, b = 20;  

// Here a and b are Global variable that can be accessed by main() function and  demo() function

int main()
{

    int x = 50;

    // x is local variable of main function , can only accessed by main function

   printf("\n Global Variables: a=%d b=%d", a,b);

   printf("\n Local Variable of main function: x = %d", x);

   demo();

   return 0;
}

void demo()
{

  int p = 100;   

  // p is a local variable of demo function, can only accessed by demo function

  printf("\n Global Variables: a = %d b = %d", a,b);
  printf("\n Local Variable of demo function: p = %d", p);
}

Output

Global Variables: a = 10 b = 20

Local Variable of main function: x = 50

Global Variables: a = 10 b = 20

Local Variable of demo function: p = 100