For Loop

In the previous lesson, you learned the while loop. Now imagine you want to print numbers 1 to 10. With a while loop, you need to write the initialization, condition, and update in three different places. The for loop combines all three into one neat line!

Analogy: Running Laps Around the Field

Your PE teacher says: “Run around the field 5 times!” You already know exactly how many times you have to run. After each lap, you count: “Lap 1… lap 2… lap 3…” until you’ve completed 5 laps.

The for loop is perfect for situations like this — when you already know how many times the loop should run.

For Loop Syntax

for (initialization; condition; update) {
    // code that repeats
}

Three parts inside the parentheses:

1. Initialization — runs once at the start (usually creates a counter)
2. Condition — checked before each iteration (continues if true, stops if false)
3. Update — runs after each iteration (usually increment/decrement)

    +----------------+
    | Initialization | (once only)
    +--------+-------+
             v
    +----------------+
    | Check condition |<-----------+
    +--------+-------+            |
        true/  \false             |
            /    \                |
   +-------+    (exit)           |
   | Code  |                     |
   | block |                     |
   +---+---+                     |
       v                         |
   +----------------+            |
   |    Update      |------------+
   +----------------+

Example 1: Print Numbers 1 to 10

#include <iostream>

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

    return 0;
}

Output:

1 2 3 4 5 6 7 8 9 10

Compare with the equivalent while loop:

// While loop
int i = 1;           // initialization
while (i <= 10) {    // condition
    std::cout << i << " ";
    i++;             // update
}

// For loop (more concise!)
for (int i = 1; i <= 10; i++) {
    std::cout << i << " ";
}

Notice: the three components (initialization, condition, update) that were scattered across the while loop are now neatly gathered in one line in the for loop.

Example 2: Running Laps Around the Field

#include <iostream>

int main() {
    int totalLaps = 5;

    std::cout << "=== Running Laps Around the Field ===" << std::endl;

    for (int lap = 1; lap <= totalLaps; lap++) {
        std::cout << "Lap " << lap << " completed!" << std::endl;
    }

    std::cout << "Tired but healthy!" << std::endl;

    return 0;
}

Output:

=== Running Laps Around the Field ===
Lap 1 completed!
Lap 2 completed!
Lap 3 completed!
Lap 4 completed!
Lap 5 completed!
Tired but healthy!

When to Use For vs While?

Example 3: Reverse Loop (Decrement)

Count down from 10 to 1:

#include <iostream>

int main() {
    std::cout << "Countdown: ";

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

    std::cout << std::endl;
    std::cout << "Happy New Year!" << std::endl;

    return 0;
}

Output:

Countdown: 10 9 8 7 6 5 4 3 2 1
Happy New Year!

Notice: initialization starts at 10, condition i >= 1 (continue as long as i is still >= 1), and update i-- (subtract 1).

Example 4: Loop with Custom Step

You don’t always have to add or subtract 1. You can use any step size!

#include <iostream>

int main() {
    // Print even numbers 2-20 (step 2)
    std::cout << "Even numbers: ";
    for (int i = 2; i <= 20; i += 2) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    // Print multiples of 5 from 5-50 (step 5)
    std::cout << "Multiples of 5: ";
    for (int i = 5; i <= 50; i += 5) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    // Print powers of 2: 1, 2, 4, 8, 16... (step x2)
    std::cout << "Powers of 2: ";
    for (int i = 1; i <= 1024; i *= 2) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output:

Even numbers: 2 4 6 8 10 12 14 16 18 20
Multiples of 5: 5 10 15 20 25 30 35 40 45 50
Powers of 2: 1 2 4 8 16 32 64 128 256 512 1024

Example 5: Multiplication Table

Make a multiplication table like the ones in math books:

#include <iostream>

int main() {
    int number = 7;

    std::cout << "=== Multiplication Table for " << number << " ===" << std::endl;

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

    return 0;
}

Output:

=== Multiplication Table for 7 ===
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

Example 6: Simple FizzBuzz

FizzBuzz is a famous programming challenge! The rules:

