How to change this for loop into while loop?

I have this code that when u pass in aList = [5,8,2,13,1,8,3,1,8] it prints countList= [0, 2, 1, 1, 0, 1, 0, 0, 3, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0]. Basically, say index 1 in countList is 2 because there are two number 1's in aList.

This is the code in for loop:

countList = 20*[0]

aList = [5,8,2,13,1,8,3,1,8]
for num in aList:
    countList[num] += 1

And I tried changing it to a while loop but prints countList= [0, 2, 2, 3, 0, 5, 0, 0, 24, 0, 0, 0, 0, 13, 0, 0, 0, 0, 0, 0] instead.

This is what I tried:

countList = 20*[0]

aList = [5,8,2,13,1,8,3,1,8]
index=0#initialize index

while index<len(aList):
    number=aList[index]
    count=0
    while count<number:
        countList[number] += 1
        count+=1
    index+=1