Python - 计算列表中项目列表的出现次数？

Question

Trying to count how many times a value from a list appears in another list. In the case of:

my_list = [4,4,4,4,4,4,5,8]
count_items = [4,5,8]

this works fine:

from collections import Counter
print (Counter(my_list))
>> Counter({4: 6, 5: 1, 8: 1})

However, if my_list does not have any entries for '4', for example

my_list = [5,8] 
count_items = [4,5,8]
print (Counter(my_list))
>> Counter({5: 1, 8: 1})

While I am looking for this output:

>> Counter({4: 0, 5: 1, , 8: 1})

Answer 1

What do you need the value for?

Because the counter you have here actually returns 0 when asked for the key 4:

my_list = [5,8] 
count_items = [4,5,8]
counter = Counter(my_list)
print(counter)
>> Counter({5: 1, 8: 1})
print(counter[4])
>> 0

Answer 2

Counter has no way to know that you would expect 4s to be counted, so by default only accounts for elements it finds in the list. An alternative approach would be:

my_list = [5,8]
count_items = [4,5,8]
counter = {i: sum(map(lambda x: 1 if x == i else 0, my_list)) for i in count_items}
print (counter)
>> {4: 0, 5: 1, 8: 1}

Answer 3

A counter is a dict and implements the update method, which preserves zeros:

>>> counter = Counter(my_list)
>>> counter
Counter({5: 1, 8: 1})
>>> counter.update(dict.fromkeys(count_items, 0))
>>> counter
Counter({5: 1, 8: 1, 4: 0})