Welcome! If you want to learn how to work with while loops in Python, then this article is for you.
While loops are very powerful programming structures that you can use in your programs to repeat a sequence of statements.
In this article, you will learn:
while True
is used for and its general syntax.break
statement to stop a while loop.You will learn how while loops work behind the scenes with examples, tables, and diagrams.
Are you ready? Let’s begin. 🔅
Let’s start with the purpose of while loops. What are they used for?
They are used to repeat a sequence of statements an unknown number of times. This type of loop runs while a given condition is True
and it only stops when the condition becomes False
.
When we write a while loop, we don’t explicitly define how many iterations will be completed, we only write the condition that has to be True
to continue the process and False
to stop it.
💡 Tip: if the while loop condition never evaluates to False
, then we will have an infinite loop, which is a loop that never stops (in theory) without external intervention.
These are some examples of real use cases of while loops:
Now that you know what while loops are used for, let’s see their main logic and how they work behind the scenes. Here we have a diagram:
While Loop
Let’s break this down in more detail:
True
or False
.True
, the statements that belong to the loop are executed.True
again, the sequence of statements runs again and the process is repeated.False
, the loop stops and the program continues beyond the loop.One of the most important characteristics of while loops is that the variables used in the loop condition are not updated automatically. We have to update their values explicitly with our code to make sure that the loop will eventually stop when the condition evaluates to False
.
Great. Now you know how while loops work, so let’s dive into the code and see how you can write a while loop in Python. This is the basic syntax:
While Loop (Syntax)
These are the main elements (in order):
while
keyword (followed by a space).True
or False
).:
) at the end of the first line.💡 Tip: The Python style guide (PEP 8) recommends using 4 spaces per indentation level. Tabs should only be used to remain consistent with code that is already indented with tabs.
Now that you know how while loops work and how to write them in Python, let’s see how they work behind the scenes with some examples.
Here we have a basic while loop that prints the value of i
while i
is less than 8 (i < 8
):
i = 4
while i < 8:
print(i)
i += 1
If we run the code, we see this output:
4
5
6
7
Let’s see what happens behind the scenes when the code runs:
i
is 4, so the condition i < 8
evaluates to True
and the loop starts to run. The value of i
is printed (4) and this value is incremented by 1. The loop starts again.i
is 5, so the condition i < 8
evaluates to True
. The body of the loop runs, the value of i
is printed (5) and this value i
is incremented by 1. The loop starts again.i
is 8
. Now the while loop condition i < 8
evaluates to False
and the loop stops immediately.💡 Tip: If the while loop condition is False
before starting the first iteration, the while loop will not even start running.
Now let’s see an example of a while loop in a program that takes user input. We will the input()
function to ask the user to enter an integer and that integer will only be appended to list if it’s even.
This is the code:
## Define the list
nums = []
## The loop will run while the length of the
## list nums is less than 4
while len(nums) < 4:
## Ask for user input and store it in a variable as an integer.
user_input = int(input("Enter an integer: "))
## If the input is an even number, add it to the list
if user_input % 2 == 0:
nums.append(user_input)
The loop condition is len(nums) < 4
, so the loop will run while the length of the list nums
is strictly less than 4.
Let’s analyze this program line by line:
nums
.nums = []
len(nums) < 4
.while len(nums) < 4:
input()
function and store it in the user_input
variable.user_input = int(input("Enter an integer: "))
💡 Tip: We need to convert (cast) the value entered by the user to an integer using the int()
function before assigning it to the variable because the input()
function returns a string (source).
if user_input % 2 == 0:
nums
list.nums.append(user_input)
If we run this code with custom user input, we get the following output:
Enter an integer: 3
Enter an integer: 4
Enter an integer: 2
Enter an integer: 1
Enter an integer: 7
Enter an integer: 6
Enter an integer: 3
Enter an integer: 4
This table summarizes what happens behind the scenes when the code runs:
💡 Tip: The initial value of len(nums)
is 0
because the list is initially empty. The last column of the table shows the length of the list at the end of the current iteration. This value is used to check the condition before the next iteration starts.
As you can see in the table, the user enters even integers in the second, third, sixth, and eight iterations and these values are appended to the nums
list.
Before a “ninth” iteration starts, the condition is checked again but now it evaluates to False
because the nums
list has four elements (length 4), so the loop stops.
If we check the value of the nums
list when the process has been completed, we see this:
>>> nums
[4, 2, 6, 4]
Exactly what we expected, the while loop stopped when the condition len(nums) < 4
evaluated to False
.
Now you know how while loops work behind the scenes and you’ve seen some practical examples, so let’s dive into a key element of while loops: the condition.
Before you start working with while loops, you should know that the loop condition plays a central role in the functionality and output of a while loop.
You must be very careful with the comparison operator that you choose because this is a very common source of bugs.
For example, common errors include:
<
(less than) instead of <=
(less than or equal to) (or vice versa).>
(greater than) instead of >=
(greater than or equal to) (or vice versa).This can affect the number of iterations of the loop and even its output.
Let’s see an example:
If we write this while loop with the condition i < 9
:
i = 6
while i < 9:
print(i)
i += 1
We see this output when the code runs:
6
7
8
The loop completes three iterations and it stops when i
is equal to 9
.
