KeyError: 'this' , why this error happened?

Question

CORPUS = [
        'this is the first document',
        'this is the second document',
        'and this is the third document',
        'is this the first document ?'
    ]

doc = CORPUS
dic = {}
for sentence in doc:
  k = list(sentence.split())
  for term in k:
    count_term = k.count(term)
    if not dic[term]:
      dic[term] = count_term
    else:
      dic[term] += count_term
print(dic)

I want to count a frequency for the terms in the sentences in CORPUS list, so I tried to make a dictionary and input the count object but KeyError: 'this'

Could you explain why the error happened?

Answer 1

If I understand it correctly, your code can be simplified to:

from collections import Counter

print(Counter(" ".join(CORPUS).split()))

which yields

Counter({'this': 4,
         'is': 4,
         'the': 4,
         'first': 2,
         'document': 4,
         'second': 1,
         'and': 1,
         'third': 1,
         '?': 1})

So, the idea is to first create one long string which avoids the loop and then use a built-in function to count the occurrences of the individual words.

The reason for the error you get is well explained in the other two answers (I upvoted both of them) :)