Over and Over Again Infinite 8
Welcome! If y'all want to learn how to work with while loops in Python, then this commodity is for you.
While loops are very powerful programming structures that you tin can use in your programs to echo a sequence of statements.
In this article, you will acquire:
- What while loops are.
- What they are used for.
- When they should exist used.
- How they work behind the scenes.
- How to write a while loop in Python.
- What infinite loops are and how to interrupt them.
- What
while Truthfulis used for and its full general syntax. - How to use a
breakstatement to stop a while loop.
You volition learn how while loops piece of work behind the scenes with examples, tables, and diagrams.
Are you ready? Let's begin. 🔅
🔹 Purpose and Use Cases for While Loops
Permit'due south kickoff 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 status is True and it only stops when the status becomes False.
When we write a while loop, we don't explicitly define how many iterations will be completed, nosotros just write the condition that has to be True to keep the process and False to finish information technology.
💡 Tip: if the while loop condition never evaluates to False, then we will accept an space loop, which is a loop that never stops (in theory) without external intervention.
These are some examples of real use cases of while loops:
- User Input: When we ask for user input, nosotros need to check if the value entered is valid. Nosotros can't possibly know in accelerate how many times the user will enter an invalid input before the programme can go along. Therefore, a while loop would exist perfect for this scenario.
- Search: searching for an element in a data structure is some other perfect use case for a while loop because nosotros can't know in accelerate how many iterations will be needed to find the target value. For example, the Binary Search algorithm can be implemented using a while loop.
- Games: In a game, a while loop could exist used to keep the main logic of the game running until the actor loses or the game ends. Nosotros tin can't know in advance when this will happen, so this is another perfect scenario for a while loop.
🔸 How While Loops Work
At present that you know what while loops are used for, let's encounter their main logic and how they piece of work behind the scenes. Here nosotros accept a diagram:
Let'south break this down in more particular:
- The process starts when a while loop is establish during the execution of the program.
- The condition is evaluated to cheque if it'southward
TrueorFalse. - If the condition is
True, the statements that belong to the loop are executed. - The while loop condition is checked again.
- If the status evaluates to
Trueover again, the sequence of statements runs once again and the process is repeated. - When the condition evaluates to
False, the loop stops and the program continues beyond the loop.
Ane of the near important characteristics of while loops is that the variables used in the loop condition are not updated automatically. We accept to update their values explicitly with our lawmaking to make sure that the loop will eventually stop when the condition evaluates to False.
🔹 General Syntax of While Loops
Not bad. Now yous know how while loops work, then let's dive into the code and encounter how you lot tin write a while loop in Python. This is the basic syntax:
These are the main elements (in order):
- The
whilekeyword (followed past a space). - A condition to determine if the loop volition continue running or not based on its truth value (
TrueorFalse). - A colon (
:) at the end of the first line. - The sequence of statements that will exist repeated. This cake of lawmaking is called the "torso" of the loop and it has to be indented. If a statement is not indented, information technology will not be considered office of the loop (delight see the diagram below).
💡 Tip: The Python fashion guide (PEP eight) recommends using iv spaces per indentation level. Tabs should only be used to remain consistent with lawmaking that is already indented with tabs.
🔸 Examples of While Loops
At present that you know how while loops work and how to write them in Python, permit's see how they work backside the scenes with some examples.
How a Basic While Loop Works
Hither we take a basic while loop that prints the value of i while i is less than viii (i < eight):
i = 4 while i < viii: print(i) i += 1 If nosotros run the lawmaking, we see this output:
4 5 6 vii Let's see what happens backside the scenes when the lawmaking runs:
- Iteration 1: initially, the value of
iis four, and then the conditioni < 8evaluates toTrueand the loop starts to run. The value ofiis printed (4) and this value is incremented by ane. The loop starts again. - Iteration 2: at present the value of
iis 5, so the conditioni < 8evaluates toTruthful. The trunk of the loop runs, the value ofiis printed (5) and this valueiis incremented by one. The loop starts over again. - Iterations 3 and 4: The same process is repeated for the tertiary and fourth iterations, then the integers 6 and 7 are printed.
- Before starting the 5th iteration, the value of
iiseight. Now the while loop conditioni < 8evaluates toSimulatedand the loop stops immediately.
💡 Tip: If the while loop status is False before starting the get-go iteration, the while loop will non fifty-fifty first running.
User Input Using a While Loop
Now let'south see an example of a while loop in a programme 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'southward even.
