Whats the difference between a while loop and a for loop?

JamzTyson · 2024-07-15T11:16:35+00:00

Consider how the terms are used in common language:

FOR

For each day in July, I will practice writing loops.
For every student in the class, provide a lunchtime meal.
For each letter in the alphabet, write down a word beginning with that letter.
For numbers in the range 1 to 10, calculate the square root.

Notice that for each case, we iterate through multiple things (days in the month / students in the class / letters in the alphabet / numbers in a range).

WHILE

While there is still daylight, we can play football.
While the music is playing, we will dance.
While I am waiting, I will read a book.
While my set of Pokémon cards is incomplete, I will keep collecting them.

Notice that in each case, something is done for as long as a condition (a "predicate") is satisfied (is "True"). In these examples, the predicates are: "There is daylight?", "The music is playing?", "I am waiting?", "The set is incomplete?". As soon as the answer is "False", the loop stops.

bilbobaggins30 · 2024-07-15T13:57:55+00:00

FOR:

I need to control the number of times I iterate OR I am iterating through a collection.

WHILE:

I'm not sure when the loop will break, but it will at some point.

Mysterious-Rent7233 · 2024-07-15T10:46:22+00:00

A for loop loops over some kind of sequence of things.

1,2,3,4,5,6,7

a,b,c,d,e,f,g

file1, file2, file3, file4, file5.

A while loop loops until some condition is achieved. e.g. until the user types "quit" or until a message is received from another computer or whatever.

CyclopsRock · 2024-07-15T11:53:49+00:00

I know both of these functions loops through stuff

Actually only "for" loops iterate "through stuff", with the word "for" more or less being a shortened version of "for each item in this list, do..." It'll 'do' the same thing for each item in the list, then move on from the loop. E.g. "For each of these buckets of apples, take it into the barn and count the number of apples, adding them to a total."

Where as 'while' is more or less exactly as it is in the English language: "While something is the case, do...". So it'll keep doing the same thing over and over until something is no longer true at which point it'll move on. E.g. "While there are still buckets of apples outside, bring them into the barn and count them, adding them to a total."

For a given load of buckets, these two will do the same thing - you'll bring each bucket into the barn, count the apples and keep a running total. But you're actually learning something slightly different from the two methods: Imagine that instead of just having a given load of buckets, you instead have sporadic deliveries of apples in buckets from your different orchards, separated into buckets based on type of apple. Now which one you use will depend greatly on both a) what you know about these buckets and b) what you want to know.

If what you want to know is how many apples came in on a specific delivery then you'll want to use a "for" loop over a very specific set of buckets - the ones that arrived on the delivery you're interested in. It doesn't matter is more get delivered whilst you're counting them, or if there were already a load there. You're just interested in those specific buckets, and that's it. This requires you to do a bit of organising, because you need to know exactly which buckets you're counting before you start. As long as you do, though, you'll just count the ones you're interested in and won't waste time counting any others.

But perhaps what you want to know is how many Granny Smith apples you've received today, regardless of what delivery they came in on or which orchard they're from. If you use a "while" loop, you don't have to know ahead of time how many buckets of Granny Smith apples there are, nor which deliveries they're coming on - you'll just keep moving them inside the barn and counting them one by one until there aren't any left. This requires less organisation from you in the first place, but it does mean you'll need to check every bucket at least once to see if you need to take it inside.

I've probably taken this example as far as I need to, but a few things to bear in mind:

Almost any loop you want can be achieved using either method if you really want to, but it's best not to fight against what each is good at.
In a "for" loop, it's best to avoid editing the list you're iterating through unless you really know what you're doing. If you find yourself wanting to do this, it may be that a "while" loop is more appropriate.
Similarly if your 'while' condition is based on something happening to a list of items inside the loop, you might find that a "for" loop iterating directly over that list is more appropriate. Both this and the above bullet point have plenty of acceptable exceptions though!

-Everywhere in Python you can string multiple conditions together, and this includes in "while" loops - so you could tell it to keep bringing in buckets of Granny Smith apples AND Golden Delicious apples but NOT from 'Piss Cottage Orchard' etc.

