Loops - Repeating Actions

What Are Loops?

Imagine you need to write "I will practice coding" 100 times. Would you type it 100 times? No way! That's where loops come in.

A loop repeats code multiple times automatically. It's like telling your computer: "Do this thing over and over until I tell you to stop."

Types of Loops

C++ has three main types of loops:

while loop - Repeat while a condition is true
for loop - Repeat a specific number of times
do-while loop - Like while, but always runs at least once

The while Loop

The while loop keeps running as long as a condition is true.

Structure:

while (condition is true) {
    do this code
}

Basic while Loop Example

#include <iostream>

int main() {
    int count = 1;

    while (count <= 5) {
        std::cout << "Count: " << count << std::endl;
        count++;
    }

    std::cout << "Done!" << std::endl;

    return 0;
}

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Done!

What's happening:

Start with count = 1
Check: Is count <= 5? Yes!
Print count
Increase count by 1
Go back to step 2
Repeat until count > 5

Counting Down

#include <iostream>

int main() {
    int countdown = 10;

    std::cout << "Rocket launch in..." << std::endl;

    while (countdown > 0) {
        std::cout << countdown << "..." << std::endl;
        countdown--;
    }

    std::cout << "Blast off! 🚀" << std::endl;

    return 0;
}

Output:

Rocket launch in...
..
..
..
..
..
..
..
..
..
..
Blast off! 🚀

User Input Example

#include <iostream>

int main() {
    int number = 0;

    while (number != 7) {
        std::cout << "Guess the secret number (1-10): ";
        std::cin >> number;

        if (number == 7) {
            std::cout << "Correct! You found it!" << std::endl;
        } else {
            std::cout << "Wrong! Try again..." << std::endl;
        }
    }

    return 0;
}

Running the program:

Guess the secret number (1-10): 3
Wrong! Try again...
Guess the secret number (1-10): 8
Wrong! Try again...
Guess the secret number (1-10): 7
Correct! You found it!

Infinite Loops (Be Careful!)

If the condition is always true, the loop never stops!

#include <iostream>

int main() {
    int number = 1;

    while (number > 0) {
        std::cout << "This will run forever!" << std::endl;
    }

    return 0;
}

This is called an infinite loop. The program will keep running until you force it to stop (Ctrl+C in terminal).

How to avoid: Make sure your loop variable changes and eventually makes the condition false.

The for Loop

The for loop is perfect when you know exactly how many times you want to repeat something.

Structure:

for (initialize; condition; update) {
    do this code
}

Basic for Loop Example

#include <iostream>

int main() {
    for (int i = 1; i <= 5; i++) {
        std::cout << "Number: " << i << std::endl;
    }

    return 0;
}

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Breaking it down:

int i = 1 - Start with i = 1 (runs once at the beginning)
i <= 5 - Keep going while i is 5 or less
i++ - After each loop, increase i by 1
i is called the "loop counter" or "iterator"

Printing Your Times Tables

#include <iostream>

int main() {
    int number = 5;

    std::cout << "Times table for " << number << ":" << std::endl;

    for (int i = 1; i <= 10; i++) {
        std::cout << number << " x " << i << " = " << (number * i) << std::endl;
    }

    return 0;
}

Output:

Times table for 5:
x 1 = 5
x 2 = 10
x 3 = 15
x 4 = 20
x 5 = 25
x 6 = 30
x 7 = 35
x 8 = 40
x 9 = 45
x 10 = 50

Counting by Different Amounts

#include <iostream>

int main() {
    std::cout << "Count by 2s:" << std::endl;
    for (int i = 0; i <= 20; i += 2) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    std::cout << "Count by 5s:" << std::endl;
    for (int i = 0; i <= 50; i += 5) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output:

Count by 2s:
0 2 4 6 8 10 12 14 16 18 20
Count by 5s:
0 5 10 15 20 25 30 35 40 45 50

Counting Backwards

#include <iostream>

int main() {
    std::cout << "Countdown from 10 to 1:" << std::endl;

    for (int i = 10; i >= 1; i--) {
        std::cout << i << " ";
    }

    std::cout << std::endl << "Go!" << std::endl;

    return 0;
}

Output:

Countdown from 10 to 1:
10 9 8 7 6 5 4 3 2 1
Go!

for Loop vs while Loop

They can often do the same thing!

