r - 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(0,1,1,0,0),
                 d = c(1,0,0,2,2))

df
#    a b c d
# 1: 1 1 0 1
# 2: 2 0 1 0
# 3: 3 2 1 0
# 4: 4 5 0 2
# 5: 5 1 0 2

I want to create a new column with non-null columns names. The result will be:

df_result <- data.table(a = c(1,2,3,4,5),
                        z = c('b_d', 'c', 'b_c', 'b_d', 'b_d'))

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

Answer 1

Assuming nrow >> ncol, you could work columnwise

ff = function(x)
{
    ans = character(nrow(x))
    for(j in seq_along(x)) {
        i = x[[j]] > 0L
        ans[i] = paste(ans[i], names(x)[[j]], sep = "_")
    }
    return(gsub("^_", "", ans))
}
ff(df[, -1L, with = FALSE]) #or, `df[, ff(.SD), .SDcols = -1L]` from David Arenburg
#[1] "b_d" "c"   "b_c" "b_d" "b_d"