Module 1: Introduction to C

• Overview of C
• Operator & If-Else Statement
• Loop & Switch-Case
• Nested Statements / Loops

Overview of C

INTRODUCTION TO C LANGUAGE

C is a general-purpose programming language that is closely related to how computer machines work. Although often considered difficult to learn, C is actually a simple language with vast capabilities.

Here are some key points to note in C:

• Case-sensitive: C distinguishes between uppercase and lowercase letters. For example, printf and Printf are two different things.
• Space-insensitive: Separators such as spaces, tabs, or new lines do not affect the program.
• Semicolon: Every statement must end with a semicolon (;).
• Multiple Statements: Several statements can be written on the same line.

---

SIMPLE C PROGRAM: PRINTING A LINE OF TEXT

The simplest C program is a program that prints text. Here is an example:

#include <stdio.h>

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

Output:

Hello, World!

Parts of the Program

1. Comment:

   • Single-line comments use //, while multi-line comments use /* ... */.

   // This is a single-line comment
   /* 
      This is a 
      multi-line comment
   */
   

2. Header File:

   • Header files like stdio.h are required to use functions such as printf() or scanf().

   #include <stdio.h>
   

3. Main Function:

   • The main() function is the program’s entry point.
   • int main() indicates that the function returns an integer (0 for success, 1 or more for failure).

   int main() {
       // Program code
       return 0; // Indicates successful program execution
   }
   

4. The printf() Function:

   • This function is used to print output to the screen.
   • \n is an escape sequence meaning newline (new row).

   printf("Hello, World!\n");
   

---

VARIABLES AND DATA TYPES

Variables are "containers" for storing values. The data type determines the kind of value that can be stored in a variable.

Types of Data in C

1. int - For integer values.

   int number1;  // Variable without initialization (random value)
   int number2 = 20;  // Variable initialized with value 20
   

2. float - For decimal values.

   float decimal = 3.14;
   

3. char - For storing a single character.

   char letter = 'A';
   

Here is a complete diagram of data types in C:

Naming Variables

• Variable names must start with a letter or an underscore (_).
• Spaces or punctuation marks (such as ?, !, etc.) are not allowed.
• Case-sensitive: name and Name are different variables.

Example:

int age = 20;      // Valid
float height = 170; // Valid
char initial = 'A'; // Valid
int ageOfFather = 45; // Valid

int 2age = 20;     // Invalid (cannot start with a number)

---

Complete Example

Here is an example program using variables and data types:

#include <stdio.h>

int main() {
    int age = 25;
    float height = 170.5;
    char initial = 'A';

    printf("Age: %d years\n", age);
    printf("Height: %.2f cm\n", height);
    printf("Initial: %c\n", initial);

    return 0;
}

Output:

Age: 25 years
Height: 170.50 cm
Initial: A

Operator & If-Else Statement

1. Arithmetic Operators

a) Addition (+)

Adds two values.

int a = 5, b = 3;
int result = a + b; // Result: 8
printf("a + b = %d\n", result); // Output: a + b = 8

b) Subtraction (-)

Subtracts one value from another.

int a = 10, b = 4;
int result = a - b; // Result: 6
printf("a - b = %d\n", result); // Output: a - b = 6

c) Multiplication (*)

Multiplies two values.

int a = 7, b = 3;
int result = a * b; // Result: 21
printf("a * b = %d\n", result); // Output: a * b = 21

d) Division (/)

Divides two values.

int a = 15, b = 5;
int result = a / b; // Result: 3
printf("a / b = %d\n", result); // Output: a / b = 3

e) Modulus (%)

Returns the remainder of a division.

int a = 10, b = 3;
int result = a % b; // Result: 1 (since 10 ÷ 3 = 3 remainder 1)
printf("a %% b = %d\n", result); // Output: a % b = 1

f) Increment (++)

Increases the value of a variable by 1.

int a = 5;
a++; // Result: 6
printf("a = %d\n", a); // Output: a = 6

g) Decrement (--)

Decreases the value of a variable by 1.

int a = 8;
a--; // Result: 7
printf("a = %d\n", a); // Output: a = 7

---

2. Logical Operators

a) Logical AND (&&)

Returns true if both conditions are true.

int a = 5, b = 10;
if (a > 3 && b < 15) {
    printf("Both are true!\n"); // Output: Both are true!
}

b) Logical OR (||)

Returns true if at least one condition is true.

int a = 7, b = 12;
if (a > 10 || b < 5) {
    printf("At least one is true!\n"); // Will not execute since both conditions are false.
}

c) Logical NOT (!)

