how to use an integer as a key in python?

Question

In the follwing code I wanted to use integers as keys in a dict:

import itertools N = {} for a,b,c,d in itertools.product(range(100), repeat=4): x = a*a + c*c y = a*b + c*d z = b*b + d*d s = x + y + y + z N[s] += 1 print N

I get a KeyError: 0 at N[s] += 1. Why is it so? The documentation says that

strings and numbers can always be keys

The wiki gives an explanation on KeyError:

Python raises a KeyError whenever a dict() object is requested (using the format a = adict[key]) and the key is not in the dictionary.

What I want to do is to build a dict with yet-unknown keys (they are computed on the fly) and keep a counter for them. I have done that in the past (with strings as keys) so what did I did wrong this time? (I know - this must be super obvious but after some time glaring at this complicated code I need help :))

Your key is not defined so you get an error. (you can't add 1 to something that doesn't exist). You probably want to use Counter, from collections import Counter; N = Counter() — monkut
– monkut, Commented Oct 8, 2013 at 7:31
Thanks - I was focusing on the existence of the key and not the value. — WoJ
– WoJ, Commented Oct 8, 2013 at 7:50
Your question is fine. You provided actual code, listed the error you received and made an attempt and deciphering the error. If people don't like your question they should leave a comment as to why. — Mark Tolonen
– Mark Tolonen, Commented Oct 8, 2013 at 7:56

Inbar Rose · Accepted Answer · 2013-10-08 07:52:38Z

Use defaultdict

import itertools from collections import defaultdict N = defaultdict(int) for a,b,c,d in itertools.product(range(100), repeat=4): x = a*a + c*c y = a*b + c*d z = b*b + d*d s = x + y + y + z N[s] += 1 print N # answer is too long to include here

YXD · Accepted Answer · 2013-10-08 07:33:06Z

In your first iteration, N is empty, however you are attempting to access N[0]

You can fix this particular problem with

if s in N: N[s] += 1 else: N[s] = 0 # or 1 or whatever

But as @monkut says in the comments you should use a Counter

Thanks - I did not know Counter, it indeed simplifies the code.

Mark Tolonen · Accepted Answer · 2013-10-08 07:53:06Z

Counter is best because it has methods to analyze the results, but you can also use a defaultdict:

from collections import defaultdict N = defaultdict(int)

If a key is not present the value will be initialized to the default for int, which is zero.

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