add DT() functional form data.table query

NAMESPACE

@@ -57,6 +57,7 @@ export(setnafill)
 export(.Last.updated)
 export(fcoalesce)
 export(substitute2)
+export(DT)  # mtcars |> DT(i,j,by)  #4872
 
 S3method("[", data.table)
 S3method("[<-", data.table)

NEWS.md

@@ -109,6 +109,12 @@
 
 21. `melt()` was pseudo generic in that `melt(DT)` would dispatch to the `melt.data.table` method but `melt(not-DT)` would explicitly redirect to `reshape2`. Now `melt()` is standard generic so that methods can be developed in other packages, [#4864](https://github.com/Rdatatable/data.table/pull/4864). Thanks to @odelmarcelle for suggesting and implementing.
 
+22. `DT(i, j, by, ...)` has been added, i.e. functional form of a `data.table` query, [#641](https://github.com/Rdatatable/data.table/issues/641) [#4872](https://github.com/Rdatatable/data.table/issues/4872). Thanks to Yike Lu and Elio Campitelli for filing requests, many others for comments and suggestions, and Matt Dowle for the PR. This enables the `data.table` general form query to be invoked on a `data.frame` without converting it to a `data.table` first. The class of the input object is retained.
+
+    ```R
+    mtcars |> DT(mpg>20, .(mean_hp=mean(hp)), by=cyl)
+    ```
+
 ## BUG FIXES
 
 1. `by=.EACHI` when `i` is keyed but `on=` different columns than `i`'s key could create an invalidly keyed result, [#4603](https://github.com/Rdatatable/data.table/issues/4603) [#4911](https://github.com/Rdatatable/data.table/issues/4911). Thanks to @myoung3 and @adamaltmejd for reporting, and @ColeMiller1 for the PR. An invalid key is where a `data.table` is marked as sorted by the key columns but the data is not sorted by those columns, leading to incorrect results from subsequent queries.

R/data.table.R

@@ -846,10 +846,10 @@ replace_dot_alias = function(e) {
           if (!is.na(nomatch)) irows = irows[irows!=0L]   # TO DO: can be removed now we have CisSortedSubset
           if (length(allbyvars)) {    ###############  TO DO  TO DO  TO DO  ###############
             if (verbose) catf("i clause present and columns used in by detected, only these subset: %s\n", brackify(allbyvars))
-            xss = x[irows,allbyvars,with=FALSE,nomatch=nomatch,mult=mult,roll=roll,rollends=rollends]
+            xss = `[.data.table`(x,irows,allbyvars,with=FALSE,nomatch=nomatch,mult=mult,roll=roll,rollends=rollends)
           } else {
             if (verbose) catf("i clause present but columns used in by not detected. Having to subset all columns before evaluating 'by': '%s'\n", deparse(by))
-            xss = x[irows,nomatch=nomatch,mult=mult,roll=roll,rollends=rollends]
+            xss = `[.data.table`(x,irows,nomatch=nomatch,mult=mult,roll=roll,rollends=rollends)
           }
           if (bysub %iscall% ':' && length(bysub)==3L) {
             byval = eval(bysub, setattr(as.list(seq_along(xss)), 'names', names(xss)), parent.frame())
@@ -1910,6 +1910,8 @@ replace_dot_alias = function(e) {
   setalloccol(ans)   # TODO: overallocate in dogroups in the first place and remove this line
 }
 
+DT = `[.data.table` #4872
+
 .optmean = function(expr) {   # called by optimization of j inside [.data.table only. Outside for a small speed advantage.
   if (length(expr)==2L)  # no parameters passed to mean, so defaults of trim=0 and na.rm=FALSE
     return(call(".External",quote(Cfastmean),expr[[2L]], FALSE))

man/data.table.Rd

@@ -5,6 +5,7 @@
 \alias{Ops.data.table}
 \alias{is.na.data.table}
 \alias{[.data.table}
+\alias{DT}
 \alias{.}
 \alias{.(}
 \alias{.()}
@@ -217,6 +218,8 @@ The way to read this out loud is: "Take \code{DT}, subset rows by \code{i}, \emp
     # see ?assign to add/update/delete columns by reference using the same consistent interface
 }
 
+A \code{data.table} query may be invoked on a \code{data.frame} using functional form \code{DT(...)}, see examples. The class of the input is retained.
+
 A \code{data.table} is a \code{list} of vectors, just like a \code{data.frame}. However :
 \enumerate{
 \item it never has or uses rownames. Rownames based indexing can be done by setting a \emph{key} of one or more columns or done \emph{ad-hoc} using the \code{on} argument (now preferred).
@@ -431,6 +434,11 @@ 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]
 
+# functional query DT(...)
+if (getRversion() >= "4.1.0") {       # native pipe |> new in R 4.1.0
+  mtcars |> DT(mpg>20, .(mean_hp=mean(hp)), by=cyl)
+}
+
 # Support guide and links:
 # https://github.com/Rdatatable/data.table/wiki/Support