Loops

Introduction

More often than not, we will have to execute some peace of code in a loop, repeating the action performed. Python offers us two ways to solve our problems without repeating the same logic. This pattern is something you will see in many programming languages. We developers always try to keep our code DRY (Don't Repeat Yourself) as much as possible.

Lesson Overview

In this lesson you will learn

• What is a loop?
• What types of loops there are in Python and why do we use each one?
• What is iteration??

What Is a Loop?

a structure, series, or process, the end of which is connected to the beginning.

• definition of loop in english language

In Python, we have two loops that behave sligly diffrently, so lets get to know them.

While Loop

While loops are used when we need to repeat some code block while some condition is True. To create while loop, we use keyword while followed by condition then a colon (:). We also have to indent the code block which we wish to execute in the loop

while condition:

We can demonstrate this in the following example:

Python SandboxView on Memory Graph ↗

Always make sure your loop exits

While loops can be a bit tricky as we must keep track and update our condition every time the loop starts a new iteration. If we fail to update the condition our program will never finish its execution and it must be killed by force.

This is a condition called infinite loop and it can be quite useful if controlled, but bad if left with no way to exit.

If you ever do find yourself in an infinite loop with no way to exit, do not panic. Use keyboard shortcut CTRL+C to kill your program.

For Loop

For loops are used when you know the number of times the code must loop. They are defined with a keyword: for followed by item which will be state of the loop, followed by keyword in, then followed by sequence of some kind. As with while loops, we must indent the code block we wish to run in loop.

for item in sequence:

range() Function

Python gives us range() function that returns a type of sequence called generator. We will learn about these at later lesson, for now, just know that we mostly use them in for loops to execute a loop specific number of times.

You can pass diffrent arguments to the function to get diffrent behaviur. Function takes in 3 arguments, with the 1 being required.

Signature of the function is as follows:

range(start, stop, step)

• start: from where it starts counting (inclusive) - default is 0
• stop: where it stops counting (exclusive) - required
• step: which step to take for each count - default is 1

Let's see a little example of using for loop and range function.

Python SandboxView on Memory Graph ↗

As you can see, Python by default always starts counting at index 0. Another thing to note is that we do not have number 5 printed, but we did get 5 iterations of the loop.

Python Sandbox

In this example you can see that our loop is now starting to print from 1, but still does not print 5. This is because start argument is inclusive but the end argument is exclusive. If we want to have 5 printed, we must set stop argument as 6.

Python Sandbox

Now we have told the function we want every number from 0 to 11 but in steps of 2. We can also count in reverse by setting step argument to negative integer.

break And continue

break

What if you need to break out of the loop early? You have found something you are looking for and the loop needs to finish? Python has a keyword break which does exactly what it's name suggests. Breaks out of the loop.

Let us demonstrate this:

Python Sandbox

continue

In some situations it's useful to continue with iteration but skipping this specific one. This is where continue keyword comes in.

Let us demonstrate this:

Python Sandbox

Both break and continue work only in loops

These statements break and continue work only in loops. If you try to use them outside the loop you will encounter an error.

else In Loops

Both for and while loops have an optional condition else which is used when loop exits without hitting a break inside the loop, but the loop exits. Extremly useful and one of the less known Python feature.

In a for loop, the else clause is executed after the loop finishes its final iteration, that is, if no break occurred.

In a while loop, it’s executed after the loop’s condition becomes false.

In either kind of loop, the else clause is not executed if the loop was terminated by a break. Of course, other ways of ending the loop early, such as a return or a raised exception, will also skip execution of the else clause.

Let us see this in action:

Python SandboxView on Memory Graph ↗

Look closely, the else clause belongs to the for loop, not if statement.

Assigment

1. Open main.py in your simple-bookstore project.
2. Create a lock for our application by:
   • Create new variable PIN_CODE and assign it some integer (eg. 1234).
   • Create another variable UNLOCK_ATTEMPTS and assign it 3.
   • We need another correct_pin_code variable to track the state, so set it to False initially.
   • Now write a for loop that runs UNLOCK_ATTEMPTS times
   • For each iteration, ask the user to enter PIN code, then check if it is correct and if it is, set correct_pin_code to True, otherwise continue with iteration.
   • If the user did not enter correct PIN code in all three iterations, print the message saying that console is locked and exit the application.
3. Write while loop that runs while correct_pin_code is True, then indent the rest of the code so it belongs to that loop.
4. Keep everything else in the loop and replace those sys.exit() calls with continue so iteration starts over if user enters the wrong information.
5. Make sure your program works, then commit and push changes.

What's Next