## Introduction

Aggregation is one of the most common uses for R. There are several ways to do so in R, which we will illustrate here.

## Aggregating with base R

For this, we will use the function aggregate, which can be used as follows:

```
aggregate(formula,function,data)
```

The following code shows various ways of using the aggregate function.

**CODE:**

```
df = data.frame(group=c("Group 1","Group 1","Group 2","Group 2","Group 2"), subgroup = c("A","A","A","A","B"),value = c(2,2.5,1,2,1.5))
# sum, grouping by one column
aggregate(value~group, FUN=sum, data=df)
# mean, grouping by one column
aggregate(value~group, FUN=mean, data=df)
# sum, grouping by multiple columns
aggregate(value~group+subgroup,FUN=sum,data=df)
# custom function, grouping by one column
# in this example we want the sum of all values larger than 2 per group.
aggregate(value~group, FUN=function(x) sum(x[x>2]), data=df)
```

**OUTPUT:**

```
> df = data.frame(group=c("Group 1","Group 1","Group 2","Group 2","Group 2"), subgroup = c("A","A","A","A","B"),value = c(2,2.5,1,2,1.5))
> print(df)
group subgroup value
1 Group 1 A 2.0
2 Group 1 A 2.5
3 Group 2 A 1.0
4 Group 2 A 2.0
5 Group 2 B 1.5
>
> # sum, grouping by one column
> aggregate(value~group, FUN=sum, data=df)
group value
1 Group 1 4.5
2 Group 2 4.5
>
> # mean, grouping by one column
> aggregate(value~group, FUN=mean, data=df)
group value
1 Group 1 2.25
2 Group 2 1.50
>
> # sum, grouping by multiple columns
> aggregate(value~group+subgroup,FUN=sum,data=df)
group subgroup value
1 Group 1 A 4.5
2 Group 2 A 3.0
3 Group 2 B 1.5
>
> # custom function, grouping by one column
> # in this example we want the sum of all values larger than 2 per group.
> aggregate(value~group, FUN=function(x) sum(x[x>2]), data=df)
group value
1 Group 1 2.5
2 Group 2 0.0
```

## Aggregating with data.table

Grouping with the data.table package is done using the syntax `dt[i, j, by]`

Which can be read out loud as: "*Take dt, subset rows using i, then calculate j, grouped by by.*" Within the dt statement, multiple calculations or groups should be put in a list. Since an alias for `list()`

is `.()`

, both can be used interchangeably. In the examples below we use `.()`

.

**CODE:**

```
# Aggregating with data.table
library(data.table)
dt = data.table(group=c("Group 1","Group 1","Group 2","Group 2","Group 2"), subgroup = c("A","A","A","A","B"),value = c(2,2.5,1,2,1.5))
print(dt)
# sum, grouping by one column
dt[,.(value=sum(value)),group]
# mean, grouping by one column
dt[,.(value=mean(value)),group]
# sum, grouping by multiple columns
dt[,.(value=sum(value)),.(group,subgroup)]
# custom function, grouping by one column
# in this example we want the sum of all values larger than 2 per group.
dt[,.(value=sum(value[value>2])),group]
```

**OUTPUT:**

```
> # Aggregating with data.table
> library(data.table)
>
> dt = data.table(group=c("Group 1","Group 1","Group 2","Group 2","Group 2"), subgroup = c("A","A","A","A","B"),value = c(2,2.5,1,2,1.5))
> print(dt)
group subgroup value
1: Group 1 A 2.0
2: Group 1 A 2.5
3: Group 2 A 1.0
4: Group 2 A 2.0
5: Group 2 B 1.5
>
> # sum, grouping by one column
> dt[,.(value=sum(value)),group]
group value
1: Group 1 4.5
2: Group 2 4.5
>
> # mean, grouping by one column
> dt[,.(value=mean(value)),group]
group value
1: Group 1 2.25
2: Group 2 1.50
>
> # sum, grouping by multiple columns
> dt[,.(value=sum(value)),.(group,subgroup)]
group subgroup value
1: Group 1 A 4.5
2: Group 2 A 3.0
3: Group 2 B 1.5
>
> # custom function, grouping by one column
> # in this example we want the sum of all values larger than 2 per group.
> dt[,.(value=sum(value[value>2])),group]
group value
1: Group 1 2.5
2: Group 2 0.0
```

## Aggregating with dplyr

Aggregating with dplyr is easy! You can use the group_by() and the summarize() functions for this. Some examples are given below.

**CODE:**

```
# Aggregating with dplyr
library(dplyr)
df = data.frame(group=c("Group 1","Group 1","Group 2","Group 2","Group 2"), subgroup = c("A","A","A","A","B"),value = c(2,2.5,1,2,1.5))
print(df)
# sum, grouping by one column
df %>% group_by(group) %>% summarize(value = sum(value)) %>% as.data.frame()
# mean, grouping by one column
df %>% group_by(group) %>% summarize(value = mean(value)) %>% as.data.frame()
# sum, grouping by multiple columns
df %>% group_by(group,subgroup) %>% summarize(value = sum(value)) %>% as.data.frame()
# custom function, grouping by one column
# in this example we want the sum of all values larger than 2 per group.
df %>% group_by(group) %>% summarize(value = sum(value[value>2])) %>% as.data.frame()
```

**OUTPUT:**

```
> library(dplyr)
>
> df = data.frame(group=c("Group 1","Group 1","Group 2","Group 2","Group 2"), subgroup = c("A","A","A","A","B"),value = c(2,2.5,1,2,1.5))
> print(df)
group subgroup value
1 Group 1 A 2.0
2 Group 1 A 2.5
3 Group 2 A 1.0
4 Group 2 A 2.0
5 Group 2 B 1.5
>
> # sum, grouping by one column
> df %>% group_by(group) %>% summarize(value = sum(value)) %>% as.data.frame()
group value
1 Group 1 4.5
2 Group 2 4.5
>
> # mean, grouping by one column
> df %>% group_by(group) %>% summarize(value = mean(value)) %>% as.data.frame()
group value
1 Group 1 2.25
2 Group 2 1.50
>
> # sum, grouping by multiple columns
> df %>% group_by(group,subgroup) %>% summarize(value = sum(value)) %>% as.data.frame()
group subgroup value
1 Group 1 A 4.5
2 Group 2 A 3.0
3 Group 2 B 1.5
>
> # custom function, grouping by one column
> # in this example we want the sum of all values larger than 2 per group.
> df %>% group_by(group) %>% summarize(value = sum(value[value>2])) %>% as.data.frame()
group value
1 Group 1 2.5
2 Group 2 0.0
```