Updating a dictionary with integer keys

Question

I'm working on a short assignment where I have to read in a .txt file and create a dictionary in which the keys are the number of words in a sentence and the values are the number of sentences of a particular length. I've read in the file and determined the length of each sentence already, but I'm having troubles creating the dictionary.

I've already initialized the dictionary and am trying to update it (within a for loop that iterates over the sentences) using the following code:

for snt in sentences:
    words = snt.split(' ')
    sDict[len(words)]+=1

It gives me a KeyError on the very first iteration. I'm sure it has to do with my syntax but I'm not sure how else to update an existing entry in the dictionary.

Answer 1

When you initialize the dictionary, it starts out empty. The next thing you do is look up a key so that you can update its value, but that key doesn't exist yet, because the dictionary is empty. The smallest change to your code is probably to use the get dictionary method. Instead of this:

sDict[len(words)]+=1

Use this:

sDict[len(words)] = sDict.get(len(words), 0) + 1

The get method looks up a key, but if the key doesn't exist, you are given a default value. The default default value is None, and you can specify a different default value, which is the second argument, 0 in this case.

The better solution is probably collections.Counter, which handles the common use case of counting occurrences:

import collections
s = map(str.split, sentences)
sDict = collections.Counter(map(len, s))