coding-mentor-agent/kb/python-course-kb-practical-python/wiki/exercises/2-18-tabulating-with-counters.md

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id	source_exercise_id	title	section	source_path	source_repo	source_commit	student_visible_solution	has_private_solution	skip
practical-python-2.18	2.18	Tabulating with Counters	2.5 collections module	02_Working_with_data/05_Collections.md	https://github.com/dabeaz-course/practical-python	93dca856b41c61a0a0f85ae334116e4c125629ea	false	false	false

Exercise 2.18: Tabulating with Counters

Source: Practical Python Programming, 02_Working_with_data/05_Collections.md.

Exercise 2.18: Tabulating with Counters

Suppose you wanted to tabulate the total number of shares of each stock. This is easy using Counter objects. Try it:

>>> portfolio = read_portfolio('Data/portfolio.csv')
>>> from collections import Counter
>>> holdings = Counter()
>>> for s in portfolio:
        holdings[s['name']] += s['shares']

>>> holdings
Counter({'MSFT': 250, 'IBM': 150, 'CAT': 150, 'AA': 100, 'GE': 95})
>>>

Carefully observe how the multiple entries for MSFT and IBM in portfolio get combined into a single entry here.

You can use a Counter just like a dictionary to retrieve individual values:

>>> holdings['IBM']
150
>>> holdings['MSFT']
250
>>>

If you want to rank the values, do this:

>>> # Get three most held stocks
>>> holdings.most_common(3)
[('MSFT', 250), ('IBM', 150), ('CAT', 150)]
>>>

Let’s grab another portfolio of stocks and make a new Counter:

>>> portfolio2 = read_portfolio('Data/portfolio2.csv')
>>> holdings2 = Counter()
>>> for s in portfolio2:
          holdings2[s['name']] += s['shares']

>>> holdings2
Counter({'HPQ': 250, 'GE': 125, 'AA': 50, 'MSFT': 25})
>>>

Finally, let’s combine all of the holdings doing one simple operation:

>>> holdings
Counter({'MSFT': 250, 'IBM': 150, 'CAT': 150, 'AA': 100, 'GE': 95})
>>> holdings2
Counter({'HPQ': 250, 'GE': 125, 'AA': 50, 'MSFT': 25})
>>> combined = holdings + holdings2
>>> combined
Counter({'MSFT': 275, 'HPQ': 250, 'GE': 220, 'AA': 150, 'IBM': 150, 'CAT': 150})
>>>

This is only a small taste of what counters provide. However, if you ever find yourself needing to tabulate values, you should consider using one.

Commentary: collections module

The collections module is one of the most useful library modules in all of Python. In fact, we could do an extended tutorial on just that. However, doing so now would also be a distraction. For now, put collections on your list of bedtime reading for later.

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关联来源

• summaries/05_Collections