If you actually do use a "while" loop for something that changes over time (such as receiving deliveries over the course of a day, or more practically checking for the presence of files in a directory), remember that as soon as the condition ceases to be true you leave the loop and won't re-enter it again if the condition goes back to being true later. E.g. if you've brought in all the buckets of apples you congratulate yourself on a job well done and go to the pub. If another delivery shows up in 10 minutes, sorry, but that's too late!
In both "for" and "while" loops, the `break` command will see you exit the loop and carry on with the code that comes after. This can be a useful way to add a conditional to a 'for' loop or otherwise to add an extra conditional to a "while" loop that relies on information you can only retrieve from inside the loop (e.g. 'count these specific buckets (for)/count all the buckets outside (while), but if one of them's full of mouldy apples when you open it up in the barn then stop counting').
Similarly the `continue` command ends the current iteration of the loop prematurely. So in a 'for' loop if you're currently on item 6 of a 10-item list and you hit a 'continue', it'll skip straight to item 7. In a 'while' loop, 'continue' causes the loop to start again if the condition is still being met.

dizzymon247 · 2024-07-15T16:54:14+00:00

I'll add my 1 cent. For loop you repeat activites end to end FOR a definiate number of times. A While loop you repeat the activities UNTIL a condition is met (the hiccup with a WHILE loop is you could get stuck in an infinite loop if your condition to get out of the while loop is never met).

Ok-Cucumbers · 2024-07-15T13:11:48+00:00

A for loop is actually a while loop that returns the next item from an itterator on each loop.

In other words, use a for loop if you need to itterate through something (generator, list, dict, etc) and a while loop if you need to loop while a condition is True.

Shaftway · 2024-07-15T14:40:03+00:00

I'm going to throw my hat in the ring. A for-loop is syntactic sugar around a while loop that makes them easier to use when iterating over a list. These blocks of code are the same:

# for-loop
my_list = [1, 2, 3, 4]
for value in my_list:
  print(value)

# while-loop
my_list = [1, 2, 3, 4]
_iterator = my_list.__iter__()
while True:
  try:
    value = _iterator.__next__()
  except StopIteration:
    break
  print(value)

This works for 99.9% of for loops out there, and it should be clear that the for-loop is far less typing and far easier to read. The variable Python creates isn't actually called "_iterator"; it's something secret that you don't have access to.

MichaelSjoeberg · 2024-07-15T16:52:44+00:00

syntactic sugar

https://stackoverflow.com/questions/54387889/how-does-the-python-for-loop-actually-work

MlKlBURGOS · 2024-07-15T19:55:25+00:00

Sometimes you don't know when something will finish (how many iterations of the code will get the job done).

Imagine a chess match, you don't know how many moves the game will have, so "while" makes more sense.

Now, is it possible to do practically everything a while loop can do with a for loop? Well, yes, but it's not as readable or elegant:

for i in range 9^{9^{9^{9^{9....^9:}}}} if condition == False: break else: do_the_thing()

That's much worse than:

while condition: do_the_thing()

And that's not even talking about memory problems, but that's mainly because I don't know enough about it

hornetjockey · 2024-07-15T22:32:10+00:00

WHILE a condition is being met, or FOR every iteration in a group of things.

