Variables and Data Types

# CHAPTER 4

Variables and Data Types #

1. Introduction #

2. Learning Objectives #

• Declare and initialize variables.

• Understand primitive data types (int, float, char, double).

• Use format specifiers in printf().

• Create constants using const and #define.

• Understand type conversion (casting).

3. What is a Variable? #

Syntax: dataType variableName = value;

int age = 25;       // 'age' is a variable holding an integer
float price = 19.99; // 'price' is a variable holding a decimal
char grade = 'A';    // 'grade' is a variable holding a character

4. Data Types in C #

5. Format Specifiers and Printing Variables #

#include <stdio.h>

int main() {
    int score = 100;
    float temp = 98.6;
    char initial = &#039;J&#039;;

    printf("Score: %d\n", score);
    printf("Temperature: %.2f\n", temp); // %.2f limits to 2 decimal places
    printf("Initial: %c\n", initial);

    return 0;
}

6. Variable Naming Rules #

• Must begin with a letter or underscore ().

• Best Practice: Use descriptive names (playerscore instead of ps).

7. Constants #

Method 1: Using const keyword

const float PI = 3.14159;
// PI = 3.14; // ERROR: assignment of read-only variable

Method 2: Using #define (Macro)

#define MAX_USERS 100
// Replaces every instance of MAX_USERS with 100 before compiling.

8. Type Conversion (Type Casting) #

Implicit Conversion (Automatic): Smaller types are safely promoted to larger types.

int num = 10;
float decimal = num; // Automatically becomes 10.000000

Explicit Conversion (Manual): You force the conversion by placing the target type in parentheses.

float price = 9.99;
int rounded = (int) price; // Forces conversion. 'rounded' becomes 9 (truncates decimal)

9. Memory-Level Explanation #

1. 1. The compiler sees int and allocates 4 contiguous bytes of memory on the Stack.

1. 2. It notes that the memory address (e.g., 0x7ffee123) is labeled age.

1. 3. It converts 25 to binary (00011001) and stores it in those 4 bytes.

Memory Stack:
Address      Value      Variable
0x1000       00000000   
0x1004       00011001   <- age (4 bytes)
0x1008       'A'        <- initial (1 byte)

10. Common Mistakes #

• Using single quotes for strings: 'Hello' is wrong. Single quotes are ONLY for single characters ('H'). Use double quotes for strings.

• Mismatched format specifiers: Printing a float using %d will result in garbage values or 0.

• Forgetting f on floats: float pi = 3.14; technically creates a double. Use float pi = 3.14f;.

11. Exercises #

1. 1. Declare variables to store your age, height in meters (float), and blood type letter. Print them out in a single sentence.

1. 2. Create a constant for the speed of light (299792458) and print it.

1. 3. Write a program that divides 5 by 2 using integers, then fix it using type casting to get the correct 2.5 answer.

12. MCQ Quiz with Answers #

13. Interview Questions #

• Q: What is the difference between #define and const?

• Q: Explain implicit vs. explicit type casting.

• Q: What happens if you assign a larger value to an integer than it can hold? (Integer overflow).

14. Summary #

15. Next Chapter Recommendation #