Python: filter list of list with another list

Question

i'm trying to filter a list, i want to extract from a list A (is a list of lists), the elements what matches they key index 0, with another list B what has a serie of values

like this

list_a = list(
  list(1, ...),
  list(5, ...),
  list(8, ...),
  list(14, ...)
)

list_b = list(5, 8)

return filter(lambda list_a: list_a[0] in list_b, list_a)

should return:

list(
    list(5, ...),
    list(8, ...)
)

How can i do this? Thanks!

Solution 1

Use a list comprehension:

result = [x for x in list_a if x[0] in list_b]

For improved performance convert list_b to a set first.

As @kevin noted in comments something like list(5,8)(unless it's not a pseudo-code) is invalid and you'll get an error.

list() accepts only one item and that item should be iterable/iterator

Solution 2

You are actually very close. Just do this:

list_a = list(
  list(1, ...),
  list(5, ...),
  list(8, ...),
  list(14, ...)
)

# Fix the syntax here
list_b = [5, 8]

return filter(lambda list_a: list_a[0] in list_b, list_a)

Comments

[Kevin]

Your solution works for me if I fix the constructors for the lists. (Hint: use [5,8] instead of list(5,8))

[Danica]

Your comment about making the function beforehand is mistaken: filter(lambda list_a: ..., list_a) would do exactly the same thing, since the lambda statement is evaluated down to a function value before being passed to filter.

[Admin]

@Dougal - Thank you. I did that because, a while back, someone got on me saying it re-created the lambda each time. I thought that sounded wrong, but assumed he knew more than me and thus went with his advice. Shows what you get for not double-checking info...

[Tom N Tech]

A more generic approach is to intersect the two lists.

[Ashwini Chaudhary]

@rjurney If you mean by using sets, then depends, do you want to lose order or preserve it? And thanks for the useful downvote.