.Last.updated value after :=, closes #1885

NAMESPACE

@@ -50,6 +50,7 @@ S3method(rollup, data.table)
 export(frollmean)
 export(nafill)
 export(setnafill)
+export(.Last.updated)
 
 S3method("[", data.table)
 S3method("[<-", data.table)

NEWS.md

@@ -46,6 +46,8 @@
 
 6. New functions `nafill` and `setnafill`, [#854](https://github.com/Rdatatable/data.table/issues/854). Thanks to Matthieu Gomez for the request and Jan Gorecki for implementing.
 
+7. New variable `.Last.updated` (similar to R's `.Last.value`) contains the number of rows affected by the most recent `:=` or `set()`, [#1885](https://github.com/Rdatatable/data.table/issues/1885).
+
 #### BUG FIXES
 
 1. `first`, `last`, `head` and `tail` by group no longer error in some cases, [#2030](https://github.com/Rdatatable/data.table/issues/2030) [#3462](https://github.com/Rdatatable/data.table/issues/3462). Thanks to @franknarf1 for reporting.

R/data.table.R

@@ -1150,6 +1150,7 @@ replace_dot_alias <- function(e) {
               cat("No rows match i. No new columns to add so not evaluating RHS of :=\n")
               cat("Assigning to 0 row subset of",nrow(x),"rows\n")
             }
+            .Call(Cassign, x, irows, NULL, NULL, NULL, FALSE) # only purpose is to write 0 to .Last.updated
             .global$print = address(x)
             return(invisible(x))
           }

R/onLoad.R

@@ -1,5 +1,7 @@
 # nocov start
 
+.Last.updated <- vector("integer", 1L) # exported variable; number of rows updated by the last := or set(), #1885
+
 .onLoad <- function(libname, pkgname) {
   # Runs when loaded but not attached to search() path; e.g., when a package just Imports (not Depends on) data.table
   if (!exists("test.data.table", .GlobalEnv, inherits=FALSE) &&    # check when installed package is loaded but skip when developing the package with cc()
@@ -102,6 +104,8 @@
   # R could feasibly in future not copy DF's vecsxp in this case. If that changes in R, we'd like to know via the warning
   # because tests will likely break too. The warning will quickly tell R-core and us why, so we can then update.
 
+  .Call(CinitLastUpdated, .Last.updated)  #1885
+
   invisible()
 }
 

inst/tests/tests.Rraw

@@ -14084,6 +14084,51 @@ test(2029.1, fread(txt), data.table(A=1:2, B=4:5, C=7:8), warning="Discarded sin
 test(2029.2, fread(txt, quote=""), data.table(A=1:2, B=4:5, C=7:8), warning="Discarded single-line footer: <<3,6>>")
 test(2029.3, fread(txt, quote="", fill=TRUE), data.table(A=1:3, B=4:6, C=c(7:8,NA)))
 
+# .Last.updated #1885
+d = data.table(a=1:4, b=2:5)
+d[, z:=5L]
+test(2030.01, .Last.updated, 4L) # new column
+d[, z:=6L]
+test(2030.02, .Last.updated, 4L) # update existing column
+d[2:3, z:=7L]
+test(2030.03, .Last.updated, 2L) # sub assign
+d[integer(), z:=8L]
+test(2030.04, .Last.updated, 0L) # empty sub-assign
+d[-1L, z:=9L]
+test(2030.05, .Last.updated, 3L) # inverse sub-assign
+d[-(1:4), z:=10L]
+test(2030.06, .Last.updated, 0L) # inverse empty sub-assign
+d[, z:=NULL]
+test(2030.07, .Last.updated, 4L) # delete column
+d[2:3, z:=11L]
+test(2030.08, .Last.updated, 2L) # new column during sub-assign
+d[, z:=NULL]
+d[integer(), z:=12L]
+test(2030.09, .Last.updated, 0L) # new columns from empty sub-assign
+d[, z:=NULL]
+d[-(1:4), z:=13L]
+test(2030.10, .Last.updated, 0L) # new columns from empty inverse sub-assign
+d[, z:=NULL][, z:=14L]
+test(2030.11, .Last.updated, 4L) # new column from chaining
+d[, z:=NULL][2:3, z:=14L]
+test(2030.12, .Last.updated, 2L) # sub-assign from chaining
+d[2:3, z:=14L][, z:=NULL]
+test(2030.13, .Last.updated, 4L) # delete column from chaining
+set(d, 1:2, "z", 15L)
+test(2030.14, .Last.updated, 2L) # set() updates .Last.updated too
+g = data.table(a=1:4, z=15L) # join
+d[g, on="a", z:=i.z]
+test(2030.15, .Last.updated, 4L) # all match of all rows
+g = data.table(a=2:4, z=16L) # join
+d[, z:=NULL][g, on="a", z:=i.z]
+test(2030.16, .Last.updated, 3L) # all match
+g = data.table(a=c(2L,4L,6L), z=17L)
+d[, z:=NULL][g, on="a", z:=i.z]
+test(2030.17, .Last.updated, 2L) # partial match
+g = data.table(a=5:6, z=18L)
+d[, z:=NULL][g, on="a", z:=i.z]
+test(2030.18, .Last.updated, 0L) # zero match
+
 
