Question

for my question I created a dummy data frame:

    set.seed(007)
 DF <- data.frame(a = rep(LETTERS[1:5], each=2), b = sample(40:49), c = sample(1:10))
 DF
   a  b  c
1  A 49  2
2  A 43  3
3  B 40  7
4  B 47  1
5  C 41  9
6  C 48  8
7  D 45  6
8  D 42  5
9  E 46 10
10 E 44  4

How can I use the aggregation function on column a so that, for instance, for “A” the following value is calculated: 49-43 / 2+3?

I started like:

aggregate(DF, by=list(DF$a), FUN=function(x) {

  ...

})

The problem I have is that I do not know how to access the 4 different cells 49, 43, 2 and 3
I tried x[[1]][1] and similar stuff but don’t get it working.

Answer 1

Inside aggregate, the function FUN is applied independently to each column of your data. Here you want to use a function that takes two columns as inputs, so a priori, you can’t use aggregate for that.

Instead, you can use ddply from the plyr package:

ddply(DF, "a", summarize, res = (b[1] - b[2]) / sum(c))
#   a        res
# 1 A  1.2000000
# 2 B -0.8750000
# 3 C -0.4117647
# 4 D  0.2727273
# 5 E  0.1428571