Reverses the condition’s result.

int a = 10;
if (!(a < 5)) {
    printf("a is not less than 5!\n"); // Output: a is not less than 5!
}

---

3. Comparison Operators

Comparison operators are used to compare two values and return a boolean (true or false).

a) == (Equal To)

Returns true if both operands are equal.

int a = 5, b = 5;
if (a == b) {
    printf("a and b are equal\n");
}

b) != (Not Equal To)

Returns true if the two operands are not equal.

int a = 5, b = 10;
if (a != b) {
    printf("a and b are different\n");
}

c) > (Greater Than)

Returns true if the left operand is greater than the right.

int a = 10, b = 5;
if (a > b) {
    printf("a is greater than b\n");
}

d) < (Less Than)

Returns true if the left operand is less than the right.

int a = 5, b = 10;
if (a < b) {
    printf("a is smaller than b\n");
}

e) >= (Greater Than or Equal To)

Returns true if the left operand is greater than or equal to the right.

int a = 10, b = 10;
if (a >= b) {
    printf("a is greater than or equal to b\n");
}

f) <= (Less Than or Equal To)

Returns true if the left operand is less than or equal to the right.

int a = 5, b = 10;
if (a <= b) {
    printf("a is smaller than or equal to b\n");
}

---

4. Assignment Operators

a) Simple Assignment (=)

Assigns a value to a variable.

int a;
a = 10; // a now holds 10
printf("a = %d\n", a); // Output: a = 10

b) Compound Assignment (+=, -=, *=, /=, %=)

Combines arithmetic operations with assignment.

int a = 5;
a += 3; // Equivalent to a = a + 3 → Result: 8
a -= 2; // Equivalent to a = a - 2 → Result: 6
a *= 4; // Equivalent to a = a * 4 → Result: 24
a /= 6; // Equivalent to a = a / 6 → Result: 4
a %= 3; // Equivalent to a = a % 3 → Result: 1
printf("Final result of a = %d\n", a); // Output: Final result of a = 1

---

5. If-Else Statement Structure

a) If

Executes code if the condition is true.

int number = 10;
if (number > 5) {
    printf("Number is greater than 5!\n"); // Output: Number is greater than 5!
}

b) If-Else

Executes an alternative code block if the condition is false.

int number = 3;
if (number > 5) {
    printf("Number is greater than 5!\n");
} else {
    printf("Number is not greater than 5!\n"); // Output: Number is not greater than 5!
}

c) Else-If

Adds additional conditions.

int number = 7;
if (number < 5) {
    printf("Number is less than 5!\n");
} else if (number == 5) {
    printf("Number is equal to 5!\n");
} else {
    printf("Number is greater than 5!\n"); // Output: Number is greater than 5!
}

Loop & Switch-Case

Loop & Switch-Case

1. While Loop

A while loop is a function used to execute the same block of code repeatedly. The loop continues execution as long as the given condition evaluates to 1 (TRUE) or more. When the condition evaluates to 0 (FALSE), the loop stops and the program proceeds to the next lines of code.

Similar to an if statement, the while loop is built into the C programming language, meaning there is no need to declare or return its value explicitly.

---

Syntax:

while (condition) {
    // Code to be executed repeatedly
}

The condition is checked before executing the loop body:

• If condition is TRUE, the code inside the loop is executed.
• If condition is FALSE, the loop terminates.

Example 1: Counting from 1 to 10

#include <stdio.h>

int main() {
    int n = 1;

    while (n <= 10) {  // Loop runs while n is less than or equal to 10
        printf("%d\n", n);
        n++;  // Increment n by 1 in each iteration
    }

    return 0;
}

Output:

1
2
3
4
5
6
7
8
9
10

---

Infinite Loop

If the condition never changes or is always TRUE, the loop will run indefinitely.

