Targeted variable recoding in R

Question

I am trying to do some text processing and need to recode the words of sentences so that a target word is identified in a particular way in the new variable. For instance, given a data frame that looks like this...

subj <- c("1", "1", "1", "2", "2", "2", "2", "2")
condition <- c("A", "A", "A", "B", "B", "B", "B", "B")
sentence <- c("1", "1", "1", "2", "2", "2", "2", "2")
word <- c("I", "like", "dogs.", "We", "don't", "like", "this", "song.")
d <- data.frame(subj,condition, sentence, word)

 subj condition sentence  word
 1         A        1     I
 1         A        1     like
 1         A        1     dogs.
 2         B        2     We
 2         B        2     don't
 2         B        2     like
 2         B        2     this
 2         B        2     song.

I need to create a new column for which every instance of the target word (in this example, when d$word="like") is marked 0, and all words before "like" in the sentence block decrement and all words after "like" increment. Each subject has multiple sentences, and sentences vary by condition, so the loop needs to consider instances of the target word per subject, per sentence. The end result should look something like this.

 subj condition sentence  word   position
 1         A        1     I        -1
 1         A        1     like      0
 1         A        1     dogs.     1
 2         B        2     We       -2
 2         B        2     don't    -1
 2         B        2     like      0
 2         B        2     this      1
 2         B        2     song.     2

Sorry if the question is poorly worded, I hope it makes sense! Note that the target is not in the same place (relative to the start of the sentence) in each sentence. I am pretty new to R and can figure out how to increment or decrement, but not do both things within each sentence block. Any suggestions on the best way to go about this? Thanks much!

Answer 1

You can add an index which you can then use for the relative positions.
Using data.table makes breaking it down by sentence very easy

library(data.table)
DT <- data.table(indx=1:nrow(d), d, key="indx")

DT[, position:=(indx - indx[word=="like"]), by=sentence]

# Results
DT
#    indx subj condition sentence  word position
# 1:    1    1         A        1     I       -1
# 2:    2    1         A        1  like        0
# 3:    3    1         A        1 dogs.        1
# 4:    4    2         B        2    We       -2
# 5:    5    2         B        2 don't       -1
# 6:    6    2         B        2  like        0
# 7:    7    2         B        2  this        1
# 8:    8    2         B        2 song.        2

Udate:

In case you have grammatically incorrect sentences, you might want to use grepl instead of ==

DT[, position:=(indx - indx[grepl("like", word)]), by=sentence]