calculate growth rate between columns in R

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My data.table consists of an a priori unknown number of columns, eg.

 DT <- data.table("columnPred" = c(1,2,3), 
                     "column1" = c(7,8,9), 
                     "column2" = c(44,55,66))

I would like to calculate the change between the columns, ie.

DT[, delta1 := ((columnPred/column1)-1)*100]
DT[, delta2 := ((column1/column2)-1)*100]

How can I do this when the number of columns is unknown ahead of time?

data.table r
1 Answers

Using data.table, and doing element-wise division on DT (with the right-most column removed on the numerator, left-most on the denominator).

delta_col_names <- paste0("delta",1:(ncol(DT)-1))

DT[, (delta_col_names) := ((DT[,1:(ncol(DT)-1)] / DT[,-1])-1)*100]

   columnPred column1 column2    delta1    delta2
1:          1       7      44 -85.71429 -84.09091
2:          2       8      55 -75.00000 -85.45455
3:          3       9      66 -66.66667 -86.36364
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