Variables, Data Types & Constants

Duration: 50 min

Variables are containers for storing data values. C is a statically-typed language, meaning you must declare the type of each variable before using it. Understanding data types is fundamental to writing efficient C programs.

Basic Data Types

C provides several fundamental data types:

#include <stdio.h>

int main() {
    int age = 25;              // Integer: -2147483648 to 2147483647
    float height = 5.9;        // Floating-point: ~6-7 decimal places
    double weight = 72.5;      // Double precision: ~15 decimal places
    char grade = 'A';          // Single character
    
    printf("Age: %d\n", age);
    printf("Height: %.1f\n", height);
    printf("Weight: %.1f\n", weight);
    printf("Grade: %c\n", grade);
    
    return 0;
}

Variable Declaration & Initialization

#include <stdio.h>

int main() {
    // Declaration without initialization
    int x;
    
    // Declaration with initialization
    int y = 10;
    
    // Multiple declarations
    int a = 5, b = 10, c = 15;
    
    // Using variables
    int sum = a + b + c;
    printf("Sum: %d\n", sum);
    
    return 0;
}

The sizeof Operator

#include <stdio.h>

int main() {
    printf("Size of int: %lu bytes\n", sizeof(int));
    printf("Size of float: %lu bytes\n", sizeof(float));
    printf("Size of double: %lu bytes\n", sizeof(double));
    printf("Size of char: %lu bytes\n", sizeof(char));
    
    // Typical output on 64-bit systems:
    // Size of int: 4 bytes
    // Size of float: 4 bytes
    // Size of double: 8 bytes
    // Size of char: 1 byte
    
    return 0;
}

Constants

Constants are variables whose values cannot be changed after initialization:

#include <stdio.h>

int main() {
    // Using const keyword
    const int MAX_USERS = 100;
    const float PI = 3.14159;
    
    // This would cause a compilation error:
    // MAX_USERS = 200;  // Error: assignment of read-only variable
    
    printf("Max users: %d\n", MAX_USERS);
    printf("Pi: %.5f\n", PI);
    
    return 0;
}

Naming Conventions & Best Practices

#include <stdio.h>

int main() {
    // Good variable names (descriptive, lowercase with underscores)
    int student_age = 20;
    float average_score = 85.5;
    char first_initial = 'J';
    
    // Constants in UPPERCASE
    const int MAX_ATTEMPTS = 3;
    const float GRAVITY = 9.8;
    
    // Avoid single letters except for loop counters
    // int x = 10;  // Not descriptive
    // int num_items = 10;  // Better
    
    return 0;
}

Quiz 1: Data Types

Quiz 2: sizeof Operator

Quiz 3: Constants

Quiz 4: Variable Initialization

Quiz 5: Float vs Double