Remove all occurrences of item(s) in list if it appears more than once

Question

I need help with a coding challenge that is asking to remove all occurrences of an item within a list that appear more than once. My code only removes one occurrence. It will not remove the item completely.

def solution(data, n):
    for x in data:
        while data.count(x) > 1:
            data.remove(x)
            continue        
    print(data)

solution([1, 2, 2, 3, 3, 4, 5, 5], 1)

expected result: [1, 4]
actual restult: [1, 2, 3, 4, 5]

Answer 1

you can use collections.Counter to count all instances of each element in data , then print only the relevant ones based on this count:

from collections import Counter


def solution(data, n):
    histogram = Counter(data)
    print([d for d in data if histogram[d] <= n])

solution([1, 2, 2, 3, 3, 4, 5, 5], 1)

or just use data.count() directly:

def solution(data, n):
    print([d for d in data if data.count(d) <= n])

solution([1, 2, 2, 3, 3, 4, 5, 5], 1)

Answer 2

As this is a coding challenge, it might be better to not rely upon Python-specific stuff, like collections or the count() function.

You could generically do this in any language using a hash table, which in Python is just a dictionary.

I assume the presence of the parameter n is to make the threshold above which you remove the number configurable, and not simply hard-coded to 1.

data = [1, 2, 2, 3, 3, 4, 5, 5]

def solution(data, n):
    occurrences = {}
    # count occurrences
    for item in data:
        occurrences[item] = occurrences.get(item, 0) + 1
    # return trimmed data
    trimmed_data = []
    for item in data:
        if occurrences[item] <= n:
            trimmed_data.append(item)
    return trimmed_data

solution(data, 1)

yields [1, 4]

This has O(n) time complexity.

You can save the second list by re-using the first one:

def solution(data, n):
    occurrences = {}
    # count occurrences
    for item in data:
        occurrences[item] = occurrences.get(item, 0) + 1
    # return trimmed data
    data.clear()
    for k, v in occurrences.items():
        if v <= n:
            data.append(k)
    return data

Answer 3

You could do:

def solution(data, n):
    for x in set(data):
        if data.count(x) > n:
            while x in data:
                data.remove(x)
    return data

Some explanation:

• set(data) makes that every value is checked only once, even if it occurs multiple times in data ;
• you should remove the continue statement, it does not do what you probably want it to do;
• look carefully what I am doing with the if statement and the while statement;
• in your code, the n argument was not used, I assumed that it should replace the 1 in the if -line.

Update: Here is the one line version!

solution = lambda data, n: [x for x in set(data) if data.count(x) <= n]

You can call the solution function the same as before. If you are puzzled about this syntax, you could search 'python list comprehensions' and 'python lambda'.