new helper function fctr to retain order of input in levels, closes #…

NAMESPACE

@@ -61,6 +61,7 @@ export(.Last.updated)
 export(fcoalesce)
 export(substitute2)
 #export(DT)  # mtcars |> DT(i,j,by)  #4872 #5472
+export(fctr)
 
 S3method("[", data.table)
 S3method("[<-", data.table)

NEWS.md

@@ -18,6 +18,28 @@
 
 6. `between()` gains the argument `ignore_tzone=FALSE`. Normally, a difference in time zone between `lower` and `upper` will produce an error, and a difference in time zone between `x` and either of the others will produce a message. Setting `ignore_tzone=TRUE` bypasses the checks, allowing both comparisons to proceed without error or message about time zones.
 
+7. New helper function `fctr` as an extended version of `factor()`, [#4837](https://github.com/Rdatatable/data.table/issues/4837). Most notably, it supports (1) retaining input level ordering by default, i.e. `levels=unique(x)` as opposed to `levels = sort(unique(x))`; (2) `rev=` to reverse the levels; and (3) `sort=` to allow more feature parity with `factor()`. The choice of default is motivated by convenience in the common case when order of elements needs be preserved, for example when using `dcast` or adding a legend to a plot. This also matches the default sort ordering of groups in `by=`.
+
+    ```r
+    d = data.table(id1=rep(1:2, each=3L), id2=letters[c(4:3,5L,3:5)], v1=1:6)
+    dcast(d, id1 ~ factor(id2))
+    #      id1     c     d     e
+    # 1:     1     2     1     3
+    # 2:     2     4     5     6
+    dcast(d, id1 ~ fctr(id2))
+    #      id1     d     c     e
+    # 1:     1     1     2     3
+    # 2:     2     5     4     6
+    dcast(d, id1 ~ fctr(id2, sort=TRUE)) # same as factor()
+    #      id1     c     d     e
+    # 1:     1     2     1     3
+    # 2:     2     4     5     6
+    dcast(d, id1 ~ fctr(id2, rev=TRUE))
+    #      id1     e     c     d
+    # 1:     1     3     2     1
+    # 2:     2     6     4     5
+    ```
+
 ### BUG FIXES
 
 1. Custom binary operators from the `lubridate` package now work with objects of class `IDate` as with a `Date` subclass, [#6839](https://github.com/Rdatatable/data.table/issues/6839). Thanks @emallickhossain for the report and @aitap for the fix.

R/utils.R

@@ -219,6 +219,17 @@ rss = function() {  #5515 #5517
   # nocov end
 }
 
+# convert char to factor retaining order #4837
+fctr = function(x, levels=unique(x), ..., sort=FALSE, rev=FALSE) {
+  if (!isTRUEorFALSE(sort))
+    stopf("argument 'sort' must be TRUE or FALSE")
+  if (!isTRUEorFALSE(rev))
+    stopf("argument 'rev' must be TRUE or FALSE")
+  if (sort) levels = sort(levels)
+  if (rev) levels = rev(levels)
+  factor(x, levels=levels, ...)
+}
+
 formula_vars = function(f, x) { # .formula2varlist is not API and seems to have appeared after R-4.2, #6841
   terms <- terms(f)
   setNames(

inst/tests/tests.Rraw

@@ -21237,3 +21237,12 @@ test(2321.4, names(DT), names(data.frame(a=integer(), b=matrix(1L, nrow=0L, ncol
 ## but respect named column vectors
 test(2321.5, DT <- data.table(a=1:2, cbind(b=3:4)), data.table(a=1:2, b=3:4))
 test(2321.6, names(DT), names(data.frame(a=1:2, cbind(b=3:4))))
+
+# New fctr() helper: like factor() but retaining order by default #4837
+test(2240.01, levels(fctr(c("b","a","c"))), c("b","a","c"))
+test(2240.02, levels(fctr(c(3,1,2))), c("3","1","2"))
+test(2240.11, levels(fctr(c("b","a","c"), rev=TRUE)), c("c","a","b"))
+test(2240.12, levels(fctr(c("b","a","c"), rev=NA)), error="TRUE or FALSE")
+test(2240.21, levels(fctr(c("b","a","c"), sort=TRUE)), c("a","b","c"))
+test(2240.22, levels(fctr(c("b","a","c"), sort=NA)), error="TRUE or FALSE")
+test(2240.31, levels(fctr(c("b","a","c"), rev=TRUE, sort=TRUE)), c("c","b","a"))

man/fctr.Rd

@@ -0,0 +1,30 @@
+\name{fctr}
+\alias{fctr}
+\alias{factor}
+\title{Create a factor retaining original ordering}
+\description{
+  Creates a code{\link[base:factor]{factor}}.
+
+  By default, the output will have its levels in the original order, i.e., \code{levels = unique(x)}, as opposed to \code{factor}'s default where \code{levels = sort(unique(x))}.
+}
+\usage{
+fctr(x, levels=unique(x), ..., sort=FALSE, rev=FALSE)
+}
+\arguments{
+  \item{x}{ Object to be turned into a factor. }
+  \item{levels}{ Levels for the new factor; \code{unique(x)} by default. }
+  \item{\dots}{ Other arguments passed to code{\link[base:factor]{factor}}. }
+  \item{sort}{ Logical, default \code{FALSE}. Should \code{levels} be sorted? }
+  \item{rev}{ Logical, default \code{FALSE}. Should \code{levels} be reversed? Applied \emph{after} \code{sort}. }
+}
+\value{
+  Factor vector having levels ordered according to the order of elements in input and arguments \code{sort}, \code{rev}.
+}
+\examples{
+levels(factor(c("b","a","c")))
+levels(fctr(c("b","a","c")))
+levels(fctr(c("b","a","c"), rev=TRUE))
+levels(fctr(c("b","a","c"), sort=TRUE))
+levels(fctr(c("b","a","c"), sort=TRUE, rev=TRUE))
+}
+\keyword{ data }