This is the code:
# Define the list nums = [] # The loop will run while the length of the # listing nums is less than 4 while len(nums) < 4: # Inquire for user input and store it in a variable as an integer. user_input = int(input("Enter an integer: ")) # If the input is an fifty-fifty number, add it to the listing 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.
Allow's analyze this program line by line:
- We start by defining an empty list and assigning information technology to a variable chosen
nums.
nums = [] - Then, nosotros define a while loop that will run while
len(nums) < 4.
while len(nums) < iv: - We ask for user input with the
input()function and store it in theuser_inputvariable.
user_input = int(input("Enter an integer: ")) 💡 Tip: Nosotros demand to convert (cast) the value entered by the user to an integer using the int() function before assigning it to the variable considering the input() function returns a string (source).
- We check if this value is even or odd.
if user_input % 2 == 0: - If it'due south fifty-fifty, we append it to the
numslist.
nums.append(user_input) - Else, if information technology's odd, the loop starts again and the status is checked to determine if the loop should continue or non.
If nosotros run this lawmaking with custom user input, we get the following output:
Enter an integer: iii Enter an integer: 4 Enter an integer: 2 Enter an integer: ane Enter an integer: 7 Enter an integer: 6 Enter an integer: 3 Enter an integer: 4 This tabular array summarizes what happens behind the scenes when the code runs:
💡 Tip: The initial value of len(nums) is 0 because the listing is initially empty. The concluding cavalcade of the table shows the length of the list at the stop of the electric current iteration. This value is used to cheque the condition before the next iteration starts.
As y'all tin can see in the table, the user enters even integers in the second, tertiary, 6th, and eight iterations and these values are appended to the nums listing.
Earlier a "ninth" iteration starts, the condition is checked again but now it evaluates to Faux because the nums list has four elements (length 4), so the loop stops.
If nosotros check the value of the nums list when the process has been completed, we see this:
>>> nums [4, ii, six, 4] Exactly what we expected, the while loop stopped when the condition len(nums) < iv evaluated to Simulated.
Now y'all know how while loops work behind the scenes and y'all've seen some practical examples, then let's dive into a primal element of while loops: the condition.
🔹 Tips for the Condition in While Loops
Before y'all start working with while loops, you should know that the loop status plays a central office in the functionality and output of a while loop.
You must exist very careful with the comparison operator that you lot choose because this is a very common source of bugs.
For example, mutual errors include:
- Using
<(less than) instead of<=(less than or equal to) (or vice versa). - Using
>(greater than) instead of>=(greater than or equal to) (or vice versa).
This can impact the number of iterations of the loop and even its output.
Let's see an example:
If nosotros write this while loop with the condition i < 9:
i = vi while i < nine: print(i) i += i We see this output when the code runs:
six 7 eight The loop completes 3 iterations and it stops when i is equal to 9.
This table illustrates what happens behind the scenes when the code runs:
- Before the first iteration of the loop, the value of
iis vi, so the conditioni < 9isTrueand the loop starts running. The value ofiis printed and then it is incremented by 1. - In the second iteration of the loop, the value of
iis 7, so the conditioni < ixisTrue. The body of the loop runs, the value ofiis printed, then information technology is incremented by i. - In the third iteration of the loop, the value of
iis 8, so the statusi < ixisTrue. The body of the loop runs, the value ofiis printed, and then information technology is incremented by 1. - The condition is checked again before a fourth iteration starts, but at present the value of
iis 9, theni < 9isFalseand the loop stops.
In this instance, nosotros used < every bit the comparison operator in the condition, but what do you think will happen if nosotros use <= instead?
i = 6 while i <= 9: print(i) i += one We come across this output:
six 7 8 ix The loop completes one more iteration because at present 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 backside the scenes:
Four iterations are completed. The condition is checked again before starting a "5th" iteration. At this point, the value of i is 10, so the condition i <= 9 is False and the loop stops.
🔸 Infinite While Loops
Now you know how while loops work, just what practise y'all think will happen if the while loop condition never evaluates to False?
What are Space While Loops?
Remember that while loops don't update variables automatically (nosotros are in accuse of doing that explicitly with our code). So there is no guarantee that the loop will finish unless we write the necessary code to make the condition Faux at some point during the execution of the loop.
If we don't do this and the condition always evaluates to True, then we volition 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 besides be acquired intentionally when we want to echo a sequence of statements indefinitely until a break argument is found.
