Iterating portions of lists - PYTHON

Question

Bit of a generic noob question. I am heavily dealing with long lists of integer/float values and wasting a lot of time.

my_list = [1,2,3,4,5,6,7,8,9,10.....] etc.

Say I want to pass a portion of that list to a function. It could be the first 3 elements, then the following 3 etc....it could also be in groups of 4,5,6....it might even be required that I take different a different amount of elements each time.

def myfunc(x,y,z):
 do something
 return something

What is the most efficient way to iterate by a specified number of values, as efficiency is always appreciated and these simple iterations are the places where I can gain something.

The python syntax to get only a portion of a list is `myList[x:y]` to get the values between `x` and `y`. Would that work for your use case? — JPeroutek, Jul 17 '15 at 16:02
It looks like this question has already been answered here: http://stackoverflow.com/questions/8671280/pythonic-way-to-iterate-over-part-of-a-list — Thomas Wetmore, Jul 17 '15 at 16:05
Also check out http://stackoverflow.com/questions/434287/what-is-the-most-pythonic-way-to-iterate-over-a-list-in-chunks?rq=1 — PM 2Ring, Jul 17 '15 at 16:08

score 0 · Answer 1 · answered Jul 17 '15 at 16:03

0

len_ml = len(my_list)
for i in range(0, len_ml, 3):
  chunk = my_list[i:min(len_ml, i+3)]

This is a way. I am not sure that it is the best, though.

answered Jul 17 '15 at 16:03

bcdan

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score 0 · Answer 2 · answered Jul 17 '15 at 16:09

With a list you can get just the items you want with list[start:end].

So to skip the first list[1:] or last list[:-1] or first 3 and last 3 list[3:-3]

if you don't know how may items are in the list to start you can do a len(list) to get number. So if you had X items and wanted 3 groups :

numberofgroups = len(list) / 3

To do for loop over only certain ones:

start_index=1

end_index=-1

for item in my_list[start_index:end_index]
    print item

Kousik · Answer 3 · 2015-07-17T16:25:17.247

0

>>>my_list = [1,2,3,4,5,6,7,8,9,10]
>>>group_len = 3 #you can change the length as per your requirement i.e 2,3,4,5,6...
>>>for i in range(0,len(my_list)):
       if i*group_len < len(my_list):
           my_list[i*group_len:(i+1)*group_len]
       else:
           break;
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
[10]

Result for group_len = 5

[1, 2, 3, 4, 5]
[6, 7, 8, 9, 10]

edited Jul 17 '15 at 16:25

answered Jul 17 '15 at 16:17

Kousik

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Iterating portions of lists - PYTHON

3 Answers3