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 of the loop you will encounter an error.

Assigment

1. Open main.py in your simple-python-shop project.
2. Set item_quantity to 0 as its initial value and remove item_price variable.
3. Use a while loop to repeatedly prompt the user for an item price.
   • If the price entered is less then 0, print message saying Invalid price, must be positive, and skipping the rest of the iteration code, asking user again to enter the price.
   • Otherwise check if the price entered is 0 and if so, break out of the loop.
   • Otherwise, check if item_stock - item_quantity is more than 1 and if not, break out of the loop, informing the user we are out of stock.
   • Otherwise, add the price user entered to total and increment item_quantity by 1.
4. When the loop finishes, determine if the total can have a discount and print out the final receipt.
5. Make sure your program works, then commit and push your code

Deepen Your Knowlege

What's Next