I have a list of number:
a=[2,3,4,5,1,3,2,4,5,6,2,6,7,5,2,7,5,6,2]
I want the longest sequence that not contain 2, so the answer is:
[3,4,5,1,3]
How can I do this in python? Thanks for helping me,
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1 Answer
You can use itertools.groupby():
from itertools import groupby
a = [2, 3, 4, 5, 1, 3, 2, 4, 5, 6, 2, 6, 7, 5, 2, 7, 5, 6, 2]
# get the subsequences not containing 2
subsequences = (list(it)
for contains_two, it in groupby(a, lambda x: x == 2)
if not contains_two)
# find the longest one among them
print(max(subsequences, key=len))
prints
[3, 4, 5, 1, 3]
5 Comments
jammon
That's why python is great! Many complex things can be done in one or two lines.
Lennart Regebro
@jammon: If you add comments. Because the above code is almost incomprehensible. :)
jammon
@Lennart: You're right, this version is much, much better. I was just pondering on a solution to the question and had something with
ifs and fors and some 5 to 10 lines in mind. Then I discovered this answer (and looked up itertools.groupby in the docs) and was once more baffled by the conciseness of a pythonic solution.Lennart Regebro
And comments now added, and the code is much more comprehensible. :)
Sven Marnach
@Lennart: I think the main reason the code is more readable is that I reformatted it and gave meaningful names to the variables. Thanks for the comments anyway!