Using while:

int i = 1;
while (i <= 5) {
    std::cout << i << " ";
    i++;
}

Using for:

for (int i = 1; i <= 5; i++) {
    std::cout << i << " ";
}

Both output: 1 2 3 4 5

When to use which:

Use for when you know how many times to loop
Use while when you loop until a condition changes

The do-while Loop

Similar to while, but it ALWAYS runs at least once!

Structure:

do {
    do this code
} while (condition is true);

do-while Example

#include <iostream>

int main() {
    int number;

    do {
        std::cout << "Enter a positive number: ";
        std::cin >> number;

        if (number <= 0) {
            std::cout << "That's not positive! Try again." << std::endl;
        }
    } while (number <= 0);

    std::cout << "Great! You entered: " << number << std::endl;

    return 0;
}

Running the program:

Enter a positive number: -5
That's not positive! Try again.
Enter a positive number: 0
That's not positive! Try again.
Enter a positive number: 10
Great! You entered: 10

while vs do-while Difference

Using while:

#include <iostream>

int main() {
    int count = 10;

    while (count < 5) {
        std::cout << "This never runs!" << std::endl;
        count++;
    }

    return 0;
}

Output: Nothing! The condition is false from the start.

Using do-while:

#include <iostream>

int main() {
    int count = 10;

    do {
        std::cout << "This runs once!" << std::endl;
        count++;
    } while (count < 5);

    return 0;
}

Output: This runs once!

The do-while runs the code first, THEN checks the condition.

Nested Loops

You can put loops inside other loops!

Drawing a Rectangle

#include <iostream>

int main() {
    int rows = 5;
    int cols = 8;

    for (int i = 1; i <= rows; i++) {
        for (int j = 1; j <= cols; j++) {
            std::cout << "* ";
        }
        std::cout << std::endl;
    }

    return 0;
}

Output:

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

How it works:

Outer loop runs 5 times (rows)
For each row, inner loop runs 8 times (cols)
Print a star for each column
After inner loop, print a new line

Multiplication Table

#include <iostream>

int main() {
    std::cout << "Multiplication Table (1-10):" << std::endl;
    std::cout << std::endl;

    for (int i = 1; i <= 10; i++) {
        for (int j = 1; j <= 10; j++) {
            std::cout << (i * j) << "\t";
        }
        std::cout << std::endl;
    }

    return 0;
}

Output:

Multiplication Table (1-10):

  2    3    4    5    6    7    8    9    10
  4    6    8    10   12   14   16   18   20
  6    9    12   15   18   21   24   27   30
  8    12   16   20   24   28   32   36   40
  10   15   20   25   30   35   40   45   50
  12   18   24   30   36   42   48   54   60
  14   21   28   35   42   49   56   63   70
  16   24   32   40   48   56   64   72   80
  18   27   36   45   54   63   72   81   90
 20   30   40   50   60   70   80   90   100

Note: \t creates a tab (spacing)

Triangle Pattern

#include <iostream>

int main() {
    int height = 5;

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

    return 0;
}

Output:

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

Loop Control: break and continue

break - Exit the Loop Early

#include <iostream>

int main() {
    for (int i = 1; i <= 10; i++) {
        std::cout << i << " ";

        if (i == 5) {
            std::cout << std::endl << "Breaking at 5!" << std::endl;
            break;
        }
    }

    return 0;
}

Output:

1 2 3 4 5
Breaking at 5!

The loop stops completely when it hits break.

#include <iostream>

int main() {
    for (int i = 1; i <= 10; i++) {
        if (i % 2 == 0) {
            continue;
        }
        std::cout << i << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output:

1 3 5 7 9

When it hits continue, it skips the rest of the loop body and goes to the next iteration.

Finding a Number

#include <iostream>

int main() {
    int target = 42;
    bool found = false;

    for (int i = 1; i <= 100; i++) {
        if (i == target) {
            std::cout << "Found " << target << " at position " << i << "!" << std::endl;
            found = true;
            break;
        }
    }

    if (!found) {
        std::cout << "Number not found!" << std::endl;
    }

    return 0;
}

Output:

Found 42 at position 42!

