This article gives a brief overview of the data.table package written by M. Dowle, T. Short, S. Lianoglou.
A data.table is an extension of a data.frame created to reduce the working time of the user in two ways
- programming time
- compute time
The data.table sintax is inspired by the R syntax matrix
A [B] where
A is a matrix and
B is a 2-column matrix.
As a data.table is a data.frame, will be compliant with all R functions and packages that accept data.frame as object.
The big advantage of a data.table than a data.frame is that it uses the tables as if they were tables in a database, with a speed of data access truly remarkable.
A data.table is created exactly like a data.frame, the sintax is the same.
DF = data.frame(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
DF e DT are identical but on DT can create an index by defining a key.
NAME NROW MB COLS KEY
[1,] DT 9 1 x,y,v x
DT have been re-ordered according to the values of x column.
A key consists of one or more columns which may be integer, factor, character or some other class.
A data.tables do not have rownames but may instead have a key of one or more columns using setkey. This key may be used for row indexing instead of rownames.
Now we can subsetting data
DT["b",] # extract data for key-column = “b”
DT[,v] # extract the v column
100+ times faster than ==
A data.table is like a data.frame but i and j can be expressions of column names directly.
Furthermore i may itself be a data.table which invokes a fast table join using binary search in O(log n).
We can easily add new data
DT[,w:=1:3] # add a w column
500+ times faster than DF[i,j] = value
or join data.table
DT[J("a",3:6)] # inner join (J is an alias of data.table)
or fast grouping
10+ times faster than tapply()
with a syntax much easier than in data.frame.
In a data.table each cell can be a different type
- each cell can be vector
- each cell can itself be a data.table
- combining list columns with i and b
In conclusion a data.table is identical to a data.frame other than:
- it doesn’t have rownames
- selecting a single row will always return a single row data.table not a vector
- the comma is optional inside , so DT returns the 3rd row as a 1 row data.table
-  is like a call to subset()
- [,...], is like a call to with()
- up to 10 times less memory
- up to 10 times faster to create, and copy
- simpler R code