Variables in C Programming

🎓 Lecture Article: Variables in C Programming

What is a Variable?
In C programming, a variable is a named memory location used to store data that can be changed during the execution of the program.

💡 Think of a variable as a labeled container where you can store, modify, and retrieve data.

🧠 Why Use Variables?

  • To store user input or calculated values.
  • To reuse and manipulate data during execution.
  • To make programs flexible, dynamic, and interactive.

🧾 Syntax for Declaring a Variable

data_type variable_name;

Or with initialization:

data_type variable_name = value;

✅ Examples:

int age;
float salary = 25000.75;
char grade = 'A';

📚 Common Data Types

Data Type Description Example Value
int Integer numbers 10, -50
float Decimal (floating point) numbers 3.14, -0.005
char Single characters 'A', 'z'
double Double-precision float 1.23456789

📌 Variable Naming Rules

  • Must begin with a letter (A–Z, a–z) or an underscore _
  • Can include letters, digits, and underscores
  • Cannot use C keywords (int, float, etc.)
  • C is case-sensitive: Total and total are different
  • Avoid using special characters (e.g., @, #, %)

✅ Valid Names:

int age;
float salary_2024;
char _grade;

❌ Invalid Names:

int 2ndItem;    // Starts with a digit
float salary$;  // Contains special character
char int;       // 'int' is a keyword

📦 Types of Variables (By Scope & Storage Class)

Type Scope Storage Location Lifetime
Local Inside function/block Stack Until block ends
Global Outside all functions Data segment Entire program
Static Local/global Data segment Entire program
Extern Declared elsewhere Data segment Entire program
Register CPU register CPU (if possible) Block

🧪 Example Program

#include <stdio.h>

int main() {
    int age = 25;
    float height = 5.9;
    char grade = 'B';

    printf("Age: %d\n", age);
    printf("Height: %.1f feet\n", height);
    printf("Grade: %c\n", grade);

    return 0;
}

🔍 Output:

Age: 25
Height: 5.9 feet
Grade: B

❗ Common Mistakes with Variables

  • Uninitialized use: int a; printf("%d", a); ❌ Unpredictable output
  • Redeclaring inside same scope: int a = 5; int a = 10; ❌ Error: redefinition
  • Using incorrect types: char name = "Alex"; ❌ Wrong: should be a string (char array)

💡 Best Practices

  • Use descriptive names: totalMarks, userAge, itemPrice
  • Avoid single-letter names (except in loops)
  • Always initialize your variables
  • Match data type to the kind of data you’re storing

📝 Summary

  • A variable is a named memory location used to store data.
  • C is a statically typed language — variable types must be declared.
  • Variable types, naming conventions, and scope are crucial to program behavior.
  • Use good naming conventions and always initialize your variables.

🎯 Practice Exercise

  1. Declare and initialize variables of each data type.
  2. Write a program to calculate the area of a rectangle using length and width variables.
  3. Try using a variable without initialization — what happens?

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