Fast row reordering of a data.table by reference
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.
Note that queries like x[order(.)] are optimised internally to use data.table's fast order.
Also note that data.table always reorders in "C-locale" (see Details). To sort by session locale, use x[base::order(.)].
bit64::integer64 type is also supported for reordering rows of a data.table.
setorder(x, ..., na.last=FALSE) setorderv(x, cols = colnames(x), order=1L, na.last=FALSE) # optimised to use data.table's internal fast order # x[order(., na.last=TRUE)]
x |
A |
... |
The columns to sort by. Do not quote column names. If |
cols |
A character vector of column names of |
order |
An integer vector with only possible values of |
na.last |
|
data.table implements its own fast radix-based ordering. See the references for some exposition on the concept of radix sort.
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). We emphasize that
this means "descending" and not "negative" because the implementation simply
reverses the sort order, as opposed to sorting the opposite of the input
(which would be inefficient).
Note that -b also works with columns of type character unlike
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 is a subset operation
as opposed to setorder or setorderv which reorders the data.table
by reference.
data.table always reorders in "C-locale".
As a consequence, the ordering may be different to that obtained by base::order.
In English locales, for example, sorting is case-sensitive in C-locale.
Thus, sorting c("c", "a", "B") returns c("B", "a", "c") in data.table
but c("a", "B", "c") in base::order. Note this makes no difference in most cases
of data; both return identical results on ids where only upper-case or lower-case letters are present ("AB123" < "AC234"
is true in both), or on country names and other proper nouns which are consistently capitalized.
For example, neither "America" < "Brazil" nor
"america" < "brazil" are affected since the first letter is consistently
capitalized.
Using C-locale makes the behaviour of sorting in data.table more consistent across sessions and locales.
The behaviour of base::order depends on assumptions about the locale of the R session.
In English locales, "america" < "BRAZIL" is true by default
but false if you either type Sys.setlocale(locale="C") or the R session has been started in a C locale
for you – which can happen on servers/services since the locale comes from the environment the R session
was started in. By contrast, "america" < "BRAZIL" is always FALSE in data.table regardless of the way your R session was started.
If setorder results in reordering of the rows of a keyed data.table,
then its key will be set to NULL.
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.
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))Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.