python：如何计算列表中给定元素的频率

Question

我了解计数器功能会计算列表中每个元素的频率。但我希望它包含给定元素的频率，即使该元素没有出现在列表中。

例如：

预期元素列表如下：

expected_element = [1,2,3,4,5,6,7]

对于数据集 a,b：

a = [1,1,1,1,6,6,6,3,3,4,5,5]
b = [2,2,7,1,7,5,3,5,5]

我想要的是计算 a 和 b 中预期元素的频率：

为：

keys() =   [1,2,3,4,5,6,7]
values() = [4,0,2,1,2,3,0]

对于 b：

keys() =   [1,2,3,4,5,6,7]
values() = [1,2,1,0,3,0,2]    

Answer 1

您可以使用set(a) 中的唯一键创建一个字典，与空值配对，然后使用Counter(b) 更新它：

from collections import Counter

a = [1,1,1,1,6,6,6,3,3,4,5,5]
b = [2,2,7,1,7,5,3,5,5]

frequencies = {key: 0 for key in set(a)}
frequencies.update(Counter(b))
print(frequencies)

输出：

{1: 1, 3: 1, 4: 0, 5: 3, 6: 0, 2: 2, 7: 2}

Answer 2

你可以使用字典

expected_element = [1,2,3,4,5,6,7]
a = [1,1,1,1,6,6,6,3,3,4,5,5]
b = [2,2,7,1,7,5,3,5,5]

a_occurance = dict()
b_occurance = dict()
for i in expected_element:
    a_occurance[i]=0
    b_occurance[i]=0
for i in a:
    a_occurance[i]+=1
for i in b:
    b_occurance[i]+=1

print a_occurance.keys()
# [1, 2, 3, 4, 5, 6, 7]
print a_occurance.values()
# [4, 0, 2, 1, 2, 3, 0]
print b_occurance.keys()
# [1, 2, 3, 4, 5, 6, 7]
print b_occurance.values()
# [1, 2, 1, 0, 3, 0, 2]