Count the amount of bigrams in sentences in a dataframe - r

Question

I have a dataset that looks a bit like this:
sentences <- c("sample text in sentence 1", "sample text in sentence 2")
id <- c(1,2)
df <- data.frame(sentences, id)
I would like to have a count where I can see the occurrence of certain bigrams. So lets say I have:
trigger_bg_1 <- "sample text"
I expect the output of 2 (as there are two occurrences of "sample text" in the two sentences. I know how to do a word count like this:
trigger_word_sentence <- 0
for(i in 1:nrow(df)){
words <- df$sentences[i]
words = strsplit(words, " ")
for(i in unlist(words)){
if(i == trigger_word_sentence){
trigger_word_sentence = trigger_word_sentence + 1
}
}
}
But I cant get something working for a bigram. Any thoughts on how I should change the code to get it working?
But as I have a long test of trigger-words which I need to count in over

Answer 1

In case you want to count the sentences where you have a match you can use grep:
length(grep(trigger_bg_1, sentences, fixed = TRUE))
#[1] 2
In case you want to count how many times you find trigger_bg_1 you can use gregexpr:
sum(unlist(lapply(gregexpr(trigger_bg_1, sentences, fixed = TRUE)
, function(x) sum(x>0))))
#[1] 2

Answer 2

You could sum a grepl
sum(grepl(trigger_bg_1, df$sentences))
[1] 2

Answer 3

If you are really interested in bigrams rather than just set word combinations, the package quanteda can offer a more substantial and systematic way forward:
Data:
sentences <- c("sample text in sentence 1", "sample text in sentence 2")
id <- c(1,2)
df <- data.frame(sentences, id)
Solution:
library(quanteda)
# strip sentences down to words (removing punctuation):
words <- tokens(sentences, remove_punct = TRUE)
# make bigrams, tabulate them and sort them in decreasing order:
bigrams <- sort(table(unlist(as.character(tokens_ngrams(words, n = 2, concatenator = " ")))), decreasing = T)
Result:
bigrams
in sentence sample text text in sentence 1 sentence 2
2 2 2 1 1
If you want to inspect the frequency count of a specific bigram:
bigrams["in sentence"]
in sentence
2