dplyr: access current group variable

Question

After using data.table for quite some time I now thought it's time to try dplyr. It's fun, but I wasn't able to figure out how to access - the current grouping variable - returning multiple values per group

The following example shows is working fine with data.table. How would you write this with dplyr

foo <- matrix(c(1, 2, 3, 4), ncol = 2)
dt <- data.table(a = c(1, 1, 2), b = c(4, 5, 6))

# data.table (expected)
dt[, .(c = foo[, a]), by = a]
   a c
1: 1 1
2: 1 2
3: 2 3
4: 2 4

# dplyr (?)
dt %>% 
  group_by(a) %>% 
  summarize(c = foo[a])

Answer 1

You can still access the group variable but it is like a normal vector with one unique value for each group, so if you put unique around it, it will work. And at same time, dplyr does not seem to expand rows like data.table automatically, you will need the unnest from tidyr package:

library(dplyr); library(tidyr)
dt %>% 
      group_by(a) %>% 
      summarize(c = list(foo[,unique(a)])) %>% 
      unnest()

# Source: local data frame [4 x 2]

#       a     c
#   <dbl> <dbl>
# 1     1     1
# 2     1     2
# 3     2     3
# 4     2     4

Or we can use first to speed up, since we've already know the group variable vector is the same for every group:

dt %>% 
      group_by(a) %>% 
      summarize(c = list(foo[,first(a)])) %>% 
      unnest()

# Source: local data frame [4 x 2]

#       a     c
#   <dbl> <dbl>
# 1     1     1
# 2     1     2
# 3     2     3
# 4     2     4

Answer 2

We can use do from dplyr. (No other packages used). The do is very handy for expanding rows. We only need to wrap with data.frame.

dt %>% 
     group_by(a) %>%
     do(data.frame(c = foo[, unique(.$a)]))
#      a     c
#  <dbl> <dbl>
#1     1     1
#2     1     2
#3     2     3
#4     2     4

---

Or instead of unique we can subset by the 1st observation

dt %>% 
    group_by(a) %>%
    do(data.frame(c = foo[, .$a[1]]))
#     a     c
#  <dbl> <dbl>
#1     1     1
#2     1     2
#3     2     3
#4     2     4

---

This can be also done without using any packages

stack(lapply(split(dt$a, dt$a), function(x) foo[,unique(x)]))[2:1]
#   ind values
#1   1      1
#2   1      2
#3   2      3
#4   2      4