Merge branch 'master' of https://github.com/Rdatatable/data.table

* 'master' of https://github.com/Rdatatable/data.table:
  merge.data.table will raise warning for 0 columns data.tables. Closes #597

R/merge.R

@@ -8,6 +8,7 @@ merge.data.table <- function(x, y, by = NULL, by.x = NULL, by.y = NULL, all = FA
             by = key(x)
         }
     }
+    if ((x0 <- length(x)==0L) | (y0 <- length(y)==0L)) warning(sprintf("You are trying to join data.tables where %s 0 columns data.table.", if(x0 & y0) "'x' and 'y' arguments are" else if(x0 & !y0) "'x' argument is" else if(!x0 & y0) "'y' argument is"))
     if (any(duplicated(names(x)))) stop("x has some duplicated column name(s): ",paste(names(x)[duplicated(names(x))],collapse=","),". Please remove or rename the duplicate(s) and try again.")
     if (any(duplicated(names(y)))) stop("y has some duplicated column name(s): ",paste(names(y)[duplicated(names(y))],collapse=","),". Please remove or rename the duplicate(s) and try again.")
 

README.md

@@ -145,6 +145,8 @@
 
   15. Added `\n` to message displayed in `melt.data.table` when duplicate names are found, [#1538](https://github.com/Rdatatable/data.table/issues/1538). Thanks @Franknarf1.
 
+  16. `merge.data.table` will raise warning if any of data.tables to join has 0 columns. Closes [#597](https://github.com/Rdatatable/data.table/issues/597).
+
 ### Changes in v1.9.6  (on CRAN 19 Sep 2015)
 
 #### NEW FEATURES

inst/tests/tests.Rraw

@@ -7401,6 +7401,13 @@ DT1 = data.table(id1 = c("c", "a", "b", "b", "b", "c"),
 DT2 = data.table(id1=c("c", "w", "b"), val=50:52)
 test(1600.2, names(DT1[DT2, .(id1=id1, val=val, bla=sum(z1, na.rm=TRUE)), on="id1"]), c("id1", "val", "bla"))
 
+# warn when merge empty data.table #597
+test(1601.1, merge(data.table(a=1),data.table(a=1), by="a"), data.table(a=1, key="a"))
+test(1601.2, tryCatch(merge(data.table(a=1),data.table(NULL), by="a"), warning = function(w) w$message), "You are trying to join data.tables where 'y' argument is 0 columns data.table.")
+test(1601.3, tryCatch(merge(data.table(NULL),data.table(a=1), by="a"), warning = function(w) w$message), "You are trying to join data.tables where 'x' argument is 0 columns data.table.")
+test(1601.4, tryCatch(merge(data.table(NULL),data.table(NULL), by="a"), warning = function(w) w$message), "You are trying to join data.tables where 'x' and 'y' arguments are 0 columns data.table.")
+
+
 ##########################
 
 # TODO: Tests involving GForce functions needs to be run with optimisation level 1 and 2, so that both functions are tested all the time.