This table illustrates what happens behind the scenes when the code runs:
i
is 6, so the condition i < 9
is True
and the loop starts running. The value of i
is printed and then it is incremented by 1.i
is 7, so the condition i < 9
is True
. The body of the loop runs, the value of i
is printed, and then it is incremented by 1.i
is 8, so the condition i < 9
is True
. The body of the loop runs, the value of i
is printed, and then it is incremented by 1.i
is 9, so i < 9
is False
and the loop stops.In this case, we used <
as the comparison operator in the condition, but what do you think will happen if we use <=
instead?
i = 6
while i <= 9:
print(i)
i += 1
We see this output:
6
7
8
9
The loop completes one more iteration because now we are using the “less than or equal to” operator <=
, so the condition is still True
when i
is equal to 9
.
This table illustrates what happens behind the scenes:
Four iterations are completed. The condition is checked again before starting a “fifth” iteration. At this point, the value of i
is 10
, so the condition i <= 9
is False
and the loop stops.
Now you know how while loops work, but what do you think will happen if the while loop condition never evaluates to False
?
Remember that while loops don’t update variables automatically (we are in charge of doing that explicitly with our code). So there is no guarantee that the loop will stop unless we write the necessary code to make the condition False
at some point during the execution of the loop.
If we don’t do this and the condition always evaluates to True
, then we will have an infinite loop, which is a while loop that runs indefinitely (in theory).
Infinite loops are typically the result of a bug, but they can also be caused intentionally when we want to repeat a sequence of statements indefinitely until a break
statement is found.
Let’s see these two types of infinite loops in the examples below.
💡 Tip: A bug is an error in the program that causes incorrect or unexpected results.
This is an example of an unintentional infinite loop caused by a bug in the program:
## Define a variable
i = 5
## Run this loop while i is less than 15
while i < 15:
## Print a message
print("Hello, World!")
Analyze this code for a moment.
Don’t you notice something missing in the body of the loop?
That’s right!
The value of the variable i
is never updated (it’s always 5
). Therefore, the condition i < 15
is always True
and the loop never stops.
If we run this code, the output will be an “infinite” sequence of Hello, World!
messages because the body of the loop print("Hello, World!")
will run indefinitely.
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
.
.
.
## Continues indefinitely
To stop the program, we will need to interrupt the loop manually by pressing CTRL + C
.
When we do, we will see a KeyboardInterrupt
error similar to this one:
To fix this loop, we will need to update the value of i
in the body of the loop to make sure that the condition i < 15
will eventually evaluate to False
.
This is one possible solution, incrementing the value of i
by 2 on every iteration:
i = 5
while i < 15:
print("Hello, World!")
## Update the value of i
i += 2
Great. Now you know how to fix infinite loops caused by a bug. You just need to write code to guarantee that the condition will eventually evaluate to False
.
Let’s start diving into intentional infinite loops and how they work.
We can generate an infinite loop intentionally using while True
. In this case, the loop will run indefinitely until the process is stopped by external intervention (CTRL + C
) or when a break
statement is found (you will learn more about break
in just a moment).
This is the basic syntax:
Instead of writing a condition after the while
keyword, we just write the truth value directly to indicate that the condition will always be True
.
Here we have an example:
>>> while True:
print(0)
0
0
0
0
0
0
0
0
0
0
0
0
0
Traceback (most recent call last):
File "<pyshell#2>", line 2, in <module>
print(0)
KeyboardInterrupt
The loop runs until CTRL + C
is pressed, but Python also has a break
statement that we can use directly in our code to stop this type of loop.
break
statementThis statement is used to stop a loop immediately. You should think of it as a red “stop sign” that you can use in your code to have more control over the behavior of the loop.
According to the Python Documentation:
The break
statement, like in C, breaks out of the innermost enclosing
foror
while loop.
This diagram illustrates the basic logic of the break
statement:
The break
statement
This is the basic logic of the break statement:
True
.break
statement is found at any point during the execution of the loop, the loop stops immediately.break
is not found, the loop continues its normal execution and it stops when the condition evaluates to False
.We can use break
to stop a while loop when a condition is met at a particular point of its execution, so you will typically find it within a conditional statement, like this:
while True:
## Code
if <condition>:
break
## Code
This stops the loop immediately if the condition is True
.
💡 Tip: You can (in theory) write a break
statement anywhere in the body of the loop. It doesn’t necessarily have to be part of a conditional, but we commonly use it to stop the loop when a given condition is True
.
Here we have an example of break
in a while True
loop:
Let’s see it in more detail:
The first line defines a while True
loop that will run indefinitely until a break
statement is found (or until it is interrupted with CTRL + C
).
while True:
The second line asks for user input. This input is converted to an integer and assigned to the variable user_input
.
user_input = int(input("Enter an integer: "))
The third line checks if the input is odd.
if user_input % 2 != 0:
If it is, the message This number is odd
is printed and the break
statement stops the loop immediately.
print("This number of odd")
break
Else, if the input is even , the message This number is even
is printed and the loop starts again.
print("This number is even")
The loop will run indefinitely until an odd integer is entered because that is the only way in which the break
statement will be found.
Here we have an example with custom user input:
Enter an integer: 4
This number is even
Enter an integer: 6
This number is even
Enter an integer: 8
This number is even
Enter an integer: 3
This number is odd
>>>
True
. They stop when the condition evaluates to False
.break
statement is found.CTRL + C
.while True
.break
statement can be used to stop a while loop immediately.I really hope you liked my article and found it helpful. Now you know how to work with While Loops in Python.
Follow me on Twitter @EstefaniaCassN and if you want to learn more about this topic, check out my online course Python Loops and Looping Techniques: Beginner to Advanced.
The Original Article can be found on freecodecamp.org
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