244 | 38 minutes ago : | ||||
1,711 | Oct-02-2023, 12:57 AM : | ||||
1,998 | Jan-10-2023, 10:58 PM : | ||||
2,251 | Feb-10-2022, 07:36 PM : | ||||
3,626 | Mar-02-2021, 08:11 PM : | ||||
4,780 | Jul-13-2020, 03:53 PM : | ||||
2,525 | Jun-11-2020, 12:45 PM : | ||||
2,602 | May-25-2020, 03:15 PM : | ||||
2,250 | Mar-08-2020, 10:15 AM : | ||||
3,615 | Dec-27-2019, 09:08 AM : |
Announcements.
Photo by Chris Ried on Unsplash
Loops are a fundamental concept in programming that allow you to repeat a block of code multiple times. Python has two main types of loops: for loops and while loops. Mastering loops is key to writing efficient Python code.
In this comprehensive guide, you‘ll learn:
So let‘s get started!
The for loop allows you to iterate over items in a sequence or other iterable object like lists, tuples, strings, dicts, and sets.
Here is the basic syntax:
Let‘s print all numbers from 0 to 9:
This prints:
We iterated through the range(10) iterable, assigning each number to the number variable and printing it.
You can loop over any iterable, not just ranges:
This loops through and prints each city in the list.
Some things you can iterate over in Python:
Now let‘s look at a real-world example.
Let‘s filter out all negative numbers from a list of integers using a for loop.
This is a tiny example of data processing – useful in machine learning!
Next let‘s learn about while loops.
The while loop runs a block of code repeatedly as long as a certain condition is true.
Here is the syntax:
It checks the condition , executes the code block if true, and repeats. Once the condition becomes false, it exits the loop.
Let‘s print numbers from 1 to 5 using a while :
This first sets num to 1. The loop runs as long as num is less/equal to 5. Each iteration prints num , then increments it by 1. Once num reaches 6, the loop exits.
The key difference between while and for is that while relies on a condition to keep looping. for loops through items directly.
Let‘s look at another useful while loop pattern.
We can keep looping continuously until some external event occurs using a flag variable.
Start an infinite while True loop:
This loops infinitely until some_event makes running == False .
For example, a game loop:
This game loop runs continuously doing game stuff until the player wins or quits.
Flags let you control while loops externally.
Now let‘s explore nesting loops and loop control statements.
We can have loops inside loops, known as nested loops. Each "inner" loop finishes all its iterations before the "outer" loop advances.
For example, nesting for publishing articles on multiple platforms:
The outer loop publishes each article, the inner handles all the publishers.
We can add loop control statements :
For example:
This prints numbers 0 to 5, skipping 3. The break exits early on 6.
These allow finer grain control over loops.
Next let‘s look at an advanced Python feature for working with loops and lists.
List comprehensions provide a concise way to apply a loop and conditionals to generate lists.
The basic syntax is:
This runs expression on each member and stores results in new_list .
For example, squares:
We can also add if conditionals:
List comps are faster, more concise alternatives to normal loops for generating lists.
Some other objects have similar syntax like dict/set/generator comprehensions .
Finally, let‘s tie it all together with some simulation examples.
Looping is useful for repeatedly running simulations to model complex phenomenon computationally.
For example, let‘s use loops to simulate virus spread in a population. We can model how a virus might spread person-to-person each day using lists to track people, randomness to decide who interacts with who, and loops to repeat the process.
Here‘s a basic example:
We loop through 15 days. Each day:
We can enhance and expand this sim into a full epidemiology model!
Loops enabled randomly running complex multi-step simulations.
Alright, let‘s wrap up with some best practices.
Here are some tips for working with loops effectively:
Mastering loops is critical to level up your Python. You can now harness the power of repeating code blocks for data processing, simulations, game programming, and almost all Python code.
Hope you enjoyed this guide to understanding, using, and mastering Python loops! Let me know if you have any other loop questions.
Happy coding!
Dr. Alex Mitchell is a dedicated coding instructor with a deep passion for teaching and a wealth of experience in computer science education. As a university professor, Dr. Mitchell has played a pivotal role in shaping the coding skills of countless students, helping them navigate the intricate world of programming languages and software development.
Beyond the classroom, Dr. Mitchell is an active contributor to the freeCodeCamp community, where he regularly shares his expertise through tutorials, code examples, and practical insights. His teaching repertoire includes a wide range of languages and frameworks, such as Python, JavaScript, Next.js, and React, which he presents in an accessible and engaging manner.
Dr. Mitchell’s approach to teaching blends academic rigor with real-world applications, ensuring that his students not only understand the theory but also how to apply it effectively. His commitment to education and his ability to simplify complex topics have made him a respected figure in both the university and online learning communities.
Sending secret messages may seem like something out of a spy movie, but cryptography – the…
The 100 Days of Code challenge has seen widespread adoption among developers looking to level up…
As a full-stack developer, I often leverage incognito or private browsing modes while testing site changes…
As developers, we have an ethical responsibility to consider the broader impacts of the code we…
With over 670 free programming and computer science courses indexed on Class Central from schools like…
As a full-stack developer and lifelong learner, I‘m fascinated by the world-class computer science education now…
Find centralized, trusted content and collaborate around the technologies you use most.
Q&A for work
Connect and share knowledge within a single location that is structured and easy to search.
Get early access and see previews of new features.
I am receiving the following error:
UnboundLocalError: local variable 'test1' referenced before assignment.
Error says that 'test1' is local variable but i thought that this variable is global
So is it global or local and how to solve this error without passing global test1 as argument to test_func ?
