data.table
inherits from data.frame
. It offers fast and nemory efficient: file reader and writer, aggregations, updates, equi, nonequi, rolling, range and interval joins, in a short and flexible syntax, for faster development.
It is inspired by A[B]
syntax in R where A
is a matrix and B
is a 2column matrix. Since a data.table
is a data.frame
, it is compatible with R functions and packages that accept only data.frame
s.
Type vignette(package="data.table")
to get started. The Introduction to data.table vignette introduces data.table
's x[i, j, by]
syntax and is a good place to start. If you have read the vignettes and the help page below, please feel free to ask questions on Stack Overflow data.table tag or on datatablehelp mailing list. To report a bug please type: bug.report(package = "data.table")
.
Please check the homepage for up to the minute live NEWS.
Tip: one of the quickest ways to learn the features is to type example(data.table)
and study the output at the prompt.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  data.table(..., keep.rownames=FALSE, check.names=FALSE, key=NULL, stringsAsFactors=FALSE)
## S3 method for class 'data.table'
x[i, j, by, keyby, with = TRUE,
nomatch = getOption("datatable.nomatch"), # default: NA_integer_
mult = "all",
roll = FALSE,
rollends = if (roll=="nearest") c(TRUE,TRUE)
else if (roll>=0) c(FALSE,TRUE)
else c(TRUE,FALSE),
which = FALSE,
.SDcols,
verbose = getOption("datatable.verbose"), # default: FALSE
allow.cartesian = getOption("datatable.allow.cartesian"), # default: FALSE
drop = NULL, on = NULL]

... 
Just as 
keep.rownames 
If 
check.names 
Just as 
key 
Character vector of one or more column names which is passed to 
stringsAsFactors 
Logical (default is 
x 
A 
i 
Integer, logical or character vector, single column numeric
If
Using When the binary operator Support for nonequi join was recently implemented, which allows for other binary operators See Keys and fast binary search based subset and Secondary indices and auto indexing. Advanced: When 
j 
When The expression '.()' is a shorthand alias to When Advanced: Advanced: When Advanced: Columns of See Introduction to data.table vignette and examples. 
by 
Column names are seen as if they are variables (as in
Advanced: When Advanced: In the 
keyby 
Same as 
with 
By default When 
nomatch 
Same as 
mult 
When 
roll 
When
Rolling joins apply to the last join column, generally a date but can be any variable. It is particularly fast using a modified binary search. A common idiom is to select a contemporaneous regular time series ( 
rollends 
A logical vector length 2 (a single logical is recycled) indicating whether values falling before the first value or after the last value for a group should be rolled as well.
When 
which 

.SDcols 
Specifies the columns of For convenient interactive use, the form 
verbose 

allow.cartesian 

drop 
Never used by 
on 
Indicate which columns in
See examples as well as Secondary indices and auto indexing. 
data.table
builds on base R functionality to reduce 2 types of time:
programming time (easier to write, read, debug and maintain), and
compute time (fast and memory efficient).
The general form of data.table syntax is:
1 2 3 4 5 
The way to read this out loud is: "Take DT
, subset rows by i
, then compute j
grouped by by
. Here are some basic usage examples expanding on this definition. See the vignette (and examples) for working examples.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16  X[, a] # return col 'a' from X as vector. If not found, search in parent frame.
X[, .(a)] # same as above, but return as a data.table.
X[, sum(a)] # return sum(a) as a vector (with same scoping rules as above)
X[, .(sum(a)), by=c] # get sum(a) grouped by 'c'.
