Day 2: Introduction to C Programming#

Overview#

Now that we have our environment set up, let’s dive into the fundamentals of C programming. We’ll learn the structure of a C program, how it works, and how to use basic input/output functions.

What We’ll Learn Today#

• The structure of a C program
• Understanding the main function
• Basic input and output (printf, scanf)
• Writing interactive programs
• Common pitfalls to avoid

The Structure of a C Program#

Every C program follows this basic structure:

#include <stdio.h>        // 1. Include libraries

int main() {              // 2. Main function
    // 3. Variable declarations
    int age = 25;

    // 4. Program statements
    printf("Hello, World!\n");

    // 5. Return statement
    return 0;
}

Breaking It Down#

1. Preprocessor Directives

#include <stdio.h>

• #include tells the compiler to include a file
• <stdio.h> - Standard Input/Output library
• Must appear at the top of the file

2. Main Function

int main()

• Every C program needs exactly one main() function
• Execution always starts here
• int means it returns an integer

3. Function Body

{
    // Code goes here
}

• Curly braces {} define the function body
• Code is executed sequentially

4. Return Statement

return 0;

• Ends the program
• 0 indicates successful execution
• Non-zero values indicate errors

Basic Output: printf()#

printf() is used to print text and values to the console.

Simple Text Output#

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

Output:

Hello, World!

Common Escape Sequences#

Printing Variables#

#include <stdio.h>

int main() {
    int age = 25;
    float height = 5.9;
    char initial = 'J';

    printf("Age: %d\n", age);
    printf("Height: %.1f\n", height);
    printf("Initial: %c\n", initial);

    return 0;
}

Output:

Age: 25
Height: 5.9
Initial: J

Format Specifiers#

Basic Input: scanf()#

scanf() is used to read input from the user.

Syntax#

scanf("format", &variable);

Important: Notice the & before the variable name! This is very important for pointers (we’ll learn more later).

Example: Reading an Integer#

#include <stdio.h>

int main() {
    int number;

    printf("Enter a number: ");
    scanf("%d", &number);

    printf("You entered: %d\n", number);

    return 0;
}

Sample Run:

Enter a number: 42
You entered: 42

Reading Different Data Types#

#include <stdio.h>

int main() {
    int age;
    float salary;
    char grade;

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Enter your salary: ");
    scanf("%f", &salary);

    printf("Enter your grade: ");
    scanf(" %c", &grade);  // Note the space before %c

    printf("\nYour Details:\n");
    printf("Age: %d\n", age);
    printf("Salary: %.2f\n", salary);
    printf("Grade: %c\n", grade);

    return 0;
}

Complete Example: Interactive Program#

Let’s write a program that calculates the area of a rectangle:

#include <stdio.h>

int main() {
    float length, width, area;

    printf("=== Rectangle Area Calculator ===\n");
    printf("Enter length: ");
    scanf("%f", &length);

    printf("Enter width: ");
    scanf("%f", &width);

    area = length * width;

    printf("\n--- Results ---\n");
    printf("Length: %.2f\n", length);
    printf("Width: %.2f\n", width);
    printf("Area: %.2f\n", area);

    return 0;
}

Sample Run:

=== Rectangle Area Calculator ===
Enter length: 5.5
Enter width: 3.2

--- Results ---
Length: 5.50
Width: 3.20
Area: 17.60

Understanding the Program Flow#

START
  ↓
Print "Enter length: "
  ↓
Read length from user
  ↓
Print "Enter width: "
  ↓
Read width from user
  ↓
Calculate: area = length × width
  ↓
Print results
  ↓
return 0
  ↓
END

Common Mistakes and How to Avoid Them#

Mistake 1: Forgetting the & in scanf()#

// ❌ Wrong
scanf("%d", number);

// ✅ Correct
scanf("%d", &number);

Mistake 2: Using Wrong Format Specifier#

// ❌ Wrong
float price = 9.99;
scanf("%d", &price);  // Should be %f

// ✅ Correct
scanf("%f", &price);

Mistake 3: Not Initializing Variables#

// ❌ Can produce garbage values
int x;
printf("%d\n", x);

// ✅ Always initialize
int x = 0;
printf("%d\n", x);

Mistake 4: Forgetting newlines#

// ❌ Output runs together
printf("Name: ");
printf("Age: ");

// ✅ Better formatting
printf("Name: \n");
printf("Age: \n");

Practice Exercises#

Exercise 1: Temperature Converter#

Write a program that:

• Asks user for temperature in Celsius
• Converts to Fahrenheit using: F = (C × 9/5) + 32
• Prints the result

Exercise 2: Simple Grading System#

Write a program that:

• Takes 3 test scores
• Calculates the average
• Prints the average

Exercise 3: Personal Information#

Write a program that:

• Asks for name (use a string - we’ll learn more tomorrow)
• Asks for age
• Asks for favorite number
• Prints all information in a nice format

Summary#

✅ Learned C program structure
✅ Understood main() function
✅ Used printf() for output
✅ Used scanf() for input
✅ Created interactive programs
✅ Learned common format specifiers

Key Points to Remember#

1. Every C program starts with main()
2. Use #include <stdio.h> for input/output
3. Always use & with scanf() (we’ll understand why later)
4. Match format specifiers to data types
5. Test your programs with different inputs

Next Steps#

Tomorrow we’ll learn about data types in detail, how to declare variables, and work with constants. We’ll also explore different ways to store and manipulate data!

→ Continue to Day 3

← Back to Day 1