Question

I have the following data frame in R:

objects   categories
   A       162
   B       162
   B       190
   C       123
   C       162
   C       185
   C       190
   C        82
   C       191
   D       185

As you see there are objects and the categories they belong to. I would like to sum up the categories of each object in comma separated list to get a data frame which would look like this:

 objects   categories
   A       162
   B       162, 190
   C       123, 162, 185, 190, 82, 191
   D       185

How could I do this?

Answer 1

This can be done with any of the aggregation tools of your choice, I’ll show an example using plyr package and paste() function. This assumes your data is named x:

library(plyr)
ddply(x, .(objects), summarize, categories = paste(categories, collapse = ","))
#-----
  objects             categories
1       A                    162
2       B                162,190
3       C 123,162,185,190,82,191
4       D                    185