X[, sum(a), by=c] # same as above, .() can be ommitted in by on single expression for convenience
X[, sum(a), by=c:f] # get sum(a) grouped by all columns in between 'c' and 'f' (both inclusive)
X[, sum(a), keyby=b] # get sum(a) grouped by 'b', and sort that result by the grouping column 'b'
X[, sum(a), by=b][order(b)] # same order as above, but by chaining compound expressions
X[c>1, sum(a), by=c] # get rows where c>1 is TRUE, and on those rows, get sum(a) grouped by 'c'
X[Y, .(a, b), on="c"] # get rows where Y$c == X$c, and select columns 'X$a' and 'X$b' for those rows
X[Y, .(a, i.a), on="c"] # get rows where Y$c == X$c, and then select 'X$a' and 'Y$a' (=i.a)
X[Y, sum(a*i.a), on="c" by=.EACHI] # for *each* 'Y$c', get sum(a*i.a) on matching rows in 'X$c'
X[, plot(a, b), by=c] # j accepts any expression, generates plot for each group and returns no data
# see ?assign to add/update/delete columns by reference using the same consistent interface

A data.table
is a list
of vectors, just like a data.frame
. However :
it never has or uses rownames. Rownames based indexing can be done by setting a key of one or more columns or done adhoc using the on
argument (now preferred).
it has enhanced functionality in [.data.table
for fast joins of keyed tables, fast aggregation, fast last observation carried forward (LOCF) and fast add/modify/delete of columns by reference with no copy at all.
See the see also
section for the several other methods that are available for operating on data.tables efficiently.
If keep.rownames
or check.names
are supplied they must be written in full because R does not allow partial argument names after '...
'. For example, data.table(DF, keep=TRUE)
will create a
column called "keep"
containing TRUE
and this is correct behaviour; data.table(DF, keep.rownames=TRUE)
was intended.
POSIXlt
is not supported as a column type because it uses 40 bytes to store a single datetime. They are implicitly converted to POSIXct
type with warning. You may also be interested in IDateTime
instead; it has methods to convert to and from POSIXlt
.
https://github.com/Rdatatable/data.table/wiki (data.table
homepage)
http://crantastic.org/packages/datatable (User reviews)
http://en.wikipedia.org/wiki/Binary_search
specialsymbols
, data.frame
, [.data.frame
, as.data.table
, setkey
, setorder
, setDT
, setDF
, J
, SJ
, CJ
, merge.data.table
, tables
, test.data.table
, IDateTime
, unique.data.table
, copy
, :=
, alloc.col
, truelength
, rbindlist
, setNumericRounding
, datatableoptimize
, fsetdiff
, funion
, fintersect
, fsetequal
, anyDuplicated
, uniqueN
, rowid
, rleid
, na.omit
, frank
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205  ## Not run:
example(data.table) # to run these examples at the prompt
## End(Not run)
DF = data.frame(x=rep(c("b","a","c"),each=3), y=c(1,3,6), v=1:9)
DT = data.table(x=rep(c("b","a","c"),each=3), y=c(1,3,6), v=1:9)
DF
DT
identical(dim(DT), dim(DF)) # TRUE
identical(DF$a, DT$a) # TRUE
is.list(DF) # TRUE
is.list(DT) # TRUE
is.data.frame(DT) # TRUE
tables()
# basic row subset operations
DT[2] # 2nd row
DT[3:2] # 3rd and 2nd row
DT[order(x)] # no need for order(DT$x)
DT[order(x), ] # same as above. The ',' is optional
DT[y>2] # all rows where DT$y > 2
DT[y>2 & v>5] # compound logical expressions
DT[!2:4] # all rows other than 2:4
