data table string concatenation of SD columns for by group values

Question

I have a big data set with many variables that looks similar to this :

 > data.table(a=letters[1:10],b=LETTERS[1:10],ID=c(1,1,1,2,2,2,2,3,3,3))
     a b          


        

Answer 1

You can concatenate all columns in using lapply.

dt[, lapply(.SD, paste0, collapse=" "), by = ID]
##    ID       a       b
## 1:  1   a b c   A B C
## 2:  2 d e f g D E F G
## 3:  3   h i j   H I J

Using newline characters as a ollapse argument instead of " " does work, but does not print as you seem to expect in your desired output.

dt[, lapply(.SD, paste0, collapse="\n"), by = ID]
##    ID          a          b
## 1:  1    a\nb\nc    A\nB\nC
## 2:  2 d\ne\nf\ng D\nE\nF\nG
## 3:  3    h\ni\nj    H\nI\nJ

As pointed out in the comments by @Frank, the question has been changed to have , as a seperator instead of \n. Of course you can just change the collapse argument to ",". If you want to have a space as well ", ", then the solution by @DavidArenburg is preferable.

dt[, lapply(.SD, paste0, collapse=","), by = ID]
dt[, lapply(.SD, toString), by = ID]