Find string match in R - r

Question

I have data.frame with 2 columns and thousands rows of random strings as:
Column1 Column2
"this is done in 1 hour" "in 1 hour"
I would like to get a new data.frame column like this:
Column3
"this is done"
So basically match the string according to the Column2 and get the remaining of Column1. How to approach this?
EDIT:
This would not solve the issues since the length of strings varies so I can't do:
substrRight <- function(x, n){
substr(x, nchar(x)-n+1, nchar(x))
}
substrRight(x, 3)
So I would need something like grepl matching.

Answer 1

You can do it with regular expression:
data <- data.frame(Column1 = "this is done in 1 hour", Column2 = "in 1 hour")
data$Column3 <- gsub(data$Column2, '', data$Column1) # Replace fist parameter by second in third.
EDIT:
For more than 1 row, you can use mapply:
data <- data.frame(Column1 = c("this is done in 1 hour", "this is a test"), Column2 = c("in 1 hour", "a test"))
data$Column3 <- mapply(gsub, data$Column2, '', data$Column1)

Answer 2

Here is an example of how you could do it:
# example data frame
testdata <- data.frame(colA=c("this is","a test"),colB=c("is","a"),stringsAsFactors=FALSE)
# adding the new column
newcol <- sapply(seq_len(nrow(testdata)),function(x) gsub(testdata[x,"colB"],"",testdata[x,"colA"],fixed=TRUE))
new.testdata <- transform(testdata,colC=newcol)
# result
new.testdata
# colA | colB | colC
# --------------------------
# 1 this is | is | th
# 2 a test | a | test
EDIT: gsub(str1,'',str2,fixed=TRUE) deletes all occurrences of str1 within str2 whereas using sub would only delete the first occurrence. Since str1 is usually interpreted as regular expression, it is important to set fixed=TRUE. Otherwise a mess happens if str1 contains characters such as .\+?*{}[]. To address the comment, the following would replace only the last occurrence of str1 in str2 leading to the desired output:
revColA <- lapply(testdata[["colA"]],function(x) paste0(substring(x,nchar(x):1,nchar(x):1)))
revColA <- lapply(revColA,paste,collapse='')
revColB <- lapply(testdata[["colB"]],function(x) paste0(substring(x,nchar(x):1,nchar(x):1)))
revColB <- lapply(revColB,paste,collapse='')
revNewCol <- sapply(seq_len(nrow(testdata)),function(x) sub(revColB[x],"",revColA[x],fixed=TRUE))
newcol <- lapply(revNewCol,function(x) paste0(substring(x,nchar(x):1,nchar(x):1)))
newcol <- sapply(newcol,paste,collapse='')
new.testdata <- transform(testdata,colC=newcol)
### output ###
# colA colB colC
------------------------
# 1 |this is | is | this
# 2 | a test | a | test