tapply() applies a function to each cell of a ragged array.

tapply(X, INDEX, FUN = NULL, ..., simplify = TRUE)

• X: vector
• INDEX: list of one of more factors
• FUN: the function
• simplify: if true, return an array of scalar, other wise an array of list
...

>Orange #R built-in dataset, Growth of Orange Trees

Tree age circumference
1 1 118 30
2 1 484 58
3 1 664 87
4 1 1004 115
5 1 1231 120
6 1 1372 142
7 1 1582 145
8 2 118 33
9 2 484 69
10 2 664 111
11 2 1004 156
12 2 1231 172
13 2 1372 203
14 2 1582 203
15 3 118 30
16 3 484 51
17 3 664 75
18 3 1004 108
19 3 1231 115
20 3 1372 139
21 3 1582 140
22 4 118 32
23 4 484 62
24 4 664 112
25 4 1004 167
26 4 1231 179
27 4 1372 209
28 4 1582 214
29 5 118 30
30 5 484 49
31 5 664 81
32 5 1004 125
33 5 1231 142
34 5 1372 174
35 5 1582 177

Calculate the mean circumference of different Tree groups:

> tapply(Orange$circumference,Orange$Tree,mean)

3 1 5 2 4
94.00000 99.57143 111.14286 135.28571 139.28571

Return a list:

> tapply(Orange$circumference,Orange$Tree,mean,simplify=FALSE)

$`3`
[1] 94
$`1`
[1] 99.57143
$`5`
[1] 111.1429
$`2`
[1] 135.2857
$`4`
[1] 139.2857