I'm getting started with Python and is currently learning about list comprehensions so this may sound really strange.
Question: Is it possible to use list comprehension to create a list of elements in t that is not found in s?
I tried the following and it gave me an error:
>>> t = [1, 2, 3, 4, 5]
>>> s = [1, 3, 5]
>>>[t for t not in s]
[t for t not in s]
^
SyntaxError: invalid syntax
4 Answers
Try this:
[x for x in t if x not in s]
You can nest any for if statements in list comprehensions. Try this identation, to get really long chains of conditionals, with a clearer intuition about what the code is doing.
my_list = [(x,a)
for x in t
if x not in s
if x > 0
for a in y
...]
See?
[item for item in t if item not in s]
I know you're asking about list comprehensions, but I wanted to point out that this specific problem would be better accomplished using sets. The result you want is the difference of set t and s:
>>> t = {1,2,3,4,5}
>>> s = {1,3,5}
>>>
>>> t - s
set([2, 4])
>>>
>>> t.difference(s)
set([2, 4])
Just hoping to expand your knowledge of the tools Python provides to you.
For better efficiency, use a set:
mySet = set(s)
result = [x for x in t if x not in mySet]
Testing membership of a set is done is O(1), but testing membership in a list is O(n).
