Given a list, How to count items in that list?

Question

Given the list

List2 = ['Apple', 'Apple', 'Apple', 'Black', 'Black', 'Black', 'Green', 'Green', 'Red', 'Yellow']

I am trying to figure out how to count how many times each element in the list appears. This has to be incredibly simple but I can't figure it out. I read in my book about the count function and I decided to try to implement it. I thought it would be..

for item in List2:
    newlist=[List2.count()] 

I thought this would lead me to what I wanted:

newlist=[3,3,2,1,1]

But I received a TypeError saying count has to have an argument. I'm extremely new to python so dumb everything down as much as possible if you could.

Answer 1

You can use collections.Counter which gives you a dict like object (in that it also has some additional functionality useful for count like purposes) that has key as the item, and a value as the number of occurrences.

from collections import Counter


>>> li = ['Apple', 'Apple', 'Apple', 'Black','Black','Black','Green','Green','Red','Yellow']    
>>> Counter(li)
Counter({'Black': 3, 'Apple': 3, 'Green': 2, 'Yellow': 1, 'Red': 1})

Then it's up to you to sort that how you'd like it presented...

One (inefficient) way to preseve the order, would be to count, then index into the original list:

>>> counts = Counter(li)
>>> [counts[key] for key in sorted(counts, key=li.index)]
[3, 3, 2, 1, 1]

An alternative is to use groupby (but this relies on the items being consecutive):

>>> from itertools import groupby
>>> [len(list(g)) for k, g in groupby(li)]
[3, 3, 2, 1, 1]

Answer 2

If you're new to Python I think that is better for you to code the solution instead of just importing something. Here's a simple and easy to understand way to do it:

counter = {}
for elem in List2:
    counter[elem] = counter.get(elem, 0) + 1