In 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)]A data.table.
The columns to sort by. Do not quote column names. If ...
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.
A character vector of column names of x, to which to order
by. Do not add "-" here. Use order argument instead.
An integer vector with only possible values of 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).
logical. If 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.
The input is modified by reference, and returned (invisibly) so it can be used
in compound statements; e.g., 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 order (descending) as well.
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
# NOT RUN {
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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