Forced To Use mapply Is There A Workaround - r

Question

I have a data.frame with a single column "Terms". This could contain a string of multiple words. Each term contains at least two words or more, no upper limit.
From this column "Terms", I would like to extract the last word and store it in a new column "Last".
# load library
library(dplyr)
library(stringi)
# read csv
df <- read("filename.txt",stringsAsFactors=F)
# show df
head(df)
# Term
# 1 this is for the
# 2 thank you for
# 3 the following
# 4 the fact that
# 5 the first
I have prepared a function LastWord which works well when a single string is given.
However, when a vector of string is given, it still works with the first string in the vector. This has forced me to use mapply when used with mutate, to add a column as seen below.
LastWord <- function(InputWord) {
stri_sub(InputWord,stri_locate_last(str=InputWord, fixed=" ")[1,1]+1, stri_length(InputWord))
}
df <- mutate(df, Last=mapply(LastWord, df$Term))
Using mapply makes the process very slow. I generally need to process around 10 to 15 million lines or terms at a time. It takes hours.
Could anyone suggest a way to create the LastWord function that works with vector rather than a string?

Answer 1

You can try:
df$LastWord <- gsub(".* ([^ ]+)$", "\\1", df$Term)
df
# Term LastWord
# 1 this is for the the
# 2 thank you for for
# 3 the following following
# 4 the fact that that
# 5 the first first
In the gsub call, the expression between the brackets matches anything that is not a space at least one time (instead of [^ ]+, [a-zA-Z]+ could work too) at the end of the string ($). The fact that it is in between brackets permit to capture the expression with \\1. So gsub only keeps what is in between brackets as replacement.
EDIT:
As #akrun mentionned in the comments, in this case, sub can also be used instead of gsub.

Answer 2

To extract the last word only, you can use a vectorized function from stringi directly which should be very fast
library(stringi)
df$LastWord <- stri_extract_last_words(df$Term)
Now if you want two new columns, one containing all words but the last and another one containing the last words, you can use some regular expression like
stri_match(df$Term, regex= "([\\w*\\s]*)\\s(\\w*)")
# [,1] [,2] [,3]
# [1,] "this is for the" "this is for" "the"
# [2,] "thank you for" "thank you" "for"
# [3,] "the following" "the" "following"
# [4,] "the fact that" "the fact" "that"
# [5,] "the first" "the" "first"
So what you want is
df[c("ExceptLast", "LastWord")] <-
stri_match(df$Term, regex= "([\\w*\\s]*)\\s(\\w*)")[, 2:3]
(Note that this won't work if df$Term contains only one word. In that case you will need to modify the regular expression, depending on which column you want it to be included in.)