Loops

Introduction

In the previous lesson, you learned how to make decisions using conditional statements. However, many programming tasks require the same operation to be performed repeatedly.

For example, a program may need to display numbers from 1 to 100, print a multiplication table, process thousands of customer records, or repeatedly ask a user for input. Writing the same statements again and again would make the program unnecessarily long and difficult to maintain.

What is a Loop?

A loop is a control structure that repeatedly executes a statement or block of statements according to a condition.

Instead of writing the same code many times, a programmer writes the code once and allows the loop to repeat it.

Start
  ↓
Check Condition
  ↓
Execute Code
  ↓
Update
  ↓
Repeat

Why Do We Need Loops?

Real-World Analogy

Imagine a teacher asking a student to write "Practice Makes Perfect" 100 times. Writing 100 separate instructions would be unnecessary.

A loop works in the same way. It repeatedly performs an instruction while controlling how and when the repetition should end.

How a Loop Works

Most counter-controlled loops can be understood through four basic parts: initialization, condition checking, execution, and update.

Types of Loops in C++

C++ provides several loop structures. The three fundamental loops introduced in this lesson are while, do...while, and for.

1. The while Loop

The while loop repeatedly executes its body while its condition evaluates to true. Because the condition is checked before the body, a while loop may execute zero times.

while (condition)
{
    // Statements
}

#include <iostream>

int main()
{
    int number = 1;

    while (number <= 5)
    {
        std::cout << number << std::endl;
        number++;
    }

    return 0;
}

1
2
3
4
5

2. The do...while Loop

The do...while loop checks its condition after executing the loop body. Therefore, the body executes at least once.

do
{
    // Statements
}
while (condition);

#include <iostream>

int main()
{
    int number = 1;

    do
    {
        std::cout << number << std::endl;
        number++;
    }
    while (number <= 5);

    return 0;
}

int number = 10;

while (number <= 5)
{
    std::cout << "while";
}

// Executes zero times

do
{
    std::cout << "do...while";
}
while (number <= 5);

// Executes once

3. The for Loop

The for loop provides a compact structure for repetition. It is especially common for counter-controlled loops because initialization, condition, and update appear together.

for (initialization; condition; update)
{
    // Statements
}

#include <iostream>

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

    return 0;
}

1
2
3
4
5

Comparison of Loops

Memory Representation

Consider the loop for (int i = 1; i <= 3; i++). The loop variable changes during each iteration.

Start:
i → 1

Iteration 1:
Condition: 1 <= 3 → true
Body executes
Update: i becomes 2

Iteration 2:
Condition: 2 <= 3 → true
Body executes
Update: i becomes 3

Iteration 3:
Condition: 3 <= 3 → true
Body executes
Update: i becomes 4

Final Check:
Condition: 4 <= 3 → false
Loop ends

Real-World Applications

Common Beginner Mistakes

int number = 1;

while (number <= 5)
{
    std::cout << number;

    // Missing:
    // number++;
}

while (true)
{
    // Repeats until some operation
    // inside the program exits the loop
}

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

// 1 >= 5 is false,
// so the loop body never executes.

while (number <= 5);
{
    std::cout << number;
}

// Prints 1 through 4
for (int i = 1; i < 5; i++)
{
    std::cout << i << std::endl;
}

// Prints 1 through 5
for (int i = 1; i <= 5; i++)
{
    std::cout << i << std::endl;
}

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

Loops are fundamental control structures that automate repetitive work. Instead of writing the same instructions repeatedly, a program can use a loop to execute them according to a condition.

The while loop checks its condition before execution, the do...while loop checks its condition after execution, and the for loop provides a compact structure for initialization, condition checking, and updating. Understanding how loops begin, repeat, update, and terminate is essential for writing reliable programs.

Loops are used throughout real applications for processing records, displaying collections, performing calculations, running simulations, handling game updates, and many other repetitive tasks. Mastering loops provides an important foundation for working with functions, arrays, collections, and more advanced programming concepts.