Aggregating all unique values of each column of data frame

Question

I have a large data frame (1616610 rows, 255 columns) and I need to paste together the unique values of each column based on a key.

For example:

>         


        

Answer 1

Moved from comments:

library(data.table)

dt <- as.data.table(data)
dt[, lapply(.SD, function(x) toString(unique(x))), by = a]

giving:

   a               b      c                  d
1: 1 apples, oranges 12, 22             Monday
2: 2          apples 45, 67 Tuesday, Wednesday
3: 3      grapefruit     28            Tuesday