Make sure OBJECT bit appropriately set when melting (#50)

NEWS.md

@@ -1,5 +1,7 @@
 # Version 1.4.0.99
 
+* `melt.data.frame()` now properly sets the OBJECT bit on `value` variable generated if attributes are copied (for example, when multiple POSIXct columns are concatenated to generate the `value` variable) (#50)
+
 * `melt.data.frame()` no longer errors when `measure.vars` is `NULL` or empty.
   (#46)
 

src/melt.cpp

@@ -267,12 +267,11 @@ List melt_dataframe(const DataFrame& data,
   output[n_id + 1] = concatenate(data, measure_ind, factorsAsStrings);
   if (!Rf_isNull(measure_attributes)) {
     SET_ATTRIB(output[n_id + 1], measure_attributes);
-  }
-
-  // Set the object bit explicitly to make sure that the 'value' is properly
-  // interpreted as a factor
-  if (valueAsFactor) {
-    SET_OBJECT(output[n_id + 1], 1);
+    // we also need to make sure the OBJECT bit is set for other 'object' types
+    // see: http://stackoverflow.com/questions/24059460/melt-data-frame-changes-behavior-how-posixct-columns-are-printed
+    // if we've entered this code block, the measure_attributes has been
+    // populated because all value variables have identical attributes
+    SET_OBJECT(output[n_id + 1], OBJECT(data[measure_ind[0]]));
   }
 
   // Make the List more data.frame like

tests/testthat/test-melt.r

@@ -183,3 +183,18 @@ test_that("melt.data.frame behaves when there are no measure variables", {
   expect_identical(df["x"], m)
 
 })
+
+test_that("melt.data.frame preserves OBJECT bit on e.g. POSIXct", {
+  t.wide <- data.frame(product=letters[1:5],
+                       result=c(2, 4, 0, 0, 1),
+                       t1=as.POSIXct("2014-05-26") + seq(0, 10800, length.out=5),
+                       t2=as.POSIXct("2014-05-27") + seq(0, 10800, length.out=5),
+                       t3=as.POSIXct("2014-05-28") + seq(0, 10800, length.out=5))
+
+  library(reshape2)
+  object_bit_set <- function(x) {
+    grepl("\\[OBJ", capture.output(.Internal(inspect(x)))[1])
+  }
+  t.long <- melt(t.wide, measure.vars=c("t1", "t2", "t3"), value.name="time")
+  expect_true(object_bit_set(t.long$time))
+})