Concatenate row-wise across specific columns of dataframe

Use unite of tidyr package:

require(tidyr)
data <- data %>% unite(id, F, E, D, C, sep = '_')

First parameter is the desired name, all next up to sep - columns to concatenate.

Try

 data$id <- paste(data$F, data$E, data$D, data$C, sep="_")

instead. The beauty of vectorized code is that you do not need row-by-row loops, or loop-equivalent *apply functions.

Edit Even better is

 data <- within(data,  id <- paste(F, E, D, C, sep=""))