In order for you to modify test1 while inside a function you will need to do define test1 as a global variable, for example:
However, if you only need to read the global variable you can print it without using the keyword global , like so:
But whenever you need to modify a global variable you must use the keyword global .
Best solution: Don't use global s
You have to specify that test1 is global:
COMMENTS
File "weird.py", line 5, in main. print f(3) UnboundLocalError: local variable 'f' referenced before assignment. Python sees the f is used as a local variable in [f for f in [1, 2, 3]], and decides that it is also a local variable in f(3). You could add a global f statement: def f(x): return x. def main():
I made this function: N = 1. while x <= N * y: R = x - N * y. N = N+1. return R. but then I receive the following error, when running my function: UnboundLocalError: local variable 'R' referenced before assignment. I searched around and found that this happens when an assigned variable in the function, is already assigned outside of it.
Output. Hangup (SIGHUP) Traceback (most recent call last): File "Solution.py", line 7, in <module> example_function() File "Solution.py", line 4, in example_function x += 1 # Trying to modify global variable 'x' without declaring it as global UnboundLocalError: local variable 'x' referenced before assignment Solution for Local variable Referenced Before Assignment in Python
Reliable monitoring for your app, databases, infrastructure, and the vendors they rely on. Ping Bot is a powerful uptime and performance monitoring tool that helps notify you and resolve issues before they affect your customers.
UnboundLocalError: local variable 'global_var' referenced before assignment ... Local variable Referenced Before Assignment in Python. ... Changing Variable Names With Python For Loops. When we write variable names play a crucial role in code readability and maintainability. However, there are situations where you might need to change variable ...
If a variable is assigned a value in a function's body, it is a local variable unless explicitly declared as global. # Local variables shadow global ones with the same name You could reference the global name variable from inside the function but if you assign a value to the variable in the function's body, the local variable shadows the global one.
The UnboundLocalError: local variable 'x' referenced before assignment occurs when you reference a variable inside a function before declaring that variable. To resolve this error, you need to use a different variable name when referencing the existing variable, or you can also specify a parameter for the function. I hope this tutorial is useful.
DJANGO - Local Variable Referenced Before Assignment [Form] The program takes information from a form filled out by a user. Accordingly, an email is sent using the information. ... Therefore, we have examined the local variable referenced before the assignment Exception in Python. The differences between a local and global variable ...
Strategy 2: Using the Global Keyword. In Python, variables declared inside a function are considered local variables. Thus, they are separate from other variables declared outside of the function.
Trying to assign a value to a variable that does not have local scope can result in this error: UnboundLocalError: local variable referenced before assignment. Python has a simple rule to determine the scope of a variable. If a variable is assigned in a function, that variable is local. This is because it is assumed that when you define a ...
Using nonlocal keyword. The nonlocal keyword is used to work with variables inside nested functions, where the variable should not belong to the inner function. It allows you to modify the value of a non-local variable in the outer scope. For example, if you have a function outer that defines a variable x, and another function inner inside outer that tries to change the value of x, you need to ...
This tutorial explains the reason and solution of the python error local variable referenced before assignment
If you run this code, you'll get. BASH. UnboundLocalError: local variable 'value' referenced before assignment. The issue is that in this line: PYTHON. value = value + 1. We are defining a local variable called value and then trying to use it before it has been assigned a value, instead of using the variable that we defined in the first line.
UnboundLocalError: local variable referenced before assignment. Example #1: Accessing a Local Variable. Solution #1: Passing Parameters to the Function. Solution #2: Use Global Keyword. Example #2: Function with if-elif statements. Solution #1: Include else statement. Solution #2: Use global keyword. Summary.
To fix this, you can either move the assignment of the variable x before the print statement, or give it an initial value before the print statement. def example (): x = 5 print (x) example()
I think you are using 'global' incorrectly. See Python reference. You should declare variable without global and then inside the function when you want to access global variable you declare it global yourvar. #!/usr/bin/python total def checkTotal(): global total total = 0 See this example:
After a loop, j and (n-1) shou… Hi, I have a question, unrelated to the whole program. The following program can work, but if I replace s_out += str(n-i) + s[n-1] with s_out += str(j+1-i) + s[j] , it won't work.
The issue is caused by this line attempts -= 1. In python there is a concept called scope, which is how it reads variables. By default it uses local scope, so it checks what variables are created within the function to see which are local, if it is not created in the function then it looks outside.
Quote:Traceback (most recent call last): File "<stdin>", line 27, in <module> File "<stdin>", line 9, in move NameError: local variable referenced before assignment because: X_right and X_left of course. but where can i do the "assignment" ? you only want to do that once, and not every time a loop is completed. thank you.
By making y a left-value with its use as an iteration variable, y is determined to be a local variable to the separate function by the Python compiler, and as such the y in the range(y+1) cannot be referenced because the local y is not yet assigned a value in that function scope. You can either rename the local y in the comprehension:
Next let's learn about while loops.. How while Loops Work. The while loop runs a block of code repeatedly as long as a certain condition is true.. Here is the syntax: while condition: # do something. It checks the condition, executes the code block if true, and repeats.Once the condition becomes false, it exits the loop.
Notice the two spaces before print in the snippet with the if, elif, else statements. These spaces are called indentation and they are important as they tell python when the condition starts and stops. Run the following examples and check that you fully understand the output. [ ]
In order for you to modify test1 while inside a function you will need to do define test1 as a global variable, for example: test1 = 0. def test_func(): global test1. test1 += 1. test_func() However, if you only need to read the global variable you can print it without using the keyword global, like so: test1 = 0.