subtracting multiple columns from each other - r

Question

I have a large dataset and I want to subtract specific columns from each other based on their position. I want to subtract column 2 from column 8, column 3 from column 9 and column 4 from column 10.
Thanks a lot
Magnus
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Answer 1

We can create 2 vectors of position and subtract the columns directly. Since you have data.table we use ..column_number to select columns by position.
library(data.table)
col1group <- 2:4
col2group <- 8:10
df[, ..col1group] - df[, ..col2group])
If you want to add them as new columns to original data you can rename them and cbind
cbind(df, setNames(df[, ..col1group] - df[, ..col2group],
paste0(names(df)[col1group], '_diff')))

Answer 2

Something like the following computes the subtractions in the question.
library(data.table)
nms <- names(df1)
iCols <- grep("^i\\.", nms, value = TRUE)
Cols <- sub("^i\\.", "", iCols)
df1[, lapply(seq_along(Cols), function(i) get(Cols[i]) - get(iCols[i]))]
# V1 V2 V3
#1: -0.0061970 0.0156564 -1.64872
#2: -0.0765590 0.0100962 0.45382
#3: -0.3198350 -0.0715609 -14.29002
#4: -0.0471500 -0.0570996 -3.01273
#5: 0.0655143 -0.0113157 1.82301
#6: -0.5673930 -0.1427161 -19.09679
Following Ronak Shah's answer I realized that the code below also works.
df1[, ..Cols] - df1[, ..iCols]
The numeric results are the same but the column names are the vector Cols.
To create new columns, try
newCols <- paste(Cols, "diff", sep = "_")
df1[, (newCols) := lapply(seq_along(Cols), function(i) get(Cols[i]) - get(iCols[i]))]

Answer 3

Base R solution:
idx <- c(2, 3, 4)
jdx <- c(8, 9, 10)
Using lapply() and column binding the list:
setNames(do.call("cbind", lapply(seq_along(idx), function(i){
df[, jdx[i], drop = FALSE] - df[, idx[i], drop = FALSE]
}
)
), c(paste("x", jdx, idx, sep = "_")))
Using sapply() and coercing vectors to a data.frame:
setNames(data.frame(sapply(seq_along(idx), function(i){
df[, jdx[i], drop = FALSE] - df[, idx[i], drop = FALSE]
}
)
), c(paste("x", jdx, idx, sep = "_")))
Using Map() and Reduce() and column binding to original data.frame:
cbind(df, setNames(Reduce(cbind, Map(function(i){
df[, jdx[i], drop = FALSE] - df[, idx[i], drop = FALSE]
}, seq_along(idx))), c(paste("x", jdx, idx, sep = "_"))))