Create a new column with non-null columns' names

Question

My data set looks like this one:

library(data.table)

df <- data.table(a = c(1,2,3,4,5),
                 b = c(1,0,2,5,1),
                 c = c(0,1,1,         


        

Answer 1

This can be a bit lengthy.

For every row finding a column whose value is not 0 and then pasting the column names together.

data.table(a= df$a, z = lapply(apply(df, 1, 
           function(x) which(x[-1]!= 0)), 
           function(x) paste0(names(x), collapse = "_")))


#   a   z
#1: 1 b_d
#2: 2   c
#3: 3 b_c
#4: 4 b_d
#5: 5 b_d