Align melt with docs for list measure.vars (#7266)

Author: Mukulyadav2004

NEWS.md

@@ -12,6 +12,8 @@
 
 1. `dcast()` now errors when `fun.aggregate` returns length != 1 (consistent with documentation), regardless of `fill`, [#6629](https://github.com/Rdatatable/data.table/issues/6629). Previously, when `fill` was not `NULL`, `dcast` warned and returned an undefined result. This change has been planned since 1.16.0 (25 Aug 2024).
 
+2. `melt()` returns an integer column for `variable` when `measure.vars` is a list of length=1, consistent with the documented behavior, [#5209](https://github.com/Rdatatable/data.table/issues/5209). Thanks to @tdhock for reporting. Any users who were relying on this behavior can change `measure.vars=list("col_name")` (output `variable` was column name, now is column index/integer) to `measure.vars="col_name"` (`variable` still is column name). This change has been planned since 1.16.0 (25 Aug 2024).
+
 ### NEW FEATURES
 
 1. New `sort_by()` method for data.tables, [#6662](https://github.com/Rdatatable/data.table/issues/6662). It uses `forder()` to improve upon the data.frame method and also matches `DT[order(...)]` behavior with respect to locale. Thanks @rikivillalba for the suggestion and PR.

inst/tests/tests.Rraw

@@ -17421,13 +17421,13 @@ exid = data.table(id=1, expected)
 test(2182.3, melt(DTid, measure.vars=list(a=c(NA,1), b=2:3), id.vars="id"), exid)
 test(2182.4, melt(DTid, measure.vars=list(a=c(NA,"a2"), b=c("b1","b2")), id.vars="id"), exid)
 test(2182.5, melt(DT.wide, measure.vars=list(a=c(NA,1), b=2:3), na.rm=TRUE), data.table(variable=factor(2), a=2, b=2))
-test(2182.6, melt(DT.wide, measure.vars=list(b=c("b1","b2"))), data.table(a2=2, variable=factor(c("b1","b2")), b=c(1,2)), warning="measure.vars is a list with length=1") # measure.vars named list length=1, #5065
+test(2182.6, melt(DT.wide, measure.vars=list(b=c("b1","b2"))), data.table(a2=2, variable=factor(c("1","2")), b=c(1,2))) # measure.vars named list length=1, #5065
 # consistency between measure.vars=list with length=1 and length>1, #5209
-test(2182.71, melt(DT.wide, measure.vars=list("a2"), variable.factor=TRUE), data.table(b1=1, b2=2, variable=factor("a2"), value=2), warning="measure.vars is a list with length=1")
+test(2182.71, melt(DT.wide, measure.vars=list("a2"), variable.factor=TRUE), data.table(b1=1, b2=2, variable=factor(1), value=2))
 test(2182.72, melt(DT.wide, measure.vars=c("a2"), variable.factor=TRUE), data.table(b1=1, b2=2, variable=factor("a2"), value=2))
-test(2182.73, melt(DT.wide, measure.vars=list("a2"), variable.factor=FALSE), data.table(b1=1, b2=2, variable="a2", value=2), warning="measure.vars is a list with length=1")
+test(2182.73, melt(DT.wide, measure.vars=list("a2"), variable.factor=FALSE), data.table(b1=1, b2=2, variable="1", value=2))
 test(2182.74, melt(DT.wide, measure.vars=c("a2"), variable.factor=FALSE), data.table(b1=1, b2=2, variable="a2", value=2))
-test(2182.75, melt(data.table(a=10, b=20), measure.vars=list(n="a"), variable.factor=FALSE), data.table(b=20, variable="a", n=10), warning="measure.vars is a list with length=1")#thanks @mnazarov
+test(2182.75, melt(data.table(a=10, b=20), measure.vars=list(n="a"), variable.factor=FALSE), data.table(b=20, variable="1", n=10))#thanks @mnazarov
 
 ### First block testing measurev
 # new variable_table attribute for measure.vars, PR#4731 for multiple issues

src/fmelt.c

@@ -598,12 +598,9 @@ SEXP getvarcols(SEXP DT, SEXP dtnames, Rboolean varfactor, Rboolean verbose, str
   if (data->lvalues==1 && length(VECTOR_ELT(data->valuecols, 0)) != data->lmax)
     internal_error(__func__, "getvarcols %d %d", length(VECTOR_ELT(data->valuecols, 0)), data->lmax);  // # nocov
   if (isNull(data->variable_table)) {
-    if ((data->lvalues == 1) & data->measure_is_list) {
-      warning(_("measure.vars is a list with length=1, which as long documented should return integer indices in the 'variable' column, but currently returns character column names. To increase consistency in the next release, we plan to change 'variable' to integer, so users who were relying on this behavior should change measure.vars=list('col_name') (output variable is column name now, but will become column index/integer) to measure.vars='col_name' (variable is column name before and after the planned change)."));
-    }
     if (!varfactor) {
       SET_VECTOR_ELT(ansvars, 0, target=allocVector(STRSXP, data->totlen));
-      if (data->lvalues == 1) {//one value column to output. TODO #5247 change to !data->measure_is_list
+      if (!data->measure_is_list) {//one value column to output.
         const int *thisvaluecols = INTEGER(VECTOR_ELT(data->valuecols, 0));
         for (int j=0, ansloc=0; j<data->lmax; ++j) {
           const int thislen = data->narm ? length(VECTOR_ELT(data->not_NA_indices, j)) : data->nrow;
@@ -622,7 +619,7 @@ SEXP getvarcols(SEXP DT, SEXP dtnames, Rboolean varfactor, Rboolean verbose, str
       SET_VECTOR_ELT(ansvars, 0, target=allocVector(INTSXP, data->totlen));
       SEXP levels;
       int *td = INTEGER(target);
-      if (data->lvalues == 1) {//one value column to output. TODO #5247 change to !data->measure_is_list
+      if (!data->measure_is_list) {//one value column to output.
         SEXP thisvaluecols = VECTOR_ELT(data->valuecols, 0);
         int len = length(thisvaluecols);
         levels = PROTECT(allocVector(STRSXP, len)); protecti++;