Keep certain number of rows per ID

Question

I would like to keep a certain number of rows per variable: For example assuming I have the following data set (the actual data set is rather large):

part<-c(1,1,1,2,2,2,3,3)
choice<-c(6,7,15,42,1,1,4,5)
data<-cbind(part,choice)
des_data<-data[c(1,2,4,5,7,8),]

      part choice
[1,]    1      6
[2,]    1      7
[3,]    1     15
[4,]    2     42
[5,]    2      1
[6,]    2      1
[7,]    3      4
[8,]    3      5

I would like to keep only the first 2 observations per "part"(icipant).

So that the final data looks like this:

      part choice
[1,]    1      6
[2,]    1      7
[3,]    2     42
[4,]    2      1
[5,]    3      4
[6,]    3      5

How can I do it?

Thanks!

Answer 1

library(dplyr)
data %>%
  as.data.frame %>% #(your example code creates a matrix)
  group_by(part) %>%
  slice(1:2)

Answer 2

An option with data.table

library(data.table)
as.data.table(data)[, .SD[1:2], part]
#    part choice
#1:    1      6
#2:    1      7
#3:    2     42
#4:    2      1
#5:    3      4
#6:    3      5