计算 Python 字典中的特定值

Question

Trying to count the amount of times a specific value occurs within a Python dictionary but can't seem to get it to work.

My dictionary is set up as so:

count = ({'John': 2, 'Sam': 1, 'Brian': 2, 'Brian': 2, 'Brian': 1, 'Sam': 2, 'John': 2, 'Henry': 2, 'Brian': 1})

I am to get the result so that if a user inputs 'Brian' the result would be:

4

Or if the user were to enter 'Sam' the result would then be:

2

number = 0
userInput = input("Please enter a player:  ")
for k, v in count.items():
        if k == userInput:
            number =+ 1
print(number)

Is there a better way of doing this, as currently if were to type in 'Sam' it would only output '1'? Thanks !

Answer 1

A dictionary can have a key only once. When you create count = ({'John': 2, 'Sam': 1, 'Brian': 2, 'Brian': 2, 'Brian': 1, 'Sam': 2, 'John': 2, 'Henry': 2, 'Brian': 1}) , Python stores {'John': 2, 'Brian': 1, 'Sam': 2, 'Henry': 2} (values can change as there is no rule on what value to keep for a key which appears more than once). Cf the Python documentation for dictionaries

So the count will always be 1.

If you want to have a key multiple times, don't use a dictionary but a list of pairs (tuples of size 2).

Answer 2

Since python dictionaries must have unique keys, counting how many times a key occurs wont work here. You can read the documentation for more comprehensive details about this data structure.

Furthermore, You can store counts for each name in a dictionary:

counts = {'Brian': 4, 'John': 2, 'Sam': 2, 'Henry': 1}

Then call each key to obtain the count value:

>>> counts['Brian']
4
>>> counts['Sam']
2

You could also just keep the names as a list, and call collections.Counter to count how many times a name occurs:

>>> from collections import Counter
>>> names = ['John', 'Sam', 'Brian', 'Brian', 'Brian', 'Sam', 'John', 'Henry', 'Brian']
>>> Counter(names)
Counter({'Brian': 4, 'John': 2, 'Sam': 2, 'Henry': 1})

Which returns a Counter() object, a subclass of dict .