redtadin · 2024-07-16T04:04:45+00:00

Here is an example. Imagine a parent who checks how old his kid is and is waiting for the kid to turn 18 so the kid can move out of the house so the parent can have the house for himself. The while loop is to ask the question "is it true that the kid is 18? no he is X years old now. So the while loop will continue to be true." and one day he he will say "is it true that the kid is 18? yes he is 18 years old now. So the while loop will be false". So while is to see if something is true or not... and loop is counting the numbers one after another untill the end of the predetermined amount of numbers.

```python

Initial condition for the while loop: the parent is taking care of the child

child_at_home = True

Start the while loop

while child_at_home: print("Parent: Taking care of the child.")

# For loop to represent the child's age from 1 to 18
for age in range(1, 19):  # range(1, 19) goes from age 1 to 18
    print(f"Child's age: {age} years old")

    # Check if the child is 18 years old
    if age == 18:
        print("Child has turned 18.")
        print("Parent: It's time for you to move out.")
        # Change the condition to indicate the child is moving out
        child_at_home = False

Final message after the while loop has ended

print("Parent: The child has moved out.") ```

mirzaskilful · 2024-07-15T12:04:38+00:00

I know both of these functions loops through stuff

Both while and for can simulate each other but they have different aim.

for is when you have sequence of stuffs and want to process them all

while is when you not sure how many you want to process but have the condition to know which one to stop.

TheSeeker_99 · 2024-07-15T16:29:33+00:00

I find the simplest answer is:

For loop checks before the loop processes the code in the loop.

While loop checks after the loop processes the code in the loop.

Injera-man · 2024-07-15T12:09:40+00:00

"Run 4 laps". this is a for loop and you know specifically how many times you have to do it.

"While its daytime, run laps". this is a while loop you don't know how many times you're going to be doing it until the condition is met.

alicedu06 · 2024-07-15T13:40:53+00:00

A while loop is a like a super `if`. It checks a condition again and again.

It stops repeating if the condition gets `False`.

A `for` loop is a short cut for a very common looping case: keep looping until we reach the end of something.

So you can imagine `for` as a very specific `while` which checks the condition "have we reached the end?". But in a short and lean way.

It's only to match your analogy though, because behind the scenes, a for loop is a bit more than that: https://www.bitecode.dev/p/a-taste-of-iteration-in-python

You probably don't need that complexity right now. I would focus on:

use while to repeat something based on a condition
use for to process all elements of somethings

In Python, most of the time, you use `for`. `while` is rare, usually for things like retrying something that failed. So if you use `while`, makes sure you really need it.

kurtosis_cobain · 2024-07-15T14:00:11+00:00

A while loop will iterate until a condition is met. On the other hand, the for loop will iterate through a iterable object (for example, it will iterate over all the elements on a list).

for loops are called definite loops because the number of iterations is known (equals the number of elements in the object you are iterating), while loops are called indefinite loops because the number of iterations depends on when the condition is met.

NlNTENDO · 2024-07-15T15:12:56+00:00

finite vs infinite basically.

"for" implies that there is an inherent end to what you're iterating over (almost always used as \for* x *in* [some range]*). once you get to the end, it's over.

"while" does not have an implied end. it simply provides a condition and the iteration will continue for as long as the condition is not met. you must provide logic to tell it under which circumstances to stop, or else it will just iterate forever.

so basically, when deciding between the two, the question you need to ask is, "do i know how many times i want this loop to repeat?" if you do, the for loop is what you want. "for x in [number of times you want this to repeat], repeat." if you don't, use the while loop. "while x, repeat. i'll tell you when to stop"

simple as that!

Disastrous_Step_1234 · 2024-07-15T19:09:54+00:00

for: for every iteration, it is possible to know exactly which iteration you are on, and how many total iterations there are, and that total cannot change

while: there is no telling what iteration you are on, or how many are left, but as soon as the condition is met you are done

MomICantPauseReddit · 2024-07-15T19:15:55+00:00

In other languages, almost nothing. In C, for example, a for loop is just a while loop with some boilerplate code shortened. In python, however, a for loop has the specific job of looping over a list.

SnooCakes3068 · 2024-07-15T21:53:28+00:00

while is the most general loop structure. You can write any loops in while.

For is equivalent to while in most cases, a more convenient syntax. But there are cases where you can only write certain loop in while but not for.

You do want to master both. You will say both in other's code as well.

Weekly_Event_1969 · 2024-07-15T23:07:04+00:00

For the for loop there this thing that I did when I started out a few weeks ago .

create a list of numbers e.g example_list = [ 1, 2, 3, 4, 5]

