Python 2.7 Counting number of dictionary items with given value

Question

first question here, so i will get right to it:

using python 2.7

I have a dictionary of items, the keys are an x,y coordinate represented as a tuple: (x,y) and all the values are Boolean values.

I am trying to figure out a quick and clean method of getting a count of how many items have a given value. I do NOT need to know which keys have the given value, just how many.

there is a similar post here: How many items in a dictionary share the same value in Python, however I do not need a dictionary returned, just an integer.

My first thought is to iterate over the items and test each one while keeping a count of each True value or something. I am just wondering, since I am still new to python and don't know all the libraries, if there is a better/faster/simpler way to do this.

thanks in advance.

Answer 1

This first part is mostly for fun -- I probably wouldn't use it in my code.

sum(d.values()) 

will get the number of True values. (Of course, you can get the number of False values by len(d) - sum(d.values())).

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Slightly more generally, you can do something like:

sum(1 for x in d.values() if some_condition(x)) 

In this case, if x works just fine in place of if some_condition(x) and is what most people would use in real-world code)

OF THE THREE SOLUTIONS I HAVE POSTED HERE, THE ABOVE IS THE MOST IDIOMATIC AND IS THE ONE I WOULD RECOMMEND

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Finally, I suppose this could be written a little more cleverly:

sum( x == chosen_value for x in d.values() ) 

This is in the same vein as my first (fun) solution as it relies on the fact that True + True == 2. Clever isn't always better. I think most people would consider this version to be a little more obscure than the one above (and therefore worse).

Answer 2

If you want a data structure that you can quickly access to check the counts, you could try using a Counter (as @mgilson points out, this relies on the values themselves being hashable):

>>> from collections import Counter >>> d = {(1, 2): 2, (3, 1): 2, (4, 4): 1, (5, 6): 4} >>> Counter(d.values()) Counter({2: 2, 1: 1, 4: 1}) 

You could then plug in a value and get the number of times it appeared:

>>> c = Counter(d.values()) >>> c[2] 2 >>> c[4] 1