In this part, you will write code to compute LM probabilities for a trigram model smoothed with “+delta” smoothing. This is just like “add-one” smoothing in the readings, except instead of adding one count to each trigram, we will add delta counts to each trigram for some small delta (e.g., delta=0.0001 in this lab). This is just about the simplest smoothing algorithm around, and this can actually work acceptably in some situations (though not in large-vocabulary ASR). To estimate the probability of a trigram P+delta(wi | wi-2 wi-1) with this smoothing, we take
Your job in this part is to fill in the function
get_prob_plus_delta(). This function should
return the value
P+delta(wi | wi-2 wi-1)
given a trigram wi-2 wi-1 wi.
You will be provided
with the count of the trigram as well as the count of the bigram history
(which you computed for Part 1), in addition to the
vocabulary size. This routine corresponds to step (B) in the pseudocode
listed in Section 2.1.
Your code will again be compiled into the program EvalLMLab3. To compile this program with your code, type
The instructions in lab3.txt will ask you to run the script lab3p2b.sh, which does the same thing as lab3p2a.sh except on a different test set.