adding to a variable in a nested list comprehension

Question

I'm attempting to make a nested list comprehension, but I can't figure out how I should do it. currently, I have a loop like this:

filtered = []
p = -1
for i in list:
  p += 1
  for k in list_of_lists[p]:
    if not k in filter:
      filtered.append(k)

While this works, it takes about 5-8 seconds for it to complete, and this amount of time is nearly unacceptable for the circumstance that it is being used. I'm trying to make it in to a list comprehension, but I can't seem to figure out a way to make the p += 1 in the list comprehension. I attempted this:

filtered = [i for i in list for k ind list_of_list[p], p+=1]

but it clearly doesn't work. I was wondering if there was anyway to get around this.

Can you provide an example of your input and desired output? — Allen Qin
– Allen Qin, Commented May 1, 2017 at 0:16
Why are you looping through list and then using list_of_lists? — Neil
– Neil, Commented May 1, 2017 at 0:20
list is built-in Python function. Why did you redefine it? — DYZ
– DYZ, Commented May 1, 2017 at 0:22
why are you keeping track of your own counter all al? Anyway, an equivalent list comprehension is only going to give you at best marginal performance increase. The problem is that your algorithm is quadratic time in the number of items you are scanning, because you check filtered each time, which is a list, so it will have linear membership testing. Use a set — juanpa.arrivillaga
– juanpa.arrivillaga, Commented May 1, 2017 at 0:46

Neil · Accepted Answer · 2017-05-01 01:04:07Z

1

I would flatten it and then convert it to a set because you can't self reference inside a list comprehension. The difference is a set can only have one of each item and order is not enforced.

list_of_lists = [["blue","green","red"],["red","yellow","white"],["orange","yellow","green"]]
filtered = set(y for x in list_of_lists for y in x)
print(filtered)

edited May 1, 2017 at 1:04

answered May 1, 2017 at 0:24

Neil

14.3k3 gold badges35 silver badges53 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

juanpa.arrivillaga Over a year ago

I think this is the right approach, but I wouldn't say that a "set and list" function "basically the same". They are two different container types with very different use-cases. Lists are sequences, so they are ordered and have indexing behavior, plus they have linear-time membership testing. Sets are unordered, only hold unique values, and have constant-time membership testing.

Collectives™ on Stack Overflow

adding to a variable in a nested list comprehension

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related