Taking means over `sam` and `dup` - r

Question

I am trying to take the means over the columns sam and dup of the following dataset:
fat co lab sam dup
1 0.62 1 1 1 1
2 0.55 1 1 1 2
3 0.34 1 1 2 1
4 0.24 1 1 2 2
5 0.80 1 1 3 1
6 0.68 1 1 3 2
7 0.76 1 1 4 1
8 0.65 1 1 4 2
9 0.30 1 2 1 1
10 0.40 1 2 1 2
11 0.33 1 2 2 1
12 0.43 1 2 2 2
13 0.39 1 2 3 1
14 0.40 1 2 3 2
15 0.29 1 2 4 1
16 0.18 1 2 4 2
17 0.46 1 3 1 1
18 0.38 1 3 1 2
19 0.27 1 3 2 1
20 0.37 1 3 2 2
21 0.37 1 3 3 1
22 0.42 1 3 3 2
23 0.45 1 3 4 1
24 0.54 1 3 4 2
25 0.18 2 1 1 1
26 0.47 2 1 1 2
27 0.53 2 1 2 1
28 0.32 2 1 2 2
29 0.40 2 1 3 1
30 0.37 2 1 3 2
31 0.31 2 1 4 1
32 0.43 2 1 4 2
33 0.35 2 2 1 1
34 0.39 2 2 1 2
35 0.37 2 2 2 1
36 0.33 2 2 2 2
37 0.42 2 2 3 1
38 0.36 2 2 3 2
39 0.20 2 2 4 1
40 0.41 2 2 4 2
41 0.37 2 3 1 1
42 0.43 2 3 1 2
43 0.28 2 3 2 1
44 0.36 2 3 2 2
45 0.18 2 3 3 1
46 0.20 2 3 3 2
47 0.26 2 3 4 1
48 0.06 2 3 4 2
The output should be this:
lab co fat
1 1 1 0.58000
2 2 1 0.34000
3 3 1 0.40750
4 1 2 0.37625
5 2 2 0.35375
6 3 2 0.26750
These are both in the form of .RData files.
How can this be done?

Answer 1

An example with part of the data you posted:
dt = read.table(text = "
fat co lab sam dup
0.62 1 1 1 1
0.55 1 1 1 2
0.34 1 1 2 1
0.24 1 1 2 2
0.80 1 1 3 1
0.68 1 1 3 2
0.76 1 1 4 1
0.65 1 1 4 2
0.30 1 2 1 1
0.40 1 2 1 2
0.33 1 2 2 1
0.43 1 2 2 2
0.39 1 2 3 1
0.40 1 2 3 2
0.29 1 2 4 1
0.18 1 2 4 2
", header= T)
library(dplyr)
dt %>%
group_by(lab, co) %>% # for each lab and co combination
summarise(fat = mean(fat)) %>% # get the mean of fat
ungroup() # forget the grouping
# # A tibble: 2 x 3
# lab co fat
# <int> <int> <dbl>
# 1 1 1 0.58
# 2 2 1 0.34