Let's run into these two types of infinite loops in the examples beneath.
💡 Tip: A bug is an error in the program that causes wrong or unexpected results.
Instance of Infinite Loop
This is an example of an unintentional infinite loop caused past a issues in the program:
# Define a variable i = v # Run this loop while i is less than fifteen while i < 15: # Impress a message impress("Hello, Earth!") Analyze this code for a moment.
Don't you discover something missing in the body of the loop?
That'south correct!
The value of the variable i is never updated (it's always 5). Therefore, the condition i < 15 is always Truthful and the loop never stops.
If we run this code, the output volition be an "infinite" sequence of Hello, World! messages because the torso of the loop print("Hello, World!") will run indefinitely.
Howdy, Globe! Hello, Globe! Hello, Earth! Hello, World! Hi, World! Hello, World! Hi, World! Hello, World! Hi, Globe! Howdy, World! Howdy, World! Howdy, World! How-do-you-do, World! Hello, World! Howdy, World! Hello, World! Hello, World! Hullo, World! . . . # Continues indefinitely To stop the programme, we will need to interrupt the loop manually by pressing CTRL + C.
When we do, we volition encounter a KeyboardInterrupt error similar to this i:
To gear up this loop, nosotros will demand to update the value of i in the trunk of the loop to make sure that the condition i < 15 will eventually evaluate to Fake.
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 += two Great. At present you know how to fix infinite loops caused by a problems. Y'all but demand to write lawmaking to guarantee that the status volition somewhen evaluate to Fake.
Allow's start diving into intentional space loops and how they piece of work.
🔹 How to Brand an Infinite Loop with While True
We can generate an infinite loop intentionally using while Truthful. In this case, the loop volition run indefinitely until the process is stopped by external intervention (CTRL + C) or when a break statement is found (you will learn more near interruption in just a moment).
This is the basic syntax:
Instead of writing a status after the while keyword, nosotros just write the truth value directly to indicate that the condition will always exist Truthful.
Here nosotros accept an case:
>>> while True: print(0) 0 0 0 0 0 0 0 0 0 0 0 0 0 Traceback (most contempo phone call last): File "<pyshell#ii>", 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 lawmaking to stop this type of loop.
The break statement
This statement is used to finish a loop immediately. Y'all should call up of information technology as a ruddy "stop sign" that you can use in your code to have more than control over the beliefs of the loop.
Co-ordinate to the Python Documentation:
Thepausestatement, like in C, breaks out of the innermost enclosingfororwhileloop.
This diagram illustrates the basic logic of the break argument:
pause argument This is the bones logic of the break statement:
- The while loop starts only if the condition evaluates to
True. - If a
breakstatement is found at any point during the execution of the loop, the loop stops immediately. - Else, if
breakis not found, the loop continues its normal execution and it stops when the condition evaluates toFalse.
We can employ pause to stop a while loop when a condition is met at a particular point of its execution, so you will typically notice it within a conditional statement, like this:
while Truthful: # Code if <condition>: break # Lawmaking This stops the loop immediately if the condition is Truthful.
💡 Tip: You tin (in theory) write a break statement anywhere in the body of the loop. It doesn't necessarily take to be office of a conditional, only we commonly apply information technology to stop the loop when a given condition is True.
Here we have an example of break in a while Truthful loop:
Let'south see it in more than detail:
The first line defines a while True loop that will run indefinitely until a suspension statement is found (or until it is interrupted with CTRL + C).
while True: The 2d 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.
impress("This number of odd") interruption Else, if the input is fifty-fifty , the message This number is fifty-fifty 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 interruption statement will be found.
Here we accept 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 >>> 🔸 In Summary
- While loops are programming structures used to repeat a sequence of statements while a condition is
True. They stop when the condition evaluates toFalse. - When you write a while loop, you need to make the necessary updates in your lawmaking to make sure that the loop will eventually cease.
- An space loop is a loop that runs indefinitely and information technology simply stops with external intervention or when a
interruptionargument is found. - You can end an infinite loop with
CTRL + C. - Y'all can generate an infinite loop intentionally with
while Truthful. - The
pausestatement can exist used to cease a while loop immediately.
I really promise you lot liked my commodity and found information technology helpful. Now you know how to work with While Loops in Python.
Follow me on Twitter @EstefaniaCassN and if yous want to larn more about this topic, cheque out my online course Python Loops and Looping Techniques: Beginner to Advanced.
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