Adding counters deletes keys

by

Counters are a kind of multiset. From the Counter() documentation:

Several mathematical operations are provided for combining Counter objects to produce multisets (counters that have counts greater than zero). Addition and subtraction combine counters by adding or subtracting the counts of corresponding elements. Intersection and union return the minimum and maximum of corresponding counts. Each operation can accept inputs with signed counts, but the output will exclude results with counts of zero or less.

Emphasis mine.

Further on it tells you gives you some more detail about the multiset nature of Counters:

Note: Counters were primarily designed to work with positive integers to represent running counts; however, care was taken to not unnecessarily preclude use cases needing other types or negative values. To help with those use cases, this section documents the minimum range and type restrictions.

[…]

  • The multiset methods are designed only for use cases with positive values. The inputs may be negative or zero, but only outputs with positive values are created. There are no type restrictions, but the value type needs to support addition, subtraction, and comparison.

So Counter objects are both; dictionaries and bags. Standard dictionaries, however, don’t support addition, but Counters do, so it’s not as if Counters are breaking a precedence set by dictionaries here.

If you wanted to retain the zeros, use Counter.update() and pass in the result of Counter.elements() of the other object:

c.update(Counter('abba').elements())

Demo:

>>> c = Counter({'a': 0, 'b': 0, 'c': 0})
>>> c.update(Counter('abba').elements())
>>> c
Counter({'a': 2, 'b': 2, 'c': 0})

More Related Contents:

  1. How do I sort a list of dictionaries by a value of the dictionary?
  2. What is the best way to implement nested dictionaries?
  3. How are Python’s Built In Dictionaries Implemented?
  4. What would a “frozen dict” be?
  5. In Python, when to use a Dictionary, List or Set?
  6. merging Python dictionaries
  7. Pandas groupby.size vs series.value_counts vs collections.Counter with multiple series
  8. What is the purpose of collections.ChainMap?
  9. Key-ordered dict in Python
  10. Creating a tree/deeply nested dict from an indented text file in python
  11. Using a Python Dictionary as a Key (Non-nested)
  12. Creating an Ordered Counter
  13. Recursive diff of two dictionaries (keys and values)?
  14. How to make a dictionary that returns key for keys missing from the dictionary instead of raising KeyError?
  15. How to get count dict of items but maintain the order in which they appear?
  16. How expensive are Python dictionaries to handle?
  17. Python: is “except KeyError” faster than “if key in dict”?
  18. How to sort a list of dictionaries by a value of the dictionary in Python?
  19. Remove string in python dictionary value
  20. Loop through all nested dictionary values?
  21. How do I convert this list of dictionaries to a csv file?
  22. Sorting a dictionary by value then key
  23. How can I initialize a dictionary of distinct empty lists in Python?
  24. How to add multiple values to a dictionary key?
  25. How do I sort a dictionary by key?
  26. Calculate difference in keys contained in two Python dictionaries
  27. How do I delete items from a dictionary while iterating over it?
  28. Counting each letter’s frequency in a string
  29. Fast way to copy dictionary in Python
  30. How to convert string representation of dictionary in Pandas DataFrame to a new columns?

Leave a Comment