R Summarize Multiple Variables by Group Using data.table Package (Example Code)

In this post you’ll learn how to group the variables of a data.table in R programming.

Setting up the Example

install.packages("data.table")               # Install & load data.table package
library("data.table")
data(iris)                                   # Load example data in R
iris_dt <- data.table(iris)                  # Convert data.frame to data.table
head(iris_dt)                                # Head of data.table
#    Sepal.Length Sepal.Width Petal.Length Petal.Width Species
# 1:          5.1         3.5          1.4         0.2  setosa
# 2:          4.9         3.0          1.4         0.2  setosa
# 3:          4.7         3.2          1.3         0.2  setosa
# 4:          4.6         3.1          1.5         0.2  setosa
# 5:          5.0         3.6          1.4         0.2  setosa
# 6:          5.4         3.9          1.7         0.4  setosa

Example: Sum Multiple Variables by Group Using data.table Package

iris_dt[, lapply(.SD, sum), by = Species]    # Grouping data.table
#       Species Sepal.Length Sepal.Width Petal.Length Petal.Width
# 1:     setosa        250.3       171.4         73.1        12.3
# 2: versicolor        296.8       138.5        213.0        66.3
# 3:  virginica        329.4       148.7        277.6       101.3

Further Resources & Related Tutorials

In the following, you may find some further resources on topics such as factors, character strings, and data conversion.

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