如何修复Python中的“索引超出范围”错误？

Question

I would like to write en auxiliar function which is counting the occurences of every integer between 0 and li[-1], saying that li is a sorted list.

I can't see my mistake... I know what this kind of error message means, but I don't know where the variable j is reaching a limit.

def aux_compter_occurence(li):
    resu = [0] * (li[-1]+1)
    i = 0
    j = 0
    while i < (li[-1] + 1):
        while li[j] == i:
            resu[i] += 1
            j +=1
        i += 1
    return resu

For example, with the input [2,4,4,4,7,8,8], the output should be [0,0,1,0,3,0,0,1,2]

Answer 1

I added "j < len(li)" to your code and now it works.

def aux_compter_occurence(li):
    resu = [0] * (li[-1]+1)
    i = 0
    j = 0
    while i < (li[-1] + 1):
        while j < len(li) and li[j] == i:
            resu[i] += 1
            j +=1
        i += 1
    return resu

Answer 2

collections.Counter can be passed an iteratable and will count each occurance, we can then use a simple comprehension to generate the result

import collections

def aux_compter_occurence(li):
    counts = collections.Counter(li)
    return [counts.get(i, 0) for i in range(li[-1] + 1)]

Or if you wish to use the previous method of incrementing the values in a list, you already know the index in the list because it is equal to the integer value. We can simplify quite a bit

def aux_compter_occurence(li):
    resu = [0] * (li[-1] + 1)
    for i in li:
        resu[i] += 1
    return resu