Variables In C

In the language C, a variable is a named memory address that is used to store and retrieve data. The variable can be used to hold many kinds of data and can be reused an unlimited number of times to store different sorts of data.

They can be thought of as the names assigned to the memory location, allowing us to access it without needing to commit the memory address to memory. The type of data that the variable stores determines its size.

int marks;    // integer variable
char ch;     // character variable
float amount;  // float variables

Variable Declaration in C

A variable declaration provides assurance to the compiler that there exists a variable with the given type and name so that the compiler can proceed for further compilation without requiring the complete detail about the variable. A variable definition has its meaning at the time of compilation only, the compiler needs actual variable definition at the time of linking the program.

Variable Definition in C

A variable definition tells the compiler where to store the variable and how much space it needs. A variable definition lists one or more variables of a certain type and specifies the type of data they hold.

type variable_list;

And in this case, type must be a valid C data type, like, char, int, float, double, bool, or any user-defined object. variable list can have one or more identifier names, separated by commas.

int    x, y, z;
char   c, ch;
float  f, amount;
double d;

Variable Initialization in C

Initialization of a variable is the process where the user assigns some value to the variable.

int marks;    // variable definition
marks = 70;  //  initialization
            // OR
int marks = 70;  //  variable definition + initialization 
    

How to use variables in C?

You can see an example of this below. Variables are declared at the top, but they are defined and set up in the main function.

#include <stdio.h>
 
int main()
{
    // declaration with definition
    int marks;
 
    printf("Marks: %d: ", marks);
 
    // initialization
    marks = 70;
 
    // declaration + definition + initialization
    int total_marks = 150;
 
    printf("Value of defined_var after initialization: %d\n",marks);
    printf("Value of total marks: %d", total_marks);
 
    return 0;
}

Marks: 0
Value of marks after initialization: 70
Value of total marks: 150

The same idea applies to declaring functions: you give them a name when you declare them, but you can give their definition anywhere else.

#include <stdio.h>

// function declaration
int func();
 
int main() {
 
   // function call
   int i = func();
   printf("Function value: %d",i);
}
 
// function definition
int func() {
   return 0;
}

Function value: 0

1. Local Variables in C

A Local variable is declared inside a function or a block of code. Its scope is limited to the block or function in which it is declared.

// declare and print local variable inside a function.
#include <stdio.h>
 
void function()
{
    int marks = 70; // local variable
    printf("%d", marks);
}
 
int main() 
{ 
    function(); 
}

Function value: 70

2. Global Variables in C

A Global variable is declared outside the function or a block of code. Its scope is the whole program i.e. we can access the global variable anywhere in the C program after it is declared.

// declare and print global variable.
#include <stdio.h>
 
int marks = 50; // global variable
 
void std1() 
{ 
  printf("std 1: %d\n", marks); 
}
 
void std2() 
{ 
  printf("std 2: %d\n", marks); 
}
 
int main()
{
  printf("The Value of marks: %d\n",marks); 
    std1();
    std2();
    return 0;
}

The Value of marks: 50
std 1: 50
std 2: 50