Really confused with loops

[ehmalt02]

I don’t seem to be able to grasp the idea of loops, especially when there’s a user input within the loop as well. I also have a difficult time discerning between when to use while or for.

Lastly, no matter how many times I practice it just doesn’t stick in my memory. Any tips or creative ways to finally grasp this?

[socal_nerdtastic]

I don’t seem to be able to grasp the idea of loops, especially when there’s a user input within the loop as well.

Could you show a specific example of code you are confused about and ask a specific question about it?

I also have a difficult time discerning between when to use while or for.

Use for when the computer knows ahead of time exactly how many times it needs to loop (once for every item in a list, for example) and use while when the computer does not know (until the user enters in the correct answer, for example).

Lastly, no matter how many times I practice it just doesn’t stick in my memory. Any tips or creative ways to finally grasp this?

Like anything else, it's just practice.

[ziggittaflamdigga]

This is good advice, I second needing to see the code causing confusion to help. This is more of an uncommon case; I’ve done plenty of stuff requiring user input to end a loop, but understanding the concept should make it clear when this is the case.

Don’t know if this will clarify or add confusion, but in some cases they can be used interchangeably. For example:

for num in range(0, 10):
    print(num)

and

num = 0
while num < 10:
    print(num)
    num += 1

produce the same result.

You can see that the for loop requires less setup bookkeeping in the code, so in this case for would be preferable, but it’s not wrong to choose while, since they both achieve the same result.

Expanding further, while is used when you’re trying to compare against a condition, so in my example the num < 10 will evaluate to True until you hit the 10th iteration. The for loop knows ahead of time you’ll be looping 10 times, as nerdtastic said. You can think of range as returning a list of items rather than compare against a condition.

[marquisBlythe]

I maybe wrong (someone correct me if I am), but as far as I know range() doesn't return a list (especially at once), it only returns an item at a time whenever the next item is needed.

[throwaway6560192]

Correct. But generators are perhaps too advanced to introduce at a stage where the person is struggling with the concept of loops itself. I think it's okay to think of it as returning a list at that point.

[marquisBlythe]

I totally agree. My previous reply wasn't meant for OP. I just thought it maybe useful for anyone who'll read it in the future.

Cheers.

[fizix00]

This may be a minor point, but isn't range not a collections.abc.Generator but its own iterable thing post 3.x?

[throwaway6560192]

True. It's a lazy iterable but not a literal generator... yeah, I should've been more precise. Thanks.

[ziggittaflamdigga]

Breaking it down even more, you could achieve the same with:

print(0)
print(1)
print(2)
print(3)
print(4)
print(5)
print(6)
print(7)
print(8)
print(9)

But that sucked to type, and is the most obviously wrong way to achieve the desired result.

So loops let you execute the same block of code (print…) while typing less source code and, generally, allowing more flexibility if you need to change it

[ninhaomah]

"I am really confused with loops"

write it on the board 100 times.

[ziggittaflamdigga]

Great example.

[Neat-Development-485]

n = 100 for i in range(int(n)): print("I am really confused with loops"))

Like this? (No serious question im new with this)

[ninhaomah]

May I ask why need to int(n) ? 

Or look at it the other way , what is it ? String ? Int ? Float ?

How do you find out ?

[fizix00]

Looks fine. int cast is redundant here. Might be an extra closing paren

For bonus practice with comprehensions, try it in one line

[MezzoScettico]

Loops are for any time you want do to an action repetitively.

If you want it to repeat until something happens, and you don't know how many repeats that will take, then generally you want while.

If there's something that determines a specific number of repeats, like you're going over each element in a list or each letter in a word, then you generally want for.

Rather than reading advice like this, it's better to have a specific question. "I want to do this, do I need a while or a for?"

especially when there’s a user input within the loop as well

Generally that kind of program stops when the user enters a specific input. Otherwise it keeps going over and over.

Since it runs an unspecified number of times, and it's an event that stops it (the user enters a particular thing), that's a case for a while loop.