DT[(2:4)] # same
# selectcompute columns data.table way
DT[, v] # v column (as vector)
DT[, list(v)] # v column (as data.table)
DT[, .(v)] # same as above, .() is a shorthand alias to list()
DT[, sum(v)] # sum of column v, returned as vector
DT[, .(sum(v))] # same, but return data.table (column autonamed V1)
DT[, .(sv=sum(v))] # same, but column named "sv"
DT[, .(v, v*2)] # return two column data.table, v and v*2
# subset rows and selectcompute data.table way
DT[2:3, sum(v)] # sum(v) over rows 2 and 3, return vector
DT[2:3, .(sum(v))] # same, but return data.table with column V1
DT[2:3, .(sv=sum(v))] # same, but return data.table with column sv
DT[2:5, cat(v, "\n")] # just for j's side effect
# select columns the data.frame way
DT[, 2, with=FALSE] # 2nd column, returns a data.table always
colNum = 2
DT[, colNum, with=FALSE] # same, equivalent to DT[, .SD, .SDcols=colNum]
DT[["v"]] # same as DT[, v] but much faster
# grouping operations  j and by
DT[, sum(v), by=x] # ad hoc by, order of groups preserved in result
DT[, sum(v), keyby=x] # same, but order the result on by cols
DT[, sum(v), by=x][order(x)] # same but by chaining expressions together
# fast ad hoc row subsets (subsets as joins)
DT["a", on="x"] # same as x == "a" but uses binary search (fast)
DT["a", on=.(x)] # same, for convenience, no need to quote every column
DT[.("a"), on="x"] # same
DT[x=="a"] # same, single "==" internally optimised to use binary search (fast)
DT[x!="b"  y!=3] # not yet optimized, currently vector scan subset
DT[.("b", 3), on=c("x", "y")] # join on columns x,y of DT; uses binary search (fast)
DT[.("b", 3), on=.(x, y)] # same, but using on=.()
DT[.("b", 1:2), on=c("x", "y")] # no match returns NA
DT[.("b", 1:2), on=.(x, y), nomatch=0] # no match row is not returned
DT[.("b", 1:2), on=c("x", "y"), roll=Inf] # locf, nomatch row gets rolled by previous row
DT[.("b", 1:2), on=.(x, y), roll=Inf] # nocb, nomatch row gets rolled by next row
DT["b", sum(v*y), on="x"] # on rows where DT$x=="b", calculate sum(v*y)
# all together now
DT[x!="a", sum(v), by=x] # get sum(v) by "x" for each i != "a"
DT[!"a", sum(v), by=.EACHI, on="x"] # same, but using subsetsasjoins
DT[c("b","c"), sum(v), by=.EACHI, on="x"] # same
DT[c("b","c"), sum(v), by=.EACHI, on=.(x)] # same, using on=.()
# joins as subsets
X = data.table(x=c("c","b"), v=8:7, foo=c(4,2))
X
DT[X, on="x"] # right join
X[DT, on="x"] # left join
DT[X, on="x", nomatch=0] # inner join
DT[!X, on="x"] # not join
DT[X, on=.(y<=foo)] # NEW nonequi join (v1.9.8+)
DT[X, on="y<=foo"] # same as above
DT[X, on=c("y<=foo")] # same as above
DT[X, on=.(y>=foo)] # NEW nonequi join (v1.9.8+)
DT[X, on=.(x, y<=foo)] # NEW nonequi join (v1.9.8+)
DT[X, .(x,y,x.y,v), on=.(x, y>=foo)] # Select x's join columns as well
DT[X, on="x", mult="first"] # first row of each group
DT[X, on="x", mult="last"] # last row of each group
DT[X, sum(v), by=.EACHI, on="x"] # join and eval j for each row in i
DT[X, sum(v)*foo, by=.EACHI, on="x"] # join inherited scope
DT[X, sum(v)*i.v, by=.EACHI, on="x"] # 'i,v' refers to X's v column
DT[X, on=.(x, v>=v), sum(y)*foo, by=.EACHI] # NEW nonequi join with by=.EACHI (v1.9.8+)
# setting keys
kDT = copy(DT) # (deep) copy DT to kDT to work with it.
setkey(kDT,x) # set a 1column key. No quotes, for convenience.
