Search code examples
pythonpython-itertools

fast deletion in a list with too many elements - PYTHON

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.

Solution

  • 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.