Delete in high element list in python-CodePudding

How can I quickly delete elements in b from inside a ?

import itertools

a=list(itertools.product("12X",repeat=15))
b=list(itertools.product("1X",repeat=15))
print("a len = ",len(a))
print("b len = ",len(b))

for x in b:
    if x in a:
        a.remove(x)

It's very slow.

CodePudding user response：

Convert the lists to sets and just subtract.

import itertools

a=list(itertools.product("123",repeat=3))
b=list(itertools.product("12",repeat=3))
print("a len = ",len(a))
print("b len = ",len(b))
c = set(a) - set(b)
print(len(c))

# This is the equivalent of
c = set(a).difference(set(b))

>>> a len =  27
>>> b len =  8
>>> 19

You can also do set(a).intersection(set(b)) in order to return those that appear in both lists.

If order matters, just use a list comprehension:

excluded_elems = set(b)
c = [x for x in a if x not in excluded_elems]
print(len(c))

Make sure to compute set(b) once, outside the comprehension, instead of inlining it in the not in check, which would compute it on every iteration.

CodePudding user response：

You can try something like :

c = set(a).difference(set(b))
a = [ x for x in a if x not in c]