8.10 — For statements

In this lesson, you'll learn about for loops, the most commonly used loop construct in C++. You'll master their syntax, understand when to use them, and discover why they're perfect for many repetitive tasks.

What is a for loop?

A for loop is a control flow statement that combines initialization, condition testing, and update in a single, compact line. It's ideal when you know how many times you want to repeat something or when working with sequences.

Syntax:

for (initialization; condition; update)
{
    // Loop body - code to execute repeatedly
}

Components:

• Initialization: Executed once before the loop starts
• Condition: Checked before each iteration; loop continues while true
• Update: Executed after each iteration

Basic for loop example

#include <iostream>

int main()
{
    for (int i = 1; i <= 5; ++i)
    {
        std::cout << "Iteration " << i << std::endl;
    }
    
    std::cout << "Loop finished!" << std::endl;
    
    return 0;
}

Output:

Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5
Loop finished!

How for loops work

Let's break down the execution:

for (int i = 0; i < 3; ++i)
{
    std::cout << "i = " << i << std::endl;
}

Step-by-step execution:

1. Initialize: int i = 0 (i becomes 0)
2. Check condition: i < 3 (0 < 3 is true, continue)
3. Execute body: Print "i = 0"
4. Update: ++i (i becomes 1)
5. Check condition: i < 3 (1 < 3 is true, continue)
6. Execute body: Print "i = 1"
7. Update: ++i (i becomes 2)
8. Check condition: i < 3 (2 < 3 is true, continue)
9. Execute body: Print "i = 2"
10. Update: ++i (i becomes 3)
11. Check condition: i < 3 (3 < 3 is false, exit loop)

For loop vs while loop

Same logic, different syntax:

Using a while loop:

#include <iostream>

int main()
{
    // While loop equivalent
    int i = 0;                    // Initialization
    while (i < 5)                 // Condition
    {
        std::cout << i << " ";
        ++i;                      // Update
    }
    
    return 0;
}

Using a for loop:

#include <iostream>

int main()
{
    // For loop - more compact
    for (int i = 0; i < 5; ++i)   // Init, condition, update in one line
    {
        std::cout << i << " ";
    }
    
    return 0;
}

Both produce: 0 1 2 3 4

The for loop is more compact and keeps all loop control in one place.

Common for loop patterns

Pattern 1: Counting up

#include <iostream>

int main()
{
    // Count from 1 to 10
    for (int count = 1; count <= 10; ++count)
    {
        std::cout << count << " ";
    }
    std::cout << std::endl;
    
    return 0;
}

Output: 1 2 3 4 5 6 7 8 9 10

Pattern 2: Counting down

#include <iostream>

int main()
{
    // Countdown from 10 to 1
    for (int countdown = 10; countdown >= 1; --countdown)
    {
        std::cout << countdown << " ";
    }
    std::cout << "Blast off!" << std::endl;
    
    return 0;
}

Output: 10 9 8 7 6 5 4 3 2 1 Blast off!

Pattern 3: Skip counting

#include <iostream>

int main()
{
    // Count by 2s
    for (int i = 0; i <= 20; i += 2)
    {
        std::cout << i << " ";
    }
    std::cout << std::endl;
    
    // Count by 5s
    for (int i = 5; i <= 50; i += 5)
    {
        std::cout << i << " ";
    }
    std::cout << std::endl;
    
    return 0;
}

Output:

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

Pattern 4: Processing sequences

#include <iostream>

int main()
{
    // Print ASCII characters
    for (char c = 'A'; c <= 'Z'; ++c)
    {
        std::cout << c << " ";
    }
    std::cout << std::endl;
    
    return 0;
}

Output: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Practical examples

Example 1: Multiplication table

#include <iostream>

int main()
{
    int number;
    std::cout << "Enter a number for multiplication table: ";
    std::cin >> number;
    
    std::cout << "Multiplication table for " << number << ":\n";
    
    for (int i = 1; i <= 12; ++i)
    {
        std::cout << number << " x " << i << " = " << (number * i) << std::endl;
    }
    
    return 0;
}

Sample Output:

Enter a number for multiplication table: 7
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
7 x 11 = 77
7 x 12 = 84

Example 2: Sum and average calculator

#include <iostream>

int main()
{
    int count;
    std::cout << "How many numbers do you want to enter? ";
    std::cin >> count;
    
    double sum = 0.0;
    
    for (int i = 1; i <= count; ++i)
    {
        double number;
        std::cout << "Enter number " << i << ": ";
        std::cin >> number;
        sum += number;
    }
    
    double average = sum / count;
    
    std::cout << "Sum: " << sum << std::endl;
    std::cout << "Average: " << average << std::endl;
    
    return 0;
}

Sample Output:

