Question

I have a number of lists that can be “opened” or “closed”, something like this:

lista = ["a", "b", "c"]
listb = ["d", "e"]
listc = ["a", "b", "e"]
listd = ["c", "d"]

I have a master list of all open items:

all_open = ["a", "b", "c", "e"]

and a list of open lists:

open_lists = ["lista", "listc"]

As the sublists are openend, their items are added to the master list:

open_lists.append("listb")
for each i in listb:
    if !(i in all_open):
        all_open.append(i)

Is there a simple algorithm to remove items from the master list when a sublist is closed? The goal is to not remove items that belong to other lists that are still open.

Answer 1

You have to keep track of how many lists each item was from. The simplest way to do that is with a map. I like to use collections.Counter for something like this.

import collections
count = collections.Counter()

# add a list
for i in listb:
    if count[i] == 0:
        all_open.append(i)
    count[i] += 1

# delete a list
for i in listb:
    count[i] -= 1
    if count[i] == 0:
        all_open.remove(i)

In addition, you can get rid of all_open altogether and use the count.keys() iterator instead.