Lesson 3: Variables & Data Types

📦 Lesson 3: Variables & Data Types

⏱️ Estimated time: 25 minutes | Difficulty: Beginner

Introduction

In C, you need to tell the computer exactly what kind of data you are storing. This is called a "type".

What Are Variables?

Variables are named containers that store data in your program. In C, you must declare the type of data a variable will hold before using it.

Declaring & Initializing Variables:

#include <stdio.h>

int main() {
    // Declaration (creating a variable)
    int age;

    // Initialization (giving it a value)
    age = 25;

    // Declaration + Initialization (in one line)
    int score = 95;
    float height = 5.9;
    char grade = 'A';

    // Print them
    printf("Age: %d\n", age);
    printf("Score: %d\n", score);
    printf("Height: %.1f\n", height);
    printf("Grade: %c\n", grade);

    return 0;
}

C Data Types

Type	Size	Format	Example
`int`	4 bytes	`%d`	42, -7, 0
`float`	4 bytes	`%f`	3.14, -0.5
`double`	8 bytes	`%lf`	3.14159265
`char`	1 byte	`%c`	'A', 'z', '9'
`short`	2 bytes	`%hd`	Small integers
`long`	8 bytes	`%ld`	Large integers

Getting User Input with scanf

#include <stdio.h>

int main() {
    int age;
    char name[50];  // String (array of characters)

    printf("Enter your name: ");
    scanf("%s", name);       // Read a string

    printf("Enter your age: ");
    scanf("%d", &age);       // Read an integer (note the &)

    printf("Hello %s! You are %d years old.\n", name, age);
    return 0;
}

⚠️ Important: Notice the & before age in scanf? That's the "address-of" operator. scanf needs the memory address to store the value. Strings (char arrays) don't need it because the array name already IS an address!

Type Casting

int a = 10, b = 3;
printf("%d\n", a / b);           // 3 (integer division!)
printf("%.2f\n", (float)a / b);  // 3.33 (cast to float first)