How many numbers do you want to enter? 4
Enter number 1: 10
Enter number 2: 20
Enter number 3: 15
Enter number 4: 25
Sum: 70
Average: 17.5

Example 3: Factorial calculator

#include <iostream>

int main()
{
    int n;
    std::cout << "Enter a positive integer: ";
    std::cin >> n;
    
    if (n < 0)
    {
        std::cout << "Factorial is not defined for negative numbers.\n";
        return 1;
    }
    
    long long factorial = 1;
    
    for (int i = 1; i <= n; ++i)
    {
        factorial *= i;
    }
    
    std::cout << n << "! = " << factorial << std::endl;
    
    return 0;
}

Sample Output:

Enter a positive integer: 6
6! = 720

Example 4: Number pattern generator

#include <iostream>

int main()
{
    int rows = 5;
    
    std::cout << "Number triangle:\n";
    for (int i = 1; i <= rows; ++i)
    {
        for (int j = 1; j <= i; ++j)
        {
            std::cout << j << " ";
        }
        std::cout << std::endl;
    }
    
    return 0;
}

Output:

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

Nested for loops

For loops can be nested inside other for loops, which is useful for working with 2D patterns, tables, or grids.

Example 1: Multiplication table grid

#include <iostream>
#include <iomanip>

int main()
{
    std::cout << "Multiplication Table (1-10):\n";
    
    // Print header
    std::cout << "   ";
    for (int col = 1; col <= 10; ++col)
    {
        std::cout << std::setw(4) << col;
    }
    std::cout << std::endl;
    
    // Print rows
    for (int row = 1; row <= 10; ++row)
    {
        std::cout << std::setw(3) << row;  // Row label
        
        for (int col = 1; col <= 10; ++col)
        {
            std::cout << std::setw(4) << (row * col);
        }
        std::cout << std::endl;
    }
    
    return 0;
}

Example 2: Star patterns

#include <iostream>

int main()
{
    int size = 5;
    
    // Right triangle
    std::cout << "Right triangle:\n";
    for (int i = 1; i <= size; ++i)
    {
        for (int j = 1; j <= i; ++j)
        {
            std::cout << "* ";
        }
        std::cout << std::endl;
    }
    
    std::cout << std::endl;
    
    // Rectangle
    std::cout << "Rectangle:\n";
    for (int i = 1; i <= 4; ++i)
    {
        for (int j = 1; j <= 8; ++j)
        {
            std::cout << "* ";
        }
        std::cout << std::endl;
    }
    
    return 0;
}

Output:

Right triangle:
* 
* * 
* * * 
* * * * 
* * * * * 

Rectangle:
* * * * * * * * 
* * * * * * * * 
* * * * * * * * 
* * * * * * * * 

Variable scope in for loops

Variables declared in the for loop initialization are only accessible within the loop:

#include <iostream>

int main()
{
    for (int i = 0; i < 5; ++i)
    {
        std::cout << "i = " << i << std::endl;
        // i is accessible here
    }
    
    // std::cout << i << std::endl;  // ERROR: i is not accessible here
    
    // If you need i after the loop, declare it outside
    int j;
    for (j = 0; j < 3; ++j)
    {
        std::cout << "j = " << j << std::endl;
    }
    
    std::cout << "Final j = " << j << std::endl;  // j is accessible here
    
    return 0;
}

Output:

i = 0
i = 1
i = 2
i = 3
i = 4
j = 0
j = 1
j = 2
Final j = 3

Advanced for loop variations

Empty sections

You can leave any section of the for loop empty:

#include <iostream>

int main()
{
    // Empty initialization (variable declared outside)
    int i = 0;
    for (; i < 5; ++i)
    {
        std::cout << i << " ";
    }
    std::cout << std::endl;
    
    // Empty update (updated inside loop body)
    for (int j = 0; j < 10;)
    {
        std::cout << j << " ";
        j += 2;  // Update inside the loop
    }
    std::cout << std::endl;
    
    // Infinite loop (empty condition means always true)
    int count = 0;
    for (;;)  // Infinite loop
    {
        std::cout << count << " ";
        if (++count >= 5) break;  // Exit with break
    }
    std::cout << std::endl;
    
    return 0;
}

Multiple variables

#include <iostream>

int main()
{
    // Multiple variables in initialization and update
    for (int i = 0, j = 10; i <= j; ++i, --j)
    {
        std::cout << "i=" << i << ", j=" << j << std::endl;
    }
    
    return 0;
}

Output:

i=0, j=10
i=1, j=9
i=2, j=8
i=3, j=7
i=4, j=6
i=5, j=5

When to use for loops

Use for loops when:

