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Loops allow you to quickly iterate over information in C++. In this lesson, we'll cover two types of loops: while loops and for loops.

Loops

A **loop** is a programming tool that *repeats* some code or a set of instructions until a specified condition is reached. As a programmer, you'll find that you rely on loops all the time! You’ll hear the generic term "iterate" when referring to loops; *iterate* simply means "to repeat".

When we see that a process has to repeat multiple times in a row, we write a loop. Loops allow us to create efficient code that automates processes to make scalable, manageable programs.

In this lesson, we will learn about two types of loops: `while` loops and `for` loops!

Introduction to Loops

Great job! <img src="https://content.codecademy.com/courses/learn-cpp/loops/review1.png" title="Congrats!" width=5% height=5% style="display:inline; margin:0"/>

Key concepts covered in this lesson:

- Loops perform repetitive actions so we don’t have to code those actions manually every time.
- How to write `while` loops with a continue condition.
- How to write `for` loops with a counter that increments or decrements.



Review

The last one we held your hand, so let's try one on your own.

As an example of iteration, we have the first program ever to run on a stored-program computer (the <a href="https://en.wikipedia.org/wiki/EDSAC">EDSAC</a>). It was written and run by David Wheeler in the computer laboratory at Cambridge University, England, on May 6th, 1948, to calculate and print a simple list of squares like the following:

```
0   0
1   1
2   4
3   9
4   16
5   25
6   36
7   49
8   64
9   81
```

On the left, there are numbers from 0 to 9. On the right are their squares. For example, for the number 9, 9 * 9 = 81.



While Loop

<img src="https://content.codecademy.com/courses/learn-cpp/loops/forloops1.png"> 

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Iterating over a sequence of numbers is so common that C++, like most other programming languages, has a special syntax for it.

When we know exactly how many times we want to iterate (or when we are counting), we can use a `for` loop instead of a `while` loop:

```cpp
for (int i = 0; i < 20; i++) 
{

  std::cout << "I will not throw paper airplanes in class.\n";

}
```

Let's take a closer look at the first line:

```cpp
for (int i = 0; i < 20; i++)
```

There are three separate parts to this separated by `;`:

- The initialization of a *counter*: `int i = 0`
- The continue condition: `i < 20`
- The change in the counter (in this case an increment): `i++`

So here we are creating a variable `i` that starts from 0. We will repeat the code inside over and over again when `i` is less than 20. At the end the `for` loop, we are adding 1 to `i` using the `++` operator.


For Loop Demo

So first up... the `while` loop!

Before we dive deep into the syntax of the `while` loop, let's do a demo.

Inside **enter_pin.cpp**, we have a program that asks and checks for a password. It uses a `while` loop to ask the user for the password over and over again.

**Note:** You don't need to understand the code right now.

While Loop Demo

So now that we got a demo of loops, let's write one!

The `while` loop looks very similar to an `if` statement. And just like an `if` statement, it executes the code inside of it if the condition is `true`. 

However, the difference is that the `while` loop will continue to execute the code inside of it, *over and over again*, as long as the condition is `true`. 

Here is what a `while` loop looks like:

```pseudo
while (condition) {

  statements

}
```

In other words, instead of executing *if* something is true, it executes *while* that thing is true.

```cpp
while (guess != 8) {
  
  std::cout << "Wrong guess, try again: ";
  std::cin >> guess;

}
```

In this example, while `guess` is not equal to 8, the program will keep on asking the user to input a new number. It will exit the `while` loop once the user types in `8` and then the program will continue.



Guess Number

In the last exercise, we saw an example of an incrementing `for` loop so here we are going to show you how to write a `for` loop where the counter goes down. When we know exactly how many times we want to iterate (or when we are counting), we can use a `for` loop instead of a `while` loop:

Incrementing `for` loop:

```cpp
for (int i = 0; i < 20; i++) 
{
  // Statements
}
```

Decrementing `for` loop:

```cpp
for (int i = 20; i > 0; i--) 
{
  // Statements
}
```