Never alter the container you're looping on, because iterators on that container are not going to be informed of your alterations and, as you've noticed, that's quite likely to produce a very different loop and/or an incorrect one. In normal cases, looping on a copy of the container helps, but in your case it's clear that you don't want that, as the container will be empty after 50 legs of the loop and if you then try popping again you'll get an exception.
What's anything BUT clear is, what behavior are you trying to achieve, if any?! Maybe you can express your desires with a while...?
i = 0
while i < len(some_list):
print i,
print some_list.pop(0),
print some_list.pop(0)
Never alter the container you're looping on, because iterators on that container are not going to be informed of your alterations and, as you've noticed, that's quite likely to produce a very different loop and/or an incorrect one. In normal cases, looping on a copy of the container helps, but in your case it's clear that you don't want that, as the container will be empty after 50 legs of the loop and if you then try popping again you'll get an exception.
What's anything BUT clear is, what behavior are you trying to achieve, if any?! Maybe you can express your desires with a while...?
i = 0
while i < len(some_list):
print i,
print some_list.pop(0),
print some_list.pop(0)
I've been bitten before by (someone else's) "clever" code that tries to modify a list while iterating over it. I resolved that I would never do it under any circumstance.
You can use the slice operator mylist[::3] to skip across to every third item in your list.
mylist = [i for i in range(100)]
for i in mylist[::3]:
print(i)
Other points about my example relate to new syntax in python 3.0.
- I use a list comprehension to define mylist because it works in Python 3.0 (see below)
- print is a function in python 3.0
Python 3.0 range() now behaves like xrange() used to behave, except it works with values of arbitrary size. The latter no longer exists.
Since the loop below only modifies elements already seen, it would be considered acceptable:
a = ['a',' b', 'c ', ' d ']
for i, s in enumerate(a):
a[i] = s.strip()
print(a) # -> ['a', 'b', 'c', 'd']
Which is different from:
a[:] = [s.strip() for s in a]
in that it doesn't require the creation of a temporary list and an assignment of it to replace the original, although it does require more indexing operations.
Caution: Although you can modify entries this way, you can't change the number of items in the list without risking the chance of encountering problems.
Here's an example of what I mean—deleting an entry messes-up the indexing from that point on:
b = ['a', ' b', 'c ', ' d ']
for i, s in enumerate(b):
if s.strip() != b[i]: # leading or trailing whitespace?
del b[i]
print(b) # -> ['a', 'c '] # WRONG!
(The result is wrong because it didn't delete all the items it should have.)
Update
Since this is a fairly popular answer, here's how to effectively delete entries "in-place" (even though that's not exactly the question):
b = ['a',' b', 'c ', ' d ']
b[:] = [entry for entry in b if entry.strip() == entry]
print(b) # -> ['a'] # CORRECT
See How to remove items from a list while iterating?.
It's considered poor form. Use a list comprehension instead, with slice assignment if you need to retain existing references to the list.
a = [1, 3, 5]
b = a
a[:] = [x + 2 for x in a]
print(b)
Help with looping through list and changing a value(s)
python - Modify a list while iterating - Stack Overflow
Trying to learn to modify a list with a for loop
Iterating over a list to modify the list
Below is the code I am having issues with. I'm trying to loop through a list, but modify the value as I iterate. I am unsure on how to access the particular element of the list that I want to change.
for recipe in recipe_list:
print(f"{recipe}? Enter Y is yes.")
if input().lower() == "y":
print("Enter the new recipe: ", end = "")
recipe = input()I feel that my issue lies line 1.
You are not modifying the list, so to speak. You are simply modifying the elements in the list. I don't believe this is a problem.
To answer your second question, both ways are indeed allowed (as you know, since you ran the code), but it would depend on the situation. Are the contents mutable or immutable?
For example, if you want to add one to every element in a list of integers, this would not work:
>>> x = [1, 2, 3, 4, 5]
>>> for i in x:
... i += 1
...
>>> x
[1, 2, 3, 4, 5]
Indeed, ints are immutable objects. Instead, you'd need to iterate over the indices and change the element at each index, like this:
>>> for i in range(len(x)):
... x[i] += 1
...
>>> x
[2, 3, 4, 5, 6]
If your items are mutable, then the first method (of directly iterating over the elements rather than the indices) is more efficient without a doubt, because the extra step of indexing is an overhead that can be avoided since those elements are mutable.
I know you should not add/remove items while iterating over a list. But can I modify an item in a list I'm iterating over if I do not change the list length?
You're not modifying the list in any way at all. What you are modifying is the elements in the list; That is perfectly fine. As long as you don't directly change the actual list, you're fine.
There's no need to iterate over the indices. In fact, that's unidiomatic. Unless you are actually trying to change the list itself, simply iterate over the list by value.
If the answer is yes, will the following snippet be valid?
lovely_numbers = [[41, 32, 17], [26, 55]] for numbers_pair in lovely_numbers: numbers_pair.pop() print(lovely_numbers) # [[41, 32], [26]]
Absolutely. For the exact same reasons as I said above. Your not modifying lovely_numbers itself. Rather, you're only modifying the elements in lovely_numbers.
list = [1,0,1,0,1,1,0,0] print(list) for i in list:
if i == 0: i = 1
print(list)
I have a list and would like to for loop to change all of the 0s to 1s
but no changes happen
the if statement is catching the 0s but not changing them to 1s
idk why "i = 1" is not working
Help! I'm in tutorial hell and can't get out!
I'm trying to write a module that I can call to work over a set of measurements. The idea is to be able to copy a bunch of measurements in short hand (e.g. 5' 6-3/16") and have python take that string, separate it into a list, and iterate over the list with a function that turns each measurement into a float value of inches. I've written the funciton, I'm working on turning the input into a list, but what I don't know how to do is take a list (call it x), and run my function (call it imperial) on each item on the list, and return the results back to x (or even some other variable). Instead python seems to be trying to run imperial on the whole list at once, which produces angry errors.