CODEWITHSUNDEEP
pythonlistloops
user3528875
user3528875

Reputation: 13

Modify list with loop in Python: Is my method of doing so wrong?

I have just started learning Python so i just wanted to get something straight. I want to make a function that repeatedly appends the sum of the current last three elements of the list "lst" to "lst", x number of "times". As far as i understand you should not change a list while iterating over it, so my second solution is wrong, right? Even though it produces the same result as the first function?

def appendsums(lst, times):
    count = 0
    for i in range(times):
        if count <= times:
            sums = sum(lst[-3:])
            lst.append(sums)
            count += 1
    return lst

Here is my second solution

def appendsums(lst, times):
    count = 0
    while count <= times:
        sums = sum(lst[-3:])
        lst.append(sums)
        count += 1
    return lst

Regards

Upvotes: 1

Views: 301

Answers (2)

Magnun Leno
Magnun Leno

Reputation: 2738

You're correct, you shouldn't iterate over a list while editing. But as pointed by other users, none of the above examples are iterating over the list lst.

Here is an example of iteration:

for item in lst:
    # Do something

If you need to iterate over a list while editing it, make a copy and iterate over the copy:

copy_lst = lst[:]
for item in copy_lst:
     # edit lst

I'd stick with the following code:

def appendsums(lst, times):
    for i in range(times):
        lst.append(sum(lst[-3:]))
    return lst

Upvotes: 2

RohitJ
RohitJ

Reputation: 563

Its generally unsafe because iterators on the container aren't informed that a change has occurred. You usually want to create a temporary list and then modify the target list at the end.

Also, as a side note, I think you may want count to be less than times (and not equal to it).

In its current form, when I set times to 5, it adds six entries. 

>>> def appendsums(lst, times):
...   count = 0
...   while count <= times:
...     sums = sum(lst[-3:])
...     lst.append(sums)
...     count += 1
...   return lst
... 
>>> appendsums([1,2,3], 5)
[1, 2, 3, 6, 11, 20, 37, 68, 125]

Upvotes: 2

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