• If a number is divisible by 3, print “Fizz”
• If a number is divisible by 5, print “Buzz”
• If divisible by both 3 AND 5, print “FizzBuzz”
• Otherwise, print the number

#include <iostream>

int main() {
    std::cout << "=== FizzBuzz 1-30 ===" << std::endl;

    for (int i = 1; i <= 30; i++) {
        if (i % 3 == 0 && i % 5 == 0) {
            std::cout << "FizzBuzz";
        } else if (i % 3 == 0) {
            std::cout << "Fizz";
        } else if (i % 5 == 0) {
            std::cout << "Buzz";
        } else {
            std::cout << i;
        }
        std::cout << " ";
    }

    std::cout << std::endl;

    return 0;
}

Output:

1 2 Fizz 4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz 13 14 FizzBuzz 16 17 Fizz 19 Buzz Fizz 22 23 Fizz Buzz 26 Fizz 28 29 FizzBuzz

Example 7: Printing a Star Pattern

A triangle pattern made of stars — this is a common exam question!

#include <iostream>

int main() {
    int height = 5;

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

    return 0;
}

Output:

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

Range-Based For Loop (C++11)

Modern C++ has a simpler way to traverse the contents of a data collection. It’s called the range-based for loop:

for (auto element : collection) {
    // use element
}

This is like saying: “for each element in the collection, do…”

#include <iostream>

int main() {
    int scores[] = {85, 90, 78, 92, 88};

    std::cout << "List of scores: ";
    for (auto n : scores) {
        std::cout << n << " ";
    }
    std::cout << std::endl;

    // Calculate average
    int total = 0;
    int count = 0;
    for (auto n : scores) {
        total += n;
        count++;
    }

    std::cout << "Average: " << total / count << std::endl;

    return 0;
}

Output:

List of scores: 85 90 78 92 88
Average: 86

Common Mistakes

1. Semicolon after for (empty loop)

// WRONG — the semicolon makes the loop empty
for (int i = 0; i < 5; i++); {
    std::cout << i << " ";  // Error: i is not recognized here!
}

// CORRECT
for (int i = 0; i < 5; i++) {
    std::cout << i << " ";
}

2. Modifying the counter inside the loop

// DANGEROUS — don't modify i inside the loop and in the update at the same time
for (int i = 0; i < 10; i++) {
    std::cout << i << " ";
    i++;  // i increases by 2 per iteration, result is confusing!
}
// Output: 0 2 4 6 8 (not 0 1 2 3 4 5 6 7 8 9)

3. Off-by-one: starting from 0 vs 1

// Starting from 0 — common in programming
for (int i = 0; i < 5; i++) {
    std::cout << i << " ";  // 0 1 2 3 4 (5 numbers)
}

// Starting from 1 — more natural for humans
for (int i = 1; i <= 5; i++) {
    std::cout << i << " ";  // 1 2 3 4 5 (5 numbers)
}

4. Loop variable cannot be accessed outside the loop

for (int i = 0; i < 5; i++) {
    std::cout << i << " ";
}
// std::cout << i;  // ERROR! i doesn't exist here

// Solution: declare i outside the loop if you need to access it later
int i;
for (i = 0; i < 5; i++) {
    std::cout << i << " ";
}
std::cout << "Value of i now: " << i;  // OK: i = 5

Exercises

Exercise 1: Create a program that prints a multiplication table from 1 to 5 using nested for loops. Format:

1 x 1 = 1    2 x 1 = 2    3 x 1 = 3    ...
1 x 2 = 2    2 x 2 = 4    3 x 2 = 6    ...
...

Exercise 2: Create a program that prints an inverted star triangle:

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

Exercise 3: Create a program that displays all prime numbers from 2 to 50. Hint: a prime number can only be divided by 1 and itself. Use nested loops — the outer loop for each number, the inner loop to check if there are any divisors besides 1 and itself.

Summary

The for loop is a programmer’s go-to weapon. You’ll use it very often in almost every program. But there’s one more type of loop you need to know — the do-while loop. Let’s continue to the next lesson!