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in a text such as

"The deal with Canada's Barrick Gold finalised in Toronto over the weekend"

When I try to break it into bigram model, I get this

"The deal"
"deal with" 
"with Canada's"
"Canada's Barrick"
"Barrick Gold"
"Gold finalised"
"finalised in"
"in Toronto"
"Toronto over"
"over the"
"the weekend"

my question

shall I include the first word and the last word as single words

"* The"
"Weekend *"
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  • $\begingroup$ Making an analogy with 2D convolutions used in computer vision, I would say you could, however I doubt here that this can improve the accuracy of your model so I would not do it. This is just my intuition to help you going. If you are not in a hurry, you can try both and compare the results. $\endgroup$ – Jeanba Nov 18 at 10:44
  • $\begingroup$ What are you intending to do with the bigram model? If you need to be able to predict the first word of a sentence for instance, you will ideally have a simple way to model that. $\endgroup$ – Neil Slater Nov 18 at 11:52
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What you describe is called padding and is indeed used frequently in language modeling. For instance if one represents the sequence "A B C" with trigrams:

# # A
# A B
A B C
B C #
C # #

The advantages of padding:

  • it makes every word/symbol appear the same number of times whether it appears in the middle of the sequence or not.
  • it marks the beginning and end of a sentence/text, so that the model can represent the probability to start/end with a particular word.
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