Real-World Example: Simple Number Guessing Game

#include <iostream>

int main() {
    int secretNumber = 7;
    int guess;
    int attempts = 0;
    int maxAttempts = 5;

    std::cout << "=== Number Guessing Game ===" << std::endl;
    std::cout << "I'm thinking of a number between 1 and 10!" << std::endl;
    std::cout << "You have " << maxAttempts << " attempts." << std::endl;
    std::cout << std::endl;

    while (attempts < maxAttempts) {
        std::cout << "Attempt " << (attempts + 1) << " - Enter your guess: ";
        std::cin >> guess;

        attempts++;

        if (guess == secretNumber) {
            std::cout << "🎉 Correct! You won in " << attempts << " attempts!" << std::endl;
            break;
        } else if (guess < secretNumber) {
            std::cout << "Too low! Try again." << std::endl;
        } else {
            std::cout << "Too high! Try again." << std::endl;
        }

        if (attempts == maxAttempts) {
            std::cout << "Game Over! The number was " << secretNumber << std::endl;
        }
    }

    return 0;
}

Sample game:

=== Number Guessing Game ===
I'm thinking of a number between 1 and 10!
You have 5 attempts.

Attempt 1 - Enter your guess: 5
Too low! Try again.
Attempt 2 - Enter your guess: 8
Too high! Try again.
Attempt 3 - Enter your guess: 7
🎉 Correct! You won in 3 attempts!

Common Mistakes

Mistake 1: Off-by-One Errors

for (int i = 1; i < 10; i++) {
    std::cout << i << " ";
}

This prints 1-9, not 1-10! Use <= instead:

for (int i = 1; i <= 10; i++) {
    std::cout << i << " ";
}

Mistake 2: Forgetting to Update Loop Variable

int i = 1;
while (i <= 5) {
    std::cout << i << " ";
}

This is an infinite loop! i never changes. Add i++:

int i = 1;
while (i <= 5) {
    std::cout << i << " ";
    i++;
}

Mistake 3: Wrong Condition

for (int i = 10; i >= 1; i++) {
    std::cout << i << " ";
}

This never runs! You're counting UP but checking if i >= 1. Use i--:

for (int i = 10; i >= 1; i--) {
    std::cout << i << " ";
}

Practice Exercises

1. Sum Calculator Ask the user how many numbers they want to add, then ask for each number and show the total.

2. Factorial Calculator Calculate the factorial of a number (5! = 5 × 4 × 3 × 2 × 1 = 120)

3. Pattern Maker Create a program that asks for a height and draws different patterns (square, triangle, pyramid)

Example solution for sum calculator:

#include <iostream>

int main() {
    int count, number, sum = 0;

    std::cout << "How many numbers do you want to add? ";
    std::cin >> count;

    for (int i = 1; i <= count; i++) {
        std::cout << "Enter number " << i << ": ";
        std::cin >> number;
        sum += number;
    }

    std::cout << "Total sum: " << sum << std::endl;

    return 0;
}

What's Next?

Now you can make your programs repeat actions automatically! Next, we'll learn:

Functions - Organizing code into reusable pieces
Parameters and return values
Making your code cleaner and easier to understand

You're becoming a real programmer! 🎯

Loops - Repeating Actions

What Are Loops?​

Types of Loops​

The while Loop​

Basic while Loop Example​

Counting Down​

User Input Example​

Infinite Loops (Be Careful!)​

The for Loop​

Basic for Loop Example​

Printing Your Times Tables​

Counting by Different Amounts​

Counting Backwards​

for Loop vs while Loop​

The do-while Loop​

do-while Example​

while vs do-while Difference​

Nested Loops​

Drawing a Rectangle​

Multiplication Table​

Triangle Pattern​

Loop Control: break and continue​

break - Exit the Loop Early​

continue - Skip to Next Iteration​

Finding a Number​

Real-World Example: Simple Number Guessing Game​

Common Mistakes​

Mistake 1: Off-by-One Errors​

Mistake 2: Forgetting to Update Loop Variable​

Mistake 3: Wrong Condition​

Practice Exercises​

What's Next?​

What Are Loops?

Types of Loops

The while Loop

Basic while Loop Example

Counting Down

User Input Example

Infinite Loops (Be Careful!)

The for Loop

Basic for Loop Example

Printing Your Times Tables

Counting by Different Amounts

Counting Backwards

for Loop vs while Loop

The do-while Loop

do-while Example

while vs do-while Difference

Nested Loops

Drawing a Rectangle

Multiplication Table

Triangle Pattern

Loop Control: break and continue

break - Exit the Loop Early

continue - Skip to Next Iteration

Finding a Number

Real-World Example: Simple Number Guessing Game

Common Mistakes

Mistake 1: Off-by-One Errors

Mistake 2: Forgetting to Update Loop Variable

Mistake 3: Wrong Condition

Practice Exercises

What's Next?