Creating a new variable when a function recognises an interaction term - r

Question

Based on this link, I wrote the following code, which is part of a function:
Sample data:
panelID = c(1:50)
year= c(2001:2010)
country = c("NLD", "BEL", "GER")
urban = c("A", "B", "C")
indust = c("D", "E", "F")
sizes = c(1,2,3,4,5)
n <- 2
library(data.table)
set.seed(123)
DT <- data.table(panelID = rep(sample(panelID), each = n),
country = rep(sample(country, length(panelID), replace = T), each = n),
year = c(replicate(length(panelID), sample(year, n))),
some_NA = sample(0:5, 6),
Factor = sample(0:5, 6),
industry = rep(sample(indust, length(panelID), replace = T), each = n),
urbanisation = rep(sample(urban, length(panelID), replace = T), each = n),
size = rep(sample(sizes, length(panelID), replace = T), each = n),
income = round(runif(100)/10,2),
sales= round(rnorm(10,10,10),2),
happiness = sample(10,10),
Sex = round(rnorm(10,0.75,0.3),2),
Age = sample(100,100),
educ = round(rnorm(10,0.75,0.3),2))
DT [, uniqueID := .I] # Creates a unique ID
DT <- as.data.frame(DT)
Code:
depvar <- "happiness"
othervar <- "factor:income"
insvar <- c("happiness","factor","income")
if (length(insvar)>2) {
DT$newvar <- DT[insvar[2]]*DT[insvar[3]]
othervar=newvar
}
The idea is that when othervar is a combination of two variables, othervar gets replaced by a new variable which is the combination of those two variables.
Right now I however get the error:
Error in `[.data.frame`(DT, insvar[2]) : undefined columns selected
How should I write this function properly?

Answer 1

If you change factor to Factor as the column is named and use DT$newvar the code runs and produces a new column, which I believe is what you are looking for.
depvar <- "happiness"
othervar <- "Factor:income"
insvar <- c("happiness","Factor","income")
if (length(insvar)>2) {
DT$newvar <- DT[insvar[2]]*DT[insvar[3]]
othervar=DT$newvar
}