CODEWITHSUNDEEP
python
Gemma Down
Gemma Down

Reputation: 100

Check if object is already in list (Python)

I have tried searching online for this, but unfortunately have not been able to come up with a solution so far, so I was hoping someone might be able to help me. My basic question is how can I check whether an object is already in a list?

I am currently working on a Rubik's Cube solver, and have created a class called MyCube, where each object has 4 properties:

def __init__(self, cp=None, co=None, ep=None, eo=None):

There are a number of methods for changing the properties, and I have also created methods for __eq__ and __ne__ as follows:

def __eq__(self, other):
    if type(other) is type(self):
        return self.__dict__ == other.__dict__
    return False

def __ne__(self, other):
    return not self.__eq__(other)

I have a list of cubes MyMoveCube = [MyCube() for i in range(18)] and each cube goes through various different transformations. I then have another cube MyNewCube and I want to check whether it is already in MyMoveCube and if not to add it to the list.

I have tried the following, but I find that this gets very slow very quickly as the size of MyMoveCube increases:

for current_move in MyMoveCube:
    if current_move == MyNewCube:
        break
    MyMoveCube.append(MyNewCube)

My question is, is there a better way to do this without looping through it each time?

Upvotes: 2

Views: 1983

Answers (2)

user2357112
user2357112

Reputation: 281949

If you want to do something if MyNewCube is already in MyMoveCube,

if MyNewCube in MyMoveCube:
    do_whatever()

or if you want to do something if it isn't,

if MyNewCube not in MyMoveCube:
    do_whatever()

(I initially didn't see the part about running into speed problems; this answer just demonstrates in syntax. Testing lists for containment is horribly slow; for efficiency, see the other answer.)

Upvotes: 2

Martin Konecny
Martin Konecny

Reputation: 59681

"but I find that this gets very slow very quickly as the size of MyMoveCube increases"

You will want to look into using a set to store your MyCube instances.

MyMoveCube = set([MyCube() for i in range(18)])

Hashing an object and checking whether it's in a set is a very efficient operation - O(1) in the average case compared to O(n) in the average case for a list.

You can still use the same in operators with a set, with much faster lookup:

if MyNewCube in MyMoveCube:
    # in cube

EDIT:

If you need to keep track of the order of the items in the set, you could possibly use a set AND a list. The list for tracking the order and the set for testing membership.

Upvotes: 1

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