What is Variable in C Language -Data Types, Rules, Example

Variables are the foundation of programming in C. They allow you to store and manipulate data. This makes your programs dynamic and interactive. When you write a program, you need variables to hold numbers, text, or other values that the program uses.

What is Variable in C

A variable is a name given to a memory location where data is stored. Every variable has three main parts:

1. Name: A unique identifier for the variable (like age or marks).
2. Type: The kind of data it can store (like numbers, decimals, or letters).
3. Value: The actual data stored in the variable (like 20 or 'A').

Example:

int age = 15;  

Here, age is the variable name, int is the data type, and 15 is the value.

Types of Variables in C

Variables in C can be of different types based on where they are declared and how long they exist.

1. Local Variables

• Declared inside a function.
• It can only be used in that function.

void myFunction() {  
    int x = 10;  // local variable  
}

2. Global Variables

• Declared outside all functions.
• It can be used anywhere in the program.

int score = 100;  // global variable  
void checkScore() {  
    printf("%d", score);  // works  
}

3. Static Variables

• Retains its value between function calls.

void counter() {  
    static int count = 0;  
    count++;  
    printf("%d", count);  
}  
// Output: 1, 2, 3... (remembers previous value)

4. Automatic Variables

• Default type (created when a function starts, destroyed when it ends).

void demo() {  
    auto int x = 5;  // same as 'int x = 5;'  
}

5. External Variables

• Used to share variables between multiple C files.

extern int globalVar;  // declared in another file

Data Types in C

Variables store different types of data. C has four main data types:

Data Type	Example	Usage
int	int age = 15;	Stores whole numbers
float	float pi = 3.14;	Stores decimal numbers
char	char grade = 'A';	Stores single characters
double	double bigNum = 1234.5678;	Stores large decimal numbers

Key Points:

• int = Whole numbers (no decimals).
• float = Decimal numbers (6-7 digit precision).
• double = Bigger decimal numbers (15-16 digit precision).

Rules for Naming Variables in C

When you create a variable, you must follow certain rules:

1. Start with a letter or underscore (_). You cannot start a variable name with a number. For example, score and _count are valid, but 1stStudent is invalid.
2. Use only letters, numbers, or underscores. Spaces and special symbols (like @, #) are not allowed. For example, total_score is valid, but total score is not.
3. C is case-sensitive. This means Age, AGE, and age are three different variables.
4. Do not use C keywords. Words like int, float, and return cannot be variable names because they have special meanings in C.

A good practice is to use meaningful names. For example, use studentAge instead of just x so that your code is easier to understand.

How to Declare and Initialize Variables

Declaring a variable means telling the compiler its name and type. Initializing means giving it a value.

Declaration

Creating a variable by specifying its type and name.

int age; // Declared but not initialized  

At this point, age has no value, and using it may give unexpected results.

Initialization

Assigning a value to the variable for the first time.

age = 20; // Now it has a value  

Declaration + Initialization (Best Way)

You can also declare and initialize in one step:

int marks = 95; // Declared and initialized

Constants in C

A constant is a variable whose value cannot be changed.

Example:

const float PI = 3.14159;  

If you try to change it later, the compiler will give an error.

Constants are useful for values that should never change, like mathematical constants.

Common Mistakes with Variables

Beginners often make these mistakes:

1. Using Uninitialized Variables

int x;  
printf("%d", x); // Prints garbage value 

Always assign a value before using a variable.

2. Wrong Data Type

float a = 10; // Works, but 'int' is better for whole numbers  

Use int for whole numbers and float for decimals.

3. Misspelling Variable Names

int age = 20;  
printf("%d", Age); // Error (case-sensitive)  

Always check spelling and case.

Practical Example of a C Program

Let’s write a simple C program using variables:

#include <stdio.h>
int main() {
    int num1, num2, sum;
    printf("Enter two numbers: ");
    scanf("%d %d", &num1, &num2);
    sum = num1 + num2;
    printf("Sum = %d", sum);
    return 0;
}

Explanation:

1. num1 and num2 store user input.
2. sum calculates and displays the result.