Lecture 6: Loops and Conditionals

1. Conditional Statements

The if Statement

Executes a block of code if a specified condition is true.

if (condition) {
  // code to execute if condition is true
}

Example:

int x = 20, y = 18;
if (x > y) {
  cout << "x is greater than y";
}

The if...else Statement

Used to execute one block if the condition is true, and another if it's false.

if (condition) {
  // if true
} else {
  // if false
}

Example:

int d;
cout << "Enter student degree (0–100): ";
cin >> d;

if (d >= 50)
  cout << "Student passed";
else
  cout << "Student failed";

The else if Ladder

Used when multiple conditions need to be tested.

if (condition1) {
  // code block 1
} else if (condition2) {
  // code block 2
} else {
  // code block 3
}

Comparison and Logical Operators

Operator	Description	Example
==	Equal to	x == y
!=	Not equal	x != y
>	Greater than	x > y
<	Less than	x < y
>=	Greater or equal	x >= y
<=	Less or equal	x <= y
&&	Logical AND	a > 4 && a < 10
|	Logical OR	a < 9 | a < 10
!	Logical NOT	!(a < 7 && a < 15)

The switch Statement

Used to execute different actions based on multiple possible values.

switch(expression) {
  case value1:
    // code block
    break;
  case value2:
    // code block
    break;
  default:
    // code if no case matches
}

Example:
A simple calculator using switch:

float n1, n2;
char op;

cout << "Enter first number: ";
cin >> n1;
cout << "Enter second number: ";
cin >> n2;
cout << "Enter operator (+, -, *, /, %): ";
cin >> op;

switch(op) {
  case '+': cout << n1 + n2; break;
  case '-': cout << n1 - n2; break;
  case '*': cout << n1 * n2; break;
  case '/':
    if (n2 != 0)
      cout << n1 / n2;
    else
      cout << "Cannot divide by zero";
    break;
  case '%':
    cout << int(n1) % int(n2);
    break;
  default:
    cout << "Invalid operator";
}

2. Loops in C++

While Loop

Repeats a block of code while a condition is true.
Used when the number of iterations is unknown.

int i = 0;
while (i < 5) {
  cout << i << "\n";
  i++;
}

Trace Example:

Step	i (before)	Condition	Action	i (after)	Output
1	0	true	print 0	1	0
2	1	true	print 1	2	1
3	2	true	print 2	3	2
4	3	true	print 3	4	3
5	4	true	print 4	5	4
6	5	false	stop	—	—

Do-While Loop

Executes the loop body at least once, then checks the condition.

int i = 0;
do {
  cout << i << "\n";
  i++;
} while (i < 5);

Trace Example:

Step	i (before)	Output	i (after)	Condition	Continue?
1	0	0	1	true	yes
2	1	1	2	true	yes
3	2	2	3	true	yes
4	3	3	4	true	yes
5	4	4	5	false	no

For Loop

Used when the number of iterations is known.

for (int i = 0; i <= 10; i += 2) {
  cout << i << "\n";
}

Trace Example:

Step	i (start)	Condition	Output	i (next)
1	0	true	0	2
2	2	true	2	4
3	4	true	4	6
4	6	true	6	8
5	8	true	8	10
6	10	true	10	12
7	12	false	—	—

3. Factorial Calculation

Formula:

$$n!=n\times (n-1)\times (n-2)\times ...\times 1$$

Example Program:

int n;
long factorial = 1;

cout << "Enter a non-negative integer: ";
cin >> n;

if (n < 0) {
  cout << "Factorial not defined for negative numbers.";
} else {
  for (int i = 1; i <= n; ++i) {
    factorial *= i;
  }
  cout << "Factorial of " << n << " is " << factorial;
}

Trace Example (n = 5):

i	Operation	factorial
1	1 × 1	1
2	1 × 2	2
3	2 × 3	6
4	6 × 4	24
5	24 × 5	120

Result: 5! = 120

4. Nested Loops (Pattern Printing)

Example: Right-Angled Triangle

int n;
cout << "Enter number of rows: ";
cin >> n;

for (int i = 1; i <= n; ++i) {
  for (int j = 1; j <= i; ++j) {
    cout << "*";
  }
  cout << endl;
}

Output (for n = 4):

*
**
***
****

Trace Explanation:

• Outer loop controls rows (i).
• Inner loop controls number of stars per row (j).
• Each new row prints one more star.

5. Practice Tasks

1. Print the following patterns:

   ****
   ***
   **
   *

2. Create a program to print a multiplication table.

Summary

• Conditional statements (if, else if, switch) allow decision making.
• Loops (while, do-while, for) enable repeated execution.
• Nested loops are used for complex repetitive patterns.
• Control structures are essential for logical flow in C++ programming.