Closes #1287 and closes #1271. by=.EACHI plays nice with mult="first"…

…/"last".

R/data.table.R

@@ -671,10 +671,13 @@ chmatch2 <- function(x, table, nomatch=NA_integer_) {
                     if (length(irows)) stop("Internal error. irows has length in by=.EACHI")
                 }
             } else {
-                irows = if (mult=="first") f__ else f__+len__-1L
-                if (identical(nomatch,0L)) irows = irows[len__>0L]  # 0s are len 0, so this removes -1 irows
-                if (length(len__)) len__ = pmin(len__,1L)  # for test 456, and consistency generally
-                                                           # the if() is for R < 2.15.1 when pmin was enhanced, see v1.8.6.
+		if (!byjoin) { # fix for #1287 and #1271
+		    irows = if (mult=="first") f__ else f__+len__-1L
+		    if (identical(nomatch,0L)) irows = irows[len__>0L]  # 0s are len 0, so this removes -1 irows
+		    if (length(len__)) len__ = pmin(len__,1L)
+		} else { if (mult == "last") f__ = f__+len__- 1L } # fix for #1287 and #1271
+		# for test 456, and consistency generally. The if() is for R < 2.15.1 when pmin was enhanced, see v1.8.6.
+		if (length(len__)) len__ = pmin(len__, 1L)
             }
             if (length(xo) && length(irows)) irows = xo[irows]   # TO DO: fsort here?
         } else {

README.md

@@ -142,6 +142,8 @@
 
   40. Not join along with `mult="first"` and `mult="last"` is handled correctly, [#1571](https://github.com/Rdatatable/data.table/issues/1571).
 
+  41. `by=.EACHI` works as expected along with `mult="first"` and `mult="last"`, [#1287](https://github.com/Rdatatable/data.table/issues/1287) and [#1271](https://github.com/Rdatatable/data.table/issues/1271).
+
 #### NOTES
 
   1. Updated error message on invalid joins to reflect the new `on=` syntax, [#1368](https://github.com/Rdatatable/data.table/issues/1368). Thanks @MichaelChirico.

inst/tests/tests.Rraw

@@ -7618,6 +7618,14 @@ test(1615.1, x[!y, on="V1", mult="first"], data.table(V1=c(1,7), V2=INT(c(1,4)))
 test(1615.2, x[!y, on="V1", mult="last"], data.table(V1=c(1,7), V2=INT(c(2,9))))
 test(1615.3, x[!y, on="V1", mult="all"], data.table(V1=c(1,1,7,7), V2=INT(c(1,2,4,9))))
 
+# fix for #1287 and #1271
+set.seed(1L)
+dt = data.table(a=c(1,1,2), b=sample(10,3), c=sample(10,3))
+test(1616.1, dt[.(1:2), if (c-b > 0L) b, on="a", by=.EACHI, mult="first"], data.table(a=c(1,2), V1=c(3L,5L)))
+test(1616.2, dt[.(1:2), if (c-b > 0L) b, on="a", by=.EACHI, mult="last"], data.table(a=c(2), V1=5L))
+test(1613.1, dt[.(1:2), c := if (c-b > 0L) b, by=.EACHI, mult="first", on="a"], 
+             data.table(a=dt$a, b=dt$b, c=c(3L,2L,5L)) )
+
 ##########################
 
 # TODO: Tests involving GForce functions needs to be run with optimisation level 1 and 2, so that both functions are tested all the time.