setkeyv(kDT,"x") # same (v in setkeyv stands for vector)
v="x"
setkeyv(kDT,v) # same
# key(kDT)<"x" # copies whole table, please use set* functions instead
haskey(kDT) # TRUE
key(kDT) # "x"
# fast *keyed* subsets
kDT["a"] # subsetasjoin on *key* column 'x'
kDT["a", on="x"] # same, being explicit using 'on=' (preferred)
# all together
kDT[!"a", sum(v), by=.EACHI] # get sum(v) for each i != "a"
# multicolumn key
setkey(kDT,x,y) # 2column key
setkeyv(kDT,c("x","y")) # same
# fast *keyed* subsets on multicolumn key
kDT["a"] # join to 1st column of key
kDT["a", on="x"] # on= is optional, but is preferred
kDT[.("a")] # same, .() is an alias for list()
kDT[list("a")] # same
kDT[.("a", 3)] # join to 2 columns
kDT[.("a", 3:6)] # join 4 rows (2 missing)
kDT[.("a", 3:6), nomatch=0] # remove missing
kDT[.("a", 3:6), roll=TRUE] # locf rolling join
kDT[.("a", 3:6), roll=Inf] # same as above
kDT[.("a", 3:6), roll=Inf] # nocb rolling join
kDT[!.("a")] # not join
kDT[!"a"] # same
# more on special symbols, see also ?"specialsymbols"
DT[.N] # last row
DT[, .N] # total number of rows in DT
DT[, .N, by=x] # number of rows in each group
DT[, .SD, .SDcols=x:y] # select columns 'x' and 'y'
DT[, .SD[1]] # first row of all columns
DT[, .SD[1], by=x] # first row of 'y' and 'v' for each group in 'x'
DT[, c(.N, lapply(.SD, sum)), by=x] # get rows *and* sum columns 'v' and 'y' by group
DT[, .I[1], by=x] # row number in DT corresponding to each group
DT[, grp := .GRP, by=x] # add a group counter column
X[, DT[.BY, y, on="x"], by=x] # join within each group
# add/update/delete by reference (see ?assign)
print(DT[, z:=42L]) # add new column by reference
print(DT[, z:=NULL]) # remove column by reference
print(DT["a", v:=42L, on="x"]) # subassign to existing v column by reference
print(DT["b", v2:=84L, on="x"]) # subassign to new column by reference (NA padded)
DT[, m:=mean(v), by=x][] # add new column by reference by group
# NB: postfix [] is shortcut to print()
# advanced usage
DT = data.table(x=rep(c("b","a","c"),each=3), v=c(1,1,1,2,2,1,1,2,2), y=c(1,3,6), a=1:9, b=9:1)
DT[, sum(v), by=.(y%%2)] # expressions in by
DT[, sum(v), by=.(bool = y%%2)] # same, using a named list to change by column name
DT[, .SD[2], by=x] # get 2nd row of each group
DT[, tail(.SD,2), by=x] # last 2 rows of each group
DT[, lapply(.SD, sum), by=x] # sum of all (other) columns for each group
DT[, .SD[which.min(v)], by=x] # nested query by group
DT[, list(MySum=sum(v),
MyMin=min(v),
MyMax=max(v)),
by=.(x, y%%2)] # by 2 expressions
DT[, .(a = .(a), b = .(b)), by=x] # list columns
DT[, .(seq = min(a):max(b)), by=x] # j is not limited to just aggregations
DT[, sum(v), by=x][V1<20] # compound query
DT[, sum(v), by=x][order(V1)] # ordering results
DT[, c(.N, lapply(.SD,sum)), by=x] # get number of observations and sum per group
DT[, {tmp < mean(y);
.(a = atmp, b = btmp)
}, by=x] # anonymous lambdain 'j', j accepts any valid
# expression. TO REMEMBER: every element of
# the list becomes a column in result.
pdf("new.pdf")
DT[, plot(a,b), by=x] # can also plot in 'j'
dev.off()
# using rleid, get max(y) and min of all cols in .SDcols for each consecutive run of 'v'
DT[, c(.(y=max(y)), lapply(.SD, min)), by=rleid(v), .SDcols=v:b]
# Follow rhelp posting guide, SUPPORT is here (*not* rhelp) :
# http://stackoverflow.com/questions/tagged/data.table
# or
# datatablehelp@lists.rforge.rproject.org
## Not run:
vignette("datatableintro")
vignette("datatablereferencesemantics")
vignette("datatablekeysfastsubset")
vignette("datatablesecondaryindicesandautoindexing")
vignette("datatablereshape")
vignette("datatablefaq")
test.data.table() # over 5700 low level tests
# keep up to date with latest stable version on CRAN
update.packages()
# get the latest devel (needs Rtools for windows, xcode for mac)
install.packages("data.table", repos = "https://Rdatatable.github.io/data.table", type = "source")
## End(Not run)

Questions? Problems? Suggestions? Tweet to @rdrrHQ or email at ian@mutexlabs.com.
All documentation is copyright its authors; we didn't write any of that.