How to compute the number of occurencies of elements in a list using list comprehensions in python?

Question

Given a list like this one

arr = [1, 1, 2, 2, 2, 3, 3]

I would like to compute the number of occurencies of each element and storing it in a new list but using a list comprehension like this:

occ = [arr.count(e) for e in arr]

but this prints out the original array: [1, 1, 2, 2, 2, 3, 3] instead of [2, 3, 2] which are respectively the number of occurences of 1, 2, and 3 in the input arry.

Why is that happening and how to fix it?

NB: it is mandatory to use a list comprehension if possible.

Answer 1

Well, the reason is tht when you do

arr = [1, 1, 2, 2, 2, 3, 3]

and arr.count(1), you'll get the number of occurencies of the number 1 in the list arr. And then when you do

occ = [arr.count(e) for e in arr]

you are actually computing arr.count(1) 2 times, then arr.count(3) three times, and then arr.count(3) two times.

So, what you have to do in order to not repeat the numbers is count the unique numbers in the list, which are just 1, 2, and 3. So the right code would be:

occ = [arr.count(e) for e in set(arr)]

and now when you print the list, you'll get [2,3,2] as expected.