 ###################################
 #  Add new tests above this line  #

src/assign.c

@@ -270,6 +270,8 @@ SEXP selfrefokwrapper(SEXP x, SEXP verbose) {
   return ScalarInteger(_selfrefok(x,FALSE,LOGICAL(verbose)[0]));
 }
 
+int *_Last_updated = NULL;
+
 SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values, SEXP verb)
 {
   // For internal use only by := in [.data.table, and set()
@@ -342,13 +344,17 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values, SEXP v
     if (verbose) Rprintf("Assigning to %d row subset of %d rows\n", numToDo, nrow);
     // TODO: include in message if any rows are assigned several times (e.g. by=.EACHI with dups in i)
     if (numToDo==0) {
-      if (!length(newcolnames)) return(dt); // all items of rows either 0 or NA. !length(newcolnames) for #759
+      if (!length(newcolnames)) {
+        *_Last_updated = 0;
+        return(dt); // all items of rows either 0 or NA. !length(newcolnames) for #759
+      }
       if (verbose) Rprintf("Added %d new column%s initialized with all-NA\n",
                            length(newcolnames), (length(newcolnames)>1)?"s":"");
     }
   }
   if (!length(cols)) {
     warning("length(LHS)==0; no columns to delete or assign RHS to.");   // test 1295 covers
+    *_Last_updated = 0;
     return(dt);
   }
   // FR #2077 - set able to add new cols by reference
@@ -624,6 +630,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values, SEXP v
     }
     memrecycle(targetcol, rows, 0, targetlen, RHS);  // also called from dogroups where these arguments are used more
   }
+  *_Last_updated = numToDo;  // the updates have taken place with no error, so update .Last.updated now
   PROTECT(assignedNames = allocVector(STRSXP, LENGTH(cols)));
   protecti++;
   for (i=0;i<LENGTH(cols);i++) SET_STRING_ELT(assignedNames,i,STRING_ELT(names,INTEGER(cols)[i]-1));

src/init.c

@@ -82,6 +82,7 @@ SEXP frollfunR();
 SEXP dllVersion();
 SEXP nafillR();
 SEXP colnamesInt();
+SEXP initLastUpdated();
 
 // .Externals
 SEXP fastmean();
@@ -166,6 +167,7 @@ R_CallMethodDef callMethods[] = {
 {"CdllVersion", (DL_FUNC) &dllVersion, -1},
 {"CnafillR", (DL_FUNC) &nafillR, -1},
 {"CcolnamesInt", (DL_FUNC) &colnamesInt, -1},
+{"CinitLastUpdated", (DL_FUNC) &initLastUpdated, -1},
 {NULL, NULL, 0}
 };
 
@@ -347,6 +349,14 @@ SEXP hasOpenMP() {
 }
 // # nocov end
 
+extern int *_Last_updated;  // assign.c
+
+SEXP initLastUpdated(SEXP var) {
+  if (!isInteger(var) || LENGTH(var)!=1) error(".Last.value in namespace is not a length 1 integer");
+  _Last_updated = INTEGER(var);
+  return R_NilValue;
+}
+
 SEXP dllVersion() {
   // .onLoad calls this and checks the same as packageVersion() to ensure no R/C version mismatch, #3056
   return(ScalarString(mkChar("1.12.3")));