What is the most efficient way to sum all columns whose name starts with a pattern?

Question

My goal is to sum all values in columns that start with the prefix skill_ in a data.table. I would prefer a solution using data.table

Answer 1

Solution using data.table and .SDcols.

require(data.table)

DT <- data.table(x=1:4, skill_a=c(0,1,0,0), skill_b=c(0,1,1,0),
                 skill_c=c(0,1,1,1))

DT[, row_idx := 1:nrow(DT)]

DT[, count_skills := Reduce(`+`, .SD), .SDcols = patterns("skill_")]
DT