📘 Section 1 — Introduction to C

Your first steps into C programming — from writing and compiling code to variables, I/O, and conditional logic.

🎯 What You’ll Learn

• How a C program is compiled and executed
• Program structure in C
• Output with printf and input with scanf
• Data types and variables
• Arithmetic and logical precedence
• Conditional statements (if, else, switch)

1. Program Execution Process

Before your code runs, it goes through several stages:

1. Editor (Word Processor) — You write your source code (text format)
2. Compiler — Attempts to translate your code to machine code (binary). If unsuccessful, it shows error messages
3. Linker — Resolves cross references among object files
4. Loader — Copies the executable file into memory and initiates execution

2. Program Structure in C

Every C program follows this basic structure:

#include <stdio.h>    // Collection of pre-compiled functions

int main() {          // Main function, where program starts
    printf("Hello World\n");
    return 0;         // Returns with code 0 (meaning the program successfully closed)
}

3. Output with printf

printf is a function used to print output to the console.

printf("Hello World\n");            // Simple text
printf("I am %d years old\n", 22); // With a format specifier

Format Specifiers

4. Data Types

5. Variable Declaration and Initialization

int n;      // Declaration
n = 10;     // Initialization

The sizeof() Function

sizeof() returns the size of a data type. It can also be used with a variable.

printf("Size of int: %zu\n", sizeof(int));

int n;
printf("Size of n: %d\n", sizeof(n));

💡 %zu is a format specifier used for size_t data type, but it is interchangeable with %d in this case.

6. Input with scanf

scanf is a function used to take input from user through the console.

int n;
scanf("%d", &n);
printf("I am %d years old.\n", n);

⚠️ Important: Be careful when using scanf to read a variable. The & symbol must be used, which indicates “address”.

7. Arithmetic and Logical Precedence

Examples on Precedence

1)  6 != 6 < 0 || 8 == 8 > 0 && 5 != 5
2)  6 != 2 * 3 || 5 * 2 && 8 % 8
3)  3 * 4 == 12 || 6 / 2 && 8 % 4

8. Conditional Statements

if Statement

if (condition) {
    // your code
}

A Tricky Example

int age = 20;
int x = 3;
if (age > x)
    printf("inside");
printf("outside");

insideoutside

if-else Statement

if (condition) {
    // body of if
} else {
    // body of else
}

Nested if Statement

if (condition1) {
    if (condition2) {
        // statements
    }
}

switch Statement

switch (expression) {
    case constant1:
        // code
        break;
    case constant2:
        // code
        break;
    default:
        // code
}

🧪 Exercises

Exercise 1: Rectangle Area & Perimeter

Task: Design a program that accepts the length and width from the user and computes the perimeter and area of a rectangle.

#include <stdio.h>

int main() {
    float l, w, area, perimeter;
    printf("Please enter the length: ");
    scanf("%f", &l);
    printf("Please enter the width: ");
    scanf("%f", &w);
    area = l * w;
    perimeter = (l + w) * 2;
    printf("Area = %f\nPerimeter = %f\n", area, perimeter);
    return 0;
}

Exercise 2: Celsius to Fahrenheit

Task: Write a program that takes degrees centigrade from the user and displays the same temperature in degrees Fahrenheit. The formula is: Fahrenheit = 9/5 × Centigrade + 32

#include <stdio.h>

int main() {
    float c, f;
    printf("Enter temperature in degrees Centigrade: ");
    scanf("%f", &c);

    f = 9 / 5.0 * c + 32;

    printf("Temperature in degrees Fahrenheit: %.3f\n", f);

    return 0;
}

Exercise 3: Trace the Code

Trace the following code and determine the output.

Part A:

int a = 3, b = -5, c;
if (a < 2 * b) {
    b = a;
    c = b;
    printf("%d\n", c);
} else if (b > 0)
    b = 0;
printf("%d\n", b);

-5

Part B:

int k, x, a;
k = 0;
a = 5;
if (k = 0)           // NOTE: This is assignment, not comparison!
    if (a > 0) {
        x = a + k;
        printf("a= %d k= %d x= %d \n", a, k, x);
    } else {
        a = k + 1;
        k = a + 1;
    }
else {
    if (a < 0)
        x = a + 1;
    else
        x = a - 1;
    printf("a= %d k= %d x= %d \n", a, k, x);
}
printf("a= %d k= %d \n", a, k);

a= 5 k= 0 x= 4 
a= 5 k= 0 

Exercise 4: Simple Calculator

Task: Write a program to input two numbers from the user and display their arithmetic results (+, -, *, /, %).

#include <stdio.h>

int main() {
    int n1, n2;
    printf("Enter the first number: ");
    scanf("%d", &n1);
    printf("Enter the second number: ");
    scanf("%d", &n2);

    printf("Addition: %d + %d = %d\n", n1, n2, n1 + n2);
    printf("Subtraction: %d - %d = %d\n", n1, n2, n1 - n2);
    printf("Multiplication: %d * %d = %d\n", n1, n2, n1 * n2);

    if (n2 != 0) {
        printf("Division: %d / %d = %f\n", n1, n2, (float)n1 / n2);
        printf("Modulus: %d %% %d = %d\n", n1, n2, n1 % n2);
    } else {
        printf("Division by zero is not possible.\n");
    }

    return 0;
}

Exercise 5: Cartesian Plane Quadrant

Task: Accept the x-y coordinates of a point in the Cartesian plane and print a message telling either an axis on which the point lies or the quadrant in which it is found.

Sample outputs:

(-1.0, -2.5) is in quadrant III
(0, 4.8) is on the y-axis

#include <stdio.h>

int main() {
    double x, y;
    printf("Enter the x coordinate: ");
    scanf("%lf", &x);
    printf("Enter the y coordinate: ");
    scanf("%lf", &y);

    if (x == 0 && y == 0) {
        printf("(0,0) is at the origin");
    } else if (x == 0) {
        printf("(0,%.1f) is on the y-axis", y);
    } else if (y == 0) {
        printf("(%.1f,0) is on the x-axis", x);
    } else if (x > 0 && y > 0) {
        printf("(%.1f,%.1f) is in quadrant I", x, y);
    } else if (x < 0 && y > 0) {
        printf("(%.1f,%.1f) is in quadrant II", x, y);
    } else if (x < 0 && y < 0) {
        printf("(%.1f,%.1f) is in quadrant III", x, y);
    } else if (x > 0 && y < 0) {
        printf("(%.1f,%.1f) is in quadrant IV", x, y);
    }

    return 0;
}

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