Beginner 10 min

For Loops

Learn the most versatile loop in C++ for counting, iterating, and repeating operations

Master the for loop - the most common loop structure for counting, iterating, and repeating operations a specific number of times.

A Simple Example

#include <iostream>

int main() {
    // Print numbers 1 to 5
    for (int i{1}; i <= 5; i++) {
        std::cout << "Iteration " << i << "\n";
    }

    // Countdown from 10 to 1
    for (int countdown{10}; countdown >= 1; countdown--) {
        std::cout << countdown << " ";
    }
    std::cout << "\n";

    // Count by twos
    for (int even{0}; even <= 10; even += 2) {
        std::cout << even << " ";  // 0 2 4 6 8 10
    }
    std::cout << "\n";

    return 0;
}

Breaking It Down

For Loop Structure: `for (init; condition; update)`

Initialization: int i{0} - runs once at the start, declares and sets the loop counter
Condition: i < 5 - checked before each iteration, continues while true
Update: i++ - runs after each iteration, typically increments the counter
Remember: All three parts are optional, but the semicolons are required

Common Loop Patterns

Count up: for (int i{0}; i < 10; i++) - most common pattern
Count down: for (int i{10}; i > 0; i--) - useful for reverse iteration
Skip values: for (int i{0}; i < 10; i += 2) - increment by any amount
Remember: The update expression can be any valid C++ statement

Loop Variable Scope

Variables declared in the init section exist only inside the loop
Example: for (int i{0}; i < 5; i++) - i is only valid within the loop body
After the loop ends, the variable is destroyed and cannot be accessed
Remember: This is good practice - it keeps variables limited to where they are needed

When to Use For vs While Loops

Use for loops when you know the number of iterations in advance
Use for loops when counting or iterating over a range
Use while loops when waiting for a condition or unknown iteration count
Remember: Any for loop can be converted to a while loop, but for is clearer for counting

Why This Matters

For loops are the workhorses of programming. Processing arrays, generating sequences, iterating over collections - these tasks appear in almost every program.
The for loop packages initialization, condition checking, and increment into one compact, readable line. This makes your intent clear and reduces errors.

Critical Insight

Any for loop can be written as a while loop, but not the other way around! For loops are just more convenient when counting:

// For loop version - compact and clear
for (int i{0}; i < 5; i++) {
    std::cout << i << "\n";
}

// Equivalent while loop - more verbose
int i{0};
while (i < 5) {
    std::cout << i << "\n";
    i++;
}

Choose for loops when: You are counting, iterating a known number of times, or working with ranges.

Choose while loops when: You are waiting for user input, processing until a sentinel value, or do not know iteration count upfront.

Best Practices

Use meaningful counter names: Instead of i, use descriptive names like dayIndex or studentNum when it makes the code clearer.

Prefer < over <= for array bounds: Use for (int i{0}; i < 10; i++) to iterate 10 times. This matches how arrays are indexed (0 to 9).

Keep loop bodies simple: If your loop body is long or complex, consider extracting it into a separate function for better readability.

Declare counter in the init section: Use for (int i{0}; ...) instead of declaring i before the loop. This limits the variable scope.

Common Mistakes

Off-by-One Errors: Using i <= 10 instead of i < 10 makes the loop run 11 times instead of 10. Always double-check your condition.

Modifying Loop Variable Inside Loop: Changing i inside the loop body can cause confusion and bugs. Let the update expression handle it.

Infinite Loops from Wrong Increment: Forgetting to increment or using the wrong operator like i-- when counting up creates infinite loops.

Semicolon After For Statement: for (int i{0}; i < 5; i++); with a semicolon creates an empty loop that does nothing. The intended body becomes separate code.

Debug Challenge

This for loop should print numbers 0 to 4, but it has a bug. Click the highlighted line to fix it:

1 #include <iostream>

3 int main() {

4 for (int i{0}; i <= 5; i++) {

5 std::cout << i << " ";

6 }

7 return 0;

8 }

Quick Quiz

How many times does this loop execute?

for (int i{0}; i < 10; i++)

10 times

9 times

11 times

What is the final value printed by this code?

for (int i{2}; i < 10; i += 2) {
    std::cout << i << " ";
}

Which loop is best for processing exactly 100 items?

for loop

while loop

Both are equally good

Practice Playground

Time to try out what you just learned! Play with the example code below, experiment by making changes and running the code to deepen your understanding.

Output:

Error:

Lesson Progress

Fix This Code
Quick Quiz
Practice Playground - run once