How to find list intersection?

Question

a = [1,2,3,4,5]
b = [1,3,5,6]
c = a and b
print c

actual output: [1,3,5,6] expected output: [1,3,5]

How can we ac

Answer 1

Make a set out of the larger one:

_auxset = set(a)

Then,

c = [x for x in b if x in _auxset]

will do what you want (preserving b's ordering, not a's -- can't necessarily preserve both) and do it fast. (Using if x in a as the condition in the list comprehension would also work, and avoid the need to build _auxset, but unfortunately for lists of substantial length it would be a lot slower).

If you want the result to be sorted, rather than preserve either list's ordering, an even neater way might be:

c = sorted(set(a).intersection(b))