字典：如何计算清单中值的频率-Python3

Question

I've got a dictionary with multiple values for each key. Therefore I'm using a list for the values. Now I want to count how often a value occurs in this list of values. For example I've got the following dictionary:

dic = {}
dic.setdefault(„Sam“, []).append(„Smith“)
dic.setdefault(„Sam“, []).append(„Miller“)
dic.setdefault(„Sam“, []).append(„Smith“)

So

for k, v in dic.items():
    print(k,v)

results in:

  Sam [Smith, Miller, Smith]

Now I want to count how often each value occurs and print the frequency next to the value itself. I want the output to look like the following:

Smith: 2, Miller: 1

For that purpose I've tried the following code without success:

D = defaultdict(list)
for i, item in enumerate(mylist):
    D[item].append(i)
D = {k: len(v) for k, v in D.items()}
print(D)

This code just works fine for a simple list like:

mylist = [10, 20, 30, 10, 30]

For such a kind of list the code above will result in what I expect:

{10: 2, 30:2, 20:1}

However it won't work with the list containing the values. Instead it shows the following error message when using „dic“ instead of „mylist“ in the 2nd line:

TypeError: unhashable type: 'list'

Would be great if someone could help.

Answer 1

That's actually a pretty interesting way of creating a "Count" feature. The issue is that your code counts items in lists, so passing a dictionary to it won't work. Instead, you should pass the values from your dic.items() . If we use what you have:

for k, v in dic.items():
    D = defaultdict(list)
    for i, item in enumerate(v):
        D[item].append(i)
    D = {k: v for k, v in D.items()}
    print(k, D)

Also, take a look at collections.Counter which is a standard library implementation of Count.

from collections import Counter
for k, v in dic.items():
    print(k, Counter(v))