Closes #1140. DT[, 0, with=FALSE returns null data.table.

R/data.table.R

@@ -754,6 +754,7 @@ chmatch2 <- function(x, table, nomatch=NA_integer_) {
                     ansvals = chmatch(ansvars, names(x))
                 }
             } else if (is.numeric(j)) {
+                if (all(j == 0L)) return (null.data.table())
                 if (any(abs(j) > ncol(x) | j==0L)) stop("j out of bounds")
                 if (any(j<0L) && any(j>0L)) stop("j mixes positive and negative")
                 if (any(j<0L)) j = seq_len(ncol(x))[j]

README.md

@@ -248,6 +248,8 @@
 
   64. Using `.GRP` unnamed in `j` now returns a variable named `GRP` instead of `.GRP` as the period was causing issues. Same for `.BY`. Closes [#1243](https://github.com/Rdatatable/data.table/issues/1243); thanks to @MichaelChirico for the PR.
 
+  65. `DT[, 0, with=FALSE]` returns null data.table to be consistent with `data.frame`'s behaviour. Closes [#1140](https://github.com/Rdatatable/data.table/issues/1140). Thanks to @franknarf1.
+
 #### NOTES
 
   1. Clearer explanation of what `duplicated()` does (borrowed from base). Thanks to @matthieugomez for pointing out. Closes [#872](https://github.com/Rdatatable/data.table/issues/872).

inst/tests/tests.Rraw

@@ -6962,8 +6962,12 @@ dt1<-data.table(X=1:3,X=1:3)
 df1<-data.frame(X=1:3,X=1:3,check.names=FALSE)
 ls1<-list("X"=1:3,"X"=1:3)
 test(1559.1, setDT(dt1, check.names=TRUE), ans)
-test(1559.1, setDT(df1, check.names=TRUE), ans)
-test(1559.1, setDT(ls1, check.names=TRUE), ans)
+test(1559.2, setDT(df1, check.names=TRUE), ans)
+test(1559.3, setDT(ls1, check.names=TRUE), ans)
+
+# Fix #1140
+test(1560.1, data.table(x=letters[1:5])[, 0, with=FALSE], null.data.table())
+test(1560.2, data.table(x=letters[1:5])[, c(0,FALSE), with=FALSE], null.data.table())
 
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