data.table Getting started with data.table

Remarks

Versions

Getting started and finding help

The package's official wiki has some essential materials:

• As a new user, you will want to check out the vignettes, FAQ and cheat sheet.

• Before asking a question -- here on StackOverflow or anywhere else -- please read the support page.

For help on individual functions, the syntax is help("fread") or ?fread . If the package has not been loaded, use the full name like ?data.table::fread .

Installation and setup

Install the stable release from CRAN:

install.packages("data.table")       
 

Or the development version from github:

install.packages("data.table", type = "source", 
  repos = "http://Rdatatable.github.io/data.table")
 

To revert from devel to CRAN, the current version must first be removed:

remove.packages("data.table")
install.packages("data.table")
 

Visit the website for full installation instructions and the latest version numbers.

Using the package

Usually you will want to load the package and all of its functions with a line like

library(data.table)
 

If you only need one or two functions, you can refer to them like data.table::fread instead.

Syntax and features

Basic syntax

DT[where, select|update|do, by] syntax is used to work with columns of a data.table.

• The "where" part is the i argument
• The "select|update|do" part is the j argument

These two arguments are usually passed by position instead of by name.

A sequence of steps can be chained like DT[...][...] .

Shortcuts, special functions and special symbols inside DT[...]

Function or symbol	Notes
.()	in several arguments, replaces list()
J()	in i , replaces list()
:=	in j , a function used to add or modify columns
.N	in i , the total number of rows in j , the number of rows in a group
.I	in j , the vector of row numbers in the table (filtered by i )
.SD	in j , the current subset of the data selected by the .SDcols argument
.GRP	in j , the current index of the subset of the data
.BY	in j , the list of by values for the current subset of data
V1, V2, ...	default names for unnamed columns created in j

Joins inside DT[...]

Notation	Notes
DT1[DT2, on, j]	join two tables
i.*	special prefix on DT2's columns after the join
by=.EACHI	special option available only with a join
DT1[!DT2, on, j]	anti-join two tables
DT1[DT2, on, roll, j]	join two tables, rolling on the last column in on=

Reshaping, stacking and splitting

Notation	Notes
melt(DT, id.vars, measure.vars)	transform to long format for multiple columns, use measure.vars = patterns(...)
dcast(DT, formula)	transform to wide format
rbind(DT1, DT2, ...)	stack enumerated data.tables
rbindlist(DT_list, idcol)	stack a list of data.tables
split(DT, by)	split a data.table into a list

Some other functions specialized for data.tables

Function(s)	Notes
foverlaps	overlap joins
merge	another way of joining two tables
set	another way of adding or modifying columns
fintersect , fsetdiff , funion , fsetequal , unique , duplicated , anyDuplicated	set-theory operations with rows as elements
CJ	the Cartesian product of vectors
uniqueN	the number of distinct rows
rowidv(DT, cols)	row ID (1 to .N) within each group determined by cols
rleidv(DT, cols)	group ID (1 to .GRP) within each group determined by runs of cols
shift(DT, n)	apply a shift operator to every column
setorder , setcolorder , setnames , setkey , setindex , setattr	modify attributes and order by reference

Other features of the package

Features	Notes
IDate and ITime	integer dates and times