parse out string, set it as a factor column in R data.table

data.table, r

Solution

You can vectorize this with regular expressions, e.g. by using `gsub()`:

Set up the data:

strings <- c("lorem pear ipsum", "apple ipsum lorem", "lorem ipsum plum")
fruit <- c("pear", "apple", "plum")

Now create a regular expression

ptn <- paste0(".*(", paste(fruit, collapse="|"), ").*")
gsub(ptn, "\\1", strings)
[1] "pear"  "apple" "plum"

The regular expression works by separating each search element with `|`, embedded inside parentheses:

ptn
[1] ".*(pear|apple|plum).*"

To do this inside a data table, as per your question is then as simple as:

library(data.table)
DT <- data.table(name=strings, value=c(4, 2, 6))
DT[, factor:=gsub(ptn, "\\1", strings)]
DT

                name value factor
1:  lorem pear ipsum     4   pear
2: apple ipsum lorem     2  apple
3:  lorem ipsum plum     6   plum

Problem

I can not really find an elegant way achieving this, please help. I have a `DT` data.table: ``` name,value "lorem pear ipsum",4 "apple ipsum lorem",2 "lorem ipsum plum",6 ``` And based on a list `Fruits <- c("pear", "apple", "plum")` I'd like to create a factor type column. ``` name,value,factor "lorem pear ipsum",4,"pear" "apple ipsum lorem",2,"apple" "lorem ipsum plum",6,"plum" ``` I guess that's basic, but I'm kinda stuck, this is how far I got: `DT[grep("apple", name, ignore.case=TRUE), factor := as.factor("apple")]` Thanks in advance.

Original source