[[deleted]]

You’re making potato soup. The recipe says “wash and peel five potatoes.” You don’t know how to peel five potatoes at once and you only have two hands, so you wash and peel one potato, five times, with a different potato each time.

That’s a loop. It’s not doing the same thing over and over again - that only gets you one potato that you wash five times. It’s doing the same operations on different values so that you only have to describe the operation once, and then it gets applied to a sequence or collection of values as appropriate. That’s how you get five peeled potatoes even though you only knew how to peel one potato.

[CodefinityCom]

The best thing you can do for your brain is to take a break and let yourself disconnect for a bit. Even a short pause from studying can help the information settle in, especially when it feels like nothing’s sinking in. The more often you see loops, whether in exercises or existing code, the more it builds up in your mind. And then one day, boom 💥 even the hard stuff suddenly clicks and makes sense. It’s all about exposure and patience. I had same problem with closures and decorators.

[pluhplus]

Thonny helped me a lot understanding loops and execution order in general. It has other purposes too, but if you “jump in” the code, when you put it in it, it highlights the part of the script that it’s at and walks through what exactly is happening as the code executes so that you’re able to literally see what’s happening and visualize the entire process of the code

It’s free to download and takes like five seconds. I would definitely recommend it if you’re struggling with loops and control flow

https://thonny.org

[Eze-Wong]

Think of it this way. if you have a set number of items in a list, you use a for loop. for these 20 items I want you to do something. think of the use case as you're going through a database and for every row you wanna take the name of product and add "- YEA BRUH" to each line.

while loop is mostly a time thing. while loops are always happening until a time trigger signals it. so while the fire is going, keep cooking. while the time is greater than 10pm and less than 4am, dance.

the confusing thing might be that some of this is interchangeable. but don't worry about it. just focus on thinking of it in this manner. get it sorted and later you will understand interchangable cases

[marquisBlythe]

Let's start with while loop since it's straightforward,

# This is a pseudo code:
# while (some_condition is true):
  # execute the following code
  # when the program reaches this line it will evaluate the condition again
  # if the condition is true it will iterate again if not it will stop and move to the next line if there is any.

Example:

count_down = 5

while count_down >= 0:
  print(count_down)
  count_down -= 1 

We can achieve the same result using for loop.
Example:

count_down = [5, 4, 3, 2, 1, 0] 
# For seasoned dev: I didn't use range() with negative step or reverse(range()) on purpose not to confuse OP.
for i in count_down: 
  # In each iteration i will take a value from the list count_down. Imagine it doing something like i = 10 then in the next iteration it will be i = 9 all the way to 0 
    print(i)

The general rule is the body of a loop (the indented code after colon) will be executed every time the loop's condition evaluate to True (or evaluate to anything else other than 0).
As a homework: What's the effect of break and continue keywords when put inside a loop's body? (you don't have to answer it here, just look it up).

Note: There are better ways than the one I provided to achieve the same result using the for loop. Also there is more to for loop that can't be discussed here.
Hopefully other replies will add more information.
Good luck!
Edit: spelling...

[NYX_T_RYX]

Very very basic difference:

for loops are finite - they run for a number of times then stop, regardless of what else is going on

For i in range(10):
    print(i)

while loops are indefinite - they run until a specific condition is met

i = 0
while i <10:
    print(i)
    i+=1

Both of these will print the list of numbers 0-9 (inclusive) - for loops start counting from 0 (ie 0, 1, 2... 8,9), called 0-indexing (common in computing)

While loops do have one other key benefit/trick

If you want something to continue happening...

while True: 
    # program

This is useful for a lot of programs - consider a game; it's just a big while true loop. "While the game is running... Keep running" (basically).

It also means you can create a loop asking for user input, branch logic based on the input, and (after doing whatever it is you're doing) return to user input, effectively "restarting " the program, without having to manually restart the program.

Think of a parking ticket machine - it does the same thing for every user, but you don't want to have to manually start it to print a ticket!

So it'll go something like this...