Example 2: Infinite Loop (Press Ctrl+C to Stop)

#include <stdio.h>

int main() {
    while (1) {  // The condition is always TRUE
        printf("This loop will run forever!\n");
    }

    return 0;
}

Output (Repeats Forever):

This loop will run forever!
This loop will run forever!
This loop will run forever!
...

---

Using break to Exit a While Loop

The break statement can be used to exit a while loop forcefully.

Example 3: Using break to Stop the Loop

#include <stdio.h>

int main() {
    int n = 1;

    while (1) {  // Infinite loop
        printf("%d\n", n);
        if (n == 5) {
            break;  // Exit the loop when n reaches 5
        }
        n++;
    }

    return 0;
}

Output:

1
2
3
4
5

---

Using continue to Skip an Iteration

The continue statement is used to skip the remaining code in the loop for a specific iteration.

Example 4: Skipping a Number (Skipping 5)

#include <stdio.h>

int main() {
    int n = 0;

    while (n < 10) {
        n++;
        
        if (n == 5) {
            continue;  // Skip printing 5
        }

        printf("%d\n", n);
    }

    return 0;
}

Output:

1
2
3
4
6
7
8
9
10

2. DO-WHILE LOOP

The do-while loop is similar to the while loop. The difference lies in the execution order:

• In a while loop, the condition is checked before executing the code.
• In a do-while loop, the code is executed at least once before checking the condition.

Syntax:

do {
    // Code to be executed
} while (condition);

Example 5: Difference Between While and Do-While

#include <stdio.h>

int main() {
    int n = 0;

    printf("Using while loop:\n");
    while (n > 0) {
        printf("This will NOT be printed.\n");
    }

    printf("\nUsing do-while loop:\n");
    do {
        printf("This WILL be printed at least once.\n");
    } while (n > 0);

    return 0;
}

Output:

Using while loop:

Using do-while loop:
This WILL be printed at least once.

Explanation:

• The while loop does not execute because n > 0 is FALSE.
• The do-while loop runs once before checking the condition.

---

3. FOR LOOP

A for loop is an advanced version of the while loop. It allows for a specific range and controlled iterations.

The for loop consists of three components:

1. Initialization (init) → Sets the starting value. (e.g., i = 1;)
2. Condition (condition) → Determines when the loop stops. (e.g., i <= 10;)
3. Increment (increment) → Updates the loop variable. (e.g., i++)

Syntax:

for (initialization; condition; increment) {
    // Code to be executed
}

---

Example 6: Printing Numbers 1 to 10

#include <stdio.h>

int main() {
    for (int i = 1; i <= 10; i++) {
        printf("%d\n", i);
    }

    return 0;
}

Output:

1
2
3
4
5
6
7
8
9
10

---

Example 7: Loop with Step Size of 2

#include <stdio.h>

int main() {
    for (int i = 0; i <= 10; i += 2) {
        printf("%d\n", i);
    }

    return 0;
}

Output:

0
2
4
6
8
10

---

4. SWITCH-CASE STATEMENT

A switch-case statement is an alternative to if-else-if for comparing a variable against multiple fixed values.

• If the variable matches a case, the corresponding block of code is executed.
• If no cases match, the default case is executed (if present).
• The break statement prevents fall-through, meaning once a match is found, execution stops.

Syntax:

switch (variable) {
    case value1:
        // Code to execute
        break;
    case value2:
        // Code to execute
        break;
    default:
        // Code if no cases match
}

---

Example 8: Simple Menu System

#include <stdio.h>

int main() {
    int choice;

    printf("Select an option:\n");
    printf("1. Start\n");
    printf("2. Settings\n");
    printf("3. Exit\n");
    printf("Enter your choice: ");
    scanf("%d", &choice);

    switch (choice) {
        case 1:
            printf("Game Starting...\n");
            break;
        case 2:
            printf("Opening Settings...\n");
            break;
        case 3:
            printf("Exiting Program...\n");
            break;
        default:
            printf("Invalid Choice!\n");
    }

    return 0;
}

Example Output:

Select an option:
1. Start
2. Settings
3. Exit
Enter your choice: 2
Opening Settings...

