data.table
parlance, all set*
functions change their input
by reference. That is, no copy is made at all, other than temporary
working memory, which is as large as one column.. The only other
data.table
operator that modifies input by reference is :=
.
Check out the See Also
section below for other set*
function
data.table
provides.setorder
(and setorderv
) reorders the rows of a data.table
based on the columns (and column order) provided. It reorders the table
by reference and is therefore very memory efficient.
Also x[order(.)]
is now optimised internally to use data.table's fast
order by default. data.table always reorders in C-locale. To sort by session
locale, use x[base::order(.)]
instead.
bit64::integer64
type is also supported for reordering rows of a
data.table
.
setorder(x, ..., na.last=FALSE)
setorderv(x, cols, order=1L, na.last=FALSE)
# optimised to use data.table's internal fast order
# x[order(., na.last=TRUE)]
data.table
. ...
is missing (ex: setorder(x)
), x
is rearranged based on all
columns in ascending order by default. To sort by a column in descending order
prefix a "-"
, i.e., setorder(x, a, -b, c)
. The -b
works
when b
is of type character
as well. x
, to which to order
by. Do not add "-"
here. Use order
argument instead.1
and
-1
, corresponding to ascending and descending order. The length of
order
must be either 1
or equal to that of cols
. If
length(order) == 1
, it's recycled to length(cols)
. TRUE
, missing values in the data are placed
last; if FALSE
, they are placed first; if NA
they are removed.
na.last=NA
is valid only for x[order(., na.last)]
and it's
default is TRUE
. setorder
and setorderv
only accept
TRUE/FALSE with default FALSE
.setorder(DT,a,-b)[, cumsum(c), by=list(a,b)]
.
If you require a copy, take a copy first (using DT2 = copy(DT)
). See
?copy
.
data.table
implements fast radix based ordering. In versions <= 1.9.2,="" it="" was="" only="" capable="" of="" increasing="" order="" (ascending).="" from="" 1.9.4="" on,="" the="" functionality="" has="" been="" extended="" to="" decreasing="" (descending)="" as="" well.="" <="" p="">setorder
accepts unquoted column names (with names preceded with a
-
sign for descending order) and reorders data.table rows
by reference, for e.g., setorder(x, a, -b, c)
. Note that
-b
also works with columns of type character
unlike
base::order
, which requires -xtfrm(y)
instead (which is slow).
setorderv
in turn accepts a character vector of column names and an
integer vector of column order separately.
Note that setkey
still requires and will always sort only in
ascending order, and is different from setorder
in that it additionally
sets the sorted
attribute.
na.last
argument, by default, is FALSE
for setorder
and
setorderv
to be consistent with data.table
's setkey
and
is TRUE
for x[order(.)]
to be consistent with base::order
.
Only x[order(.)]
can have na.last = NA
as it's a subset operation
as opposed to setorder
or setorderv
which reorders the data.table
by reference.
If setorder
results in reordering of the rows of a keyed data.table
,
then it's key will be set to NULL
.
setkey
, setcolorder
, setattr
,
setnames
, set
, :=
, setDT
,
setDF
, copy
, setNumericRounding
set.seed(45L)
DT = data.table(A=sample(3, 10, TRUE),
B=sample(letters[1:3], 10, TRUE), C=sample(10))
# setorder
setorder(DT, A, -B)
# same as above, but using setorderv
setorderv(DT, c("A", "B"), c(1, -1))
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