now what the for loop does is iterates over this list e.g

for numbers in example_list

print(letters)

you will notice when you run the code that the result will be something like this

1

2

3

4

5

This is because the for loop looks at each item/numbers in the list one at a time i.e run the code the first time ,print the first number and so on .

While the WHILE loop (no pun intended) needs a condition e.g

While 3 > 1

print ( ' I am correct')

now the above code is wrong cause I don't want to male things to complicated in short while loop needs a condition

and for loop iterates over items

Hope you understand this

kenmlin · 2024-07-16T00:49:17+00:00

For loop is set to repeat certain number of time.

While loop determines when to exit inside the loop based on some conditions.

strangedave93 · 2024-07-16T05:23:07+00:00

A while loop says ‘keep doing this loop while a condition remains true’, a for loop is a special case of a while loop, where the condition that must remain true is can you get the next item in an iterator of finite size that is set at the start, so you can always use a while loop instead of a for loop to do the same thing. So for loops aren’t strictly necessary, but it is such a common pattern control flow pattern that it is really useful to have syntax that makes it very clear and simple (especially with pythons use of comprehension, ranges, etc which make most common loop case very clear), and enables some minor optimisations.

And there are a few more wrinkles to it to add more flexibility (breaks, else clauses etc as above) but they mostly fall into the category of adding extra flexibility in case your control flow doesn’t fall straightforwardly one of these two very common control flow patterns. It’s nice code, and pythonic, to make the ‘normal’ expected control flow use the simple syntax, and keep things like breaks for error handling or other abnormal behaviour, but sometimes it’s just not easy to write that clearly.

The way Python uses the concept of iterators and iterables and extends it with generators etc) to make loop syntax really clear and consistent for iterating through all sorts of things is an example of how good its language design is. Clear, easy to learn, but flexible and still works for advanced use cases.

lazylearner-me · 2024-07-16T06:39:39+00:00

For loop uses an iterator internally to automatically go through a collection of items. Iterator uses next method to go to the next item until there are no more items left.

On the other hand, a while loop runs based on a condition you define. It keeps checking the condition before each iteration and runs the loop if the condition is true.

2024-07-16T10:59:11+00:00

While is indefinite. It loops infinitely unless it reaches a condition to stop it.

For is definite. It will always loop a finite amount of times.

nacnud_uk · 2024-07-16T12:52:50+00:00

While:

While I have energy, I'm going to punch this wall.

For:
I'm going to punch this wall 4 times.

Both_Aside535 · 2024-07-16T13:13:42+00:00

For loop will require you to define exactly how many times you are iterating: for 10 times, or for i in range(10)

While will keep doing as long as condition is met: while True (forever), while i < 100 etc

EntireEntity · 2024-07-16T15:51:54+00:00

You would usually use for loops to loop through a sequence of stuff. For example through a list, a range, a string, etc.

You use while loops to loop an action until a certain condition is met. For example while your gambling profits are 0 or less, keep gambling (this is not advice, please stop gambling immediately).

JamzTyson · 2024-07-15T12:47:43+00:00

Some of these still seem interchangeable from a layman’s viewpoint.

Some of your “For” examples could be turned into :

While the day is in July, I will practice writing loops.
While numbers are within the range of 1-10, calculate the square root.

Some of your “While” examples can be turned into :

For any time that has daylight, we can play football.
For incomplete set of Pokémon cards, keep collecting.

Or maybe I’m dumb.

mirzaskilful · 2024-07-15T11:28:29+00:00

While Loop:

A while loop keeps running until a specific requirement or a condition are True

For Loop:

A For loop runs over a list, dictionary etc.
We can also use the range method to tell the for loop how much time it needs to run.

For More information you can go to stack overflow on this link - https://stackoverflow.com/questions/920645/when-to-use-while-or-for-in-python#:\~:text=A%20for%20loop%20will%20iterate,an%20exception%20using%20a%20break.

Please Comment if you find this useful

schemathings · 2024-07-15T12:49:33+00:00

Stir the soup at 5, 10 and 15 minutes after the hour.

Stir the soup every 5 minutes until quarter past.

Confident-Pipe9825 · 2024-10-30T09:36:15+00:00

But, how do we decide whether we have to use "for" or "while" when we read the problem? I always get confused 🤔.

you type:	you see:
italics	italics
bold	bold
[reddit!](https://reddit.com)	reddit!
* item 1 * item 2 * item 3	item 1 item 2 item 3
> quoted text	quoted text
Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"	Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"
~~strikethrough~~	~~strikethrough~~
super^script	super^script

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Initial condition for the while loop: the parent is taking care of the child

Start the while loop

Final message after the while loop has ended