---

Example 9: Days of the Week

#include <stdio.h>

int main() {
    int day;

    printf("Enter a number (1-7) for the day of the week: ");
    scanf("%d", &day);

    switch (day) {
        case 1:
            printf("Sunday\n");
            break;
        case 2:
            printf("Monday\n");
            break;
        case 3:
            printf("Tuesday\n");
            break;
        case 4:
            printf("Wednesday\n");
            break;
        case 5:
            printf("Thursday\n");
            break;
        case 6:
            printf("Friday\n");
            break;
        case 7:
            printf("Saturday\n");
            break;
        default:
            printf("Invalid input! Enter a number between 1 and 7.\n");
    }

    return 0;
}

Example Output:

Enter a number (1-7) for the day of the week: 5
Thursday

Nested Statements / Loops

1. NESTED IF STATEMENT

A nested if statement is an if condition inside another if block. This allows checking multiple conditions in a hierarchical manner.

Syntax:

if (condition1) {
    if (condition2) {
        // Code to execute if both conditions are true
    }
}

---

Example 1: Nested If Statement

#include <stdio.h>

int main() {
    int num = 10;

    if (num > 0) {  // Outer if
        printf("The number is positive.\n");

        if (num % 2 == 0) {  // Inner if
            printf("The number is even.\n");
        }
    }

    return 0;
}

Output:

The number is positive.
The number is even.

---

2. NESTED WHILE LOOP

A nested while loop is a while loop inside another while loop. The inner loop executes completely for each iteration of the outer loop.

Syntax:

while (condition1) {
    while (condition2) {
        // Code to execute
    }
}

---

Example 2: Multiplication Table using Nested While Loop

#include <stdio.h>

int main() {
    int i = 1, j;

    while (i <= 5) {
        j = 1;
        while (j <= 5) {
            printf("%d\t", i * j);
            j++;
        }
        printf("\n");
        i++;
    }

    return 0;
}

Output:

1   2   3   4   5
2   4   6   8   10
3   6   9   12  15
4   8   12  16  20
5   10  15  20  25

---

3. NESTED DO-WHILE LOOP

A nested do-while loop is a do-while loop inside another do-while loop. The inner loop will always execute at least once before checking the condition.

Syntax:

do {
    do {
        // Code to execute
    } while (condition2);
} while (condition1);

---

Example 3: Number Grid using Nested Do-While

#include <stdio.h>

int main() {
    int i = 1, j;

    do {
        j = 1;
        do {
            printf("%d ", j);
            j++;
        } while (j <= 5);

        printf("\n");
        i++;
    } while (i <= 5);

    return 0;
}

Output:

1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5

---

4. NESTED FOR LOOP

A nested for loop is a for loop inside another for loop. The inner loop runs completely for each iteration of the outer loop.

Syntax:

for (initialization; condition1; increment) {
    for (initialization; condition2; increment) {
        // Code to execute
    }
}

---

Example 4: Printing a Square Pattern

#include <stdio.h>

int main() {
    for (int i = 1; i <= 5; i++) {
        for (int j = 1; j <= 5; j++) {
            printf("* ");
        }
        printf("\n");
    }

    return 0;
}

Output:

* * * * *
* * * * *
* * * * *
* * * * *
* * * * *

---

5. NESTED SWITCH-CASE

A nested switch-case is when a switch statement is placed inside another switch statement.

Syntax:

switch (variable1) {
    case value1:
        switch (variable2) {
            case value2:
                // Code to execute
                break;
        }
        break;
}

---

Example 5: Nested Switch-Case for User Role and Permission

#include <stdio.h>

int main() {
    int role = 1; // 1 = Admin, 2 = User
    int action = 2; // 1 = View, 2 = Edit

    switch (role) {
        case 1:
            printf("Role: Admin\n");
            switch (action) {
                case 1:
                    printf("Action: Viewing data\n");
                    break;
                case 2:
                    printf("Action: Editing data\n");
                    break;
                default:
                    printf("Invalid action!\n");
            }
            break;

        case 2:
            printf("Role: User\n");
            switch (action) {
                case 1:
                    printf("Action: Viewing data\n");
                    break;
                default:
                    printf("Users cannot edit data!\n");
            }
            break;

        default:
            printf("Invalid role!\n");
    }

    return 0;
}

Example Output:

Role: Admin
Action: Editing data