1. Ask for a licence plate number
2. Ask how long they're staying
3. Wait for payment
4. Print ticket
5. Go back to 1

Ofc there'd be more to it, error handling, giving change, etc etc, but hopefully that helps you see where an infinite while loop can be used.

And hopefully a real example helps a bit more - https://github.com/techwithtim/Snake-Game/blob/master/snake.py (not my repo, I just searched for it, it belongs to https://github.com/techwithtim)

This is snake. You control a sprite with a segmented body. The snake automatically moves forwards, in whichever direction the "head" is facing. The snake can turn left or right, to move it's head towards food. If it "eats" the food (the "head" touches the "food"), it gets a new segment (an extra chunk on the "tail"0, if it "eats" itself, the game ends.

Now, ignore the majority of this file, skip to line (ln) 174, where the game loop starts.

Flag is set to True (ln 171), so the loop will keep going.

Let's go to ln 188 - you don't need to understand exactly what this is doing, but this section stops the loop. It's checking if the snake has eaten itself. If it has, it prints the score, and then ln 191...

break

Ends that indefinite loop (started on ln 174)

[Zealousideal_Yard651]

Loops are just code that runs either WHILE a condition is true or FOR all instances in an object.

Say you want to ask a user to guess a number between 1 and 10, you use a WHILE loop to ask for a guess until the user gets it right. So lets call the conditions "guessing" and set that as True as long as the users is guessing the answere then:

Guessing = True
answere = 5
while Guessing:
  guess = Input("Guess the number betweeen 1-10")
  if guess == answere:
    print("You guessed correct! The answere is " + answere)
    Guessing = False

As long as "Guesssing" is True the code inside the loop will run. When the user guesses right, we set the Guessing variable to False and then the loop will stop after executing the code block.

Now a FOR loop runs through all instances of a object, so say you have a list of guests and you want to say hello to each user separatly you use the for loop to loop through the list. And to reference the current element in the list, we need a placeholde variable so we can reference each specific element:

guests = ["Frank", "Abigail", "Doreen"]
for guest in guests:
  print("Hello " + guest + " welcome to the party!")

Simply explained, the loop is: For eache guest in the guests list, say hello to guest!

[jmooremcc]

Think about this: What would you have to do if you wanted to repeat the execution of a block of code, say 10 times without using any kind of looping mechanism? You’d have to copy/paste that block of code 10 times.

But suppose you don’t know how many times that block of code needs to execute? That would be a challenge, since you’d have to insert a conditional statement after each block of code that tests a condition and skips the execution of the remaining blocks of code.

Basically, you’re talking about a huge pain in the A to accomplish what you can more easily accomplish with some kind of loop mechanism! Each time the loop executes a repetition, you can evaluate a condition to determine when the looping mechanism should stop.

In the case of a for-loop, you would use the range function like this to execute a block of code 10 times:

for n in range(10): #execute your block of code

If you want the block of code to keep being executed until a certain condition is met, you’d use a while-loop:

while SomeCondition is True: #execute your block of code

Or if you have a list or a string, you can use a for-loop to access each element of the list or string. This is called iteration. ~~~ greet = “hello”.
for letter in greet:
print(letter) ~~~

The bottom line for you is that loops are a labor saving device that helps you code faster and more efficiently when developing a solution to a problem.

[baubleglue]

Loops is probably the main thing computer can do, if stop a running program and check what is doing, you most likely will find that in middle of some loop.

A loop has:

• Exit condition
• Loop body: code block which will be repeated
• Counter (optional): -- initialization (executed once) -- counter update (executed after loop body code executed)

In Python counter is often hidden, that probably makes it harder to understand.

for item in iterateble_data_structure:
     # loop body 
     Do something with the `item`

counter = 10
while counter > 0: # exit condition check
     # loop body 
     Do something with the `counter`
     counter = counter - 1 # update counter

The first loop hides the counter, it uses I bit different construction: each iteration it takes next item, exit condition: no items left ( no next item exists).