• You know the number of iterations in advance
• You're working with sequences or ranges
• You need a counter variable
• You're iterating through arrays or containers
• You're generating patterns or tables

Perfect for loop scenarios:

#include <iostream>

int main()
{
    // 1. Known number of iterations
    for (int i = 0; i < 10; ++i)
    {
        // Process exactly 10 items
    }
    
    // 2. Array processing (we'll learn about arrays later)
    int numbers[] = {1, 2, 3, 4, 5};
    for (int i = 0; i < 5; ++i)
    {
        std::cout << numbers[i] << " ";
    }
    std::cout << std::endl;
    
    // 3. Mathematical sequences
    for (int i = 1; i <= 1000; i *= 2)
    {
        std::cout << i << " ";  // Powers of 2
    }
    std::cout << std::endl;
    
    return 0;
}

Common for loop mistakes

Mistake 1: Off-by-one errors

// WRONG - goes one too far
for (int i = 1; i <= 10; ++i)
{
    // This runs 10 times (1 through 10)
    // If you want exactly 10 iterations starting from 0, this is wrong
}

// CORRECT - for 10 iterations starting from 0
for (int i = 0; i < 10; ++i)
{
    // This runs 10 times (0 through 9)
}

Mistake 2: Modifying the loop variable incorrectly

#include <iostream>

int main()
{
    // Can cause confusion
    for (int i = 0; i < 10; ++i)
    {
        if (i == 5)
        {
            i = 8;  // Modifying loop variable inside loop
        }
        std::cout << i << " ";
    }
    // Output: 0 1 2 3 4 8 9 (skips 5, 6, 7)
    
    return 0;
}

Mistake 3: Wrong increment/decrement

// WRONG - infinite loop
for (int i = 10; i > 0; ++i)  // Incrementing when we should decrement
{
    std::cout << i << std::endl;  // i gets larger, never reaches 0
}

// CORRECT
for (int i = 10; i > 0; --i)  // Decrement to reach the exit condition
{
    std::cout << i << std::endl;
}

Best practices for for loops

1. Use meaningful variable names

// Good - clear purpose
for (int studentCount = 0; studentCount < totalStudents; ++studentCount)
{
    processStudent(studentCount);
}

// Acceptable for simple cases
for (int i = 0; i < 10; ++i)
{
    // Simple counting
}

2. Keep the loop condition simple

// Good - simple condition
for (int i = 0; i < maxItems; ++i)

// Avoid - complex condition
for (int i = 0; i < calculateDynamicMax() && isStillValid(); ++i)

3. Use const for the limit when possible

const int MAX_ITERATIONS = 100;
for (int i = 0; i < MAX_ITERATIONS; ++i)
{
    // Clear intent and prevents accidental modification
}

4. Prefer ++i over i++

// Better - pre-increment
for (int i = 0; i < 10; ++i)  // Slightly more efficient

// Also correct - post-increment
for (int i = 0; i < 10; i++)  // But creates temporary copy

For basic types like int, the difference is minimal, but it's good practice.

Summary

For loops provide compact and powerful iteration control:

Structure:

• Initialization: Set up loop variables (runs once)
• Condition: Test before each iteration (continues while true)
• Update: Modify variables after each iteration

Key advantages:

• Compact syntax keeps all loop control in one place
• Perfect for known iteration counts
• Excellent for working with sequences and ranges
• Natural fit for array and container processing

Common patterns:

• Counting up/down
• Skip counting (increment by values other than 1)
• Processing sequences
• Generating patterns
• Nested loops for 2D operations

Best practices:

• Use meaningful variable names
• Keep conditions simple
• Watch for off-by-one errors
• Prefer pre-increment (++i)
• Use const for limits when possible

For loops are the most commonly used loop construct in C++ because they're perfect for the majority of repetitive tasks where you know how many iterations you need.

Quiz

1. What are the three components of a for loop?
2. When is each component of a for loop executed?
3. What's the scope of a variable declared in the for loop initialization?
4. What happens if you leave the condition section empty in a for loop?
5. What's the difference between ++i and i++ in a for loop?

Practice exercises

1. Prime number checker: Write a program that checks if a number is prime by testing divisibility from 2 to the square root of the number.

2. Fibonacci sequence: Generate and print the first n numbers in the Fibonacci sequence (1, 1, 2, 3, 5, 8, 13, ...).

3. Diamond pattern: Create a program that prints a diamond shape using asterisks, with the size determined by user input.

4. Grade statistics: Write a program that reads grades for a class and calculates statistics like average, highest, lowest, and the number of passing grades.