Fix #6512: Informative error message for missing measure.vars

Author: ayushsingh9720

R/fmelt.R

@@ -195,6 +195,15 @@ melt.data.table = function(data, id.vars, measure.vars, variable.name = "variabl
       }
     }
   }
+
+# GSoC Fix for #6512
+  if (is.character(measure.vars)) {
+    if (length(invalid_vars <- setdiff(measure.vars, names(data)))) {
+      stopf("One or more values in 'measure.vars' is invalid; please fix by removing [%s]", 
+            paste(invalid_vars, collapse = ", "))
+    }
+  }
+
   if (is.list(measure.vars)) {
     meas.nm = names(measure.vars)
     if (is.null(meas.nm)) {

inst/tests/tests.Rraw

@@ -21959,3 +21959,8 @@ test(2355.1, fread(txt, skip=0),              data.table(V1 = c("b1", "c1"), a1
 test(2355.2, fread(txt, skip=0, header=TRUE), data.table(V1 = c("b1", "c1"), a1 = c("b2", "c2"), a2 = c("b3", "c3")), warning="Added an extra default column name")
 test(2355.3, fread(txt, skip=0, header=FALSE), data.table(V1=character(), V2=character(), V3=character()), warning="Consider fill=TRUE")
 test(2355.4, fread(txt, skip=0, fill=TRUE), data.table(V1 = c("a1", "b1", "c1"), V2 = c("a2", "b2", "c2"), V3 = c("", "b3", "c3")))
+
+# Test for issue #6512: Informative error message for missing measure.vars
+DT = data.table(x1=1, x2=2)
+test(2154.1, melt(DT, measure.vars=c("x1", "z1")), 
+     error="One or more values in 'measure.vars' is invalid; please fix by removing [z1]")

src/fmelt.c

@@ -179,7 +179,7 @@ bool is_default_measure(SEXP vec) {
 // maybe unlist, then unique, then set_diff.
 SEXP uniq_diff(SEXP int_or_list, int ncol, bool is_measure) {
   SEXP int_vec = PROTECT(isNewList(int_or_list) ? unlist_(int_or_list) : int_or_list);
-  SEXP is_duplicated = PROTECT(duplicated(int_vec, FALSE)); 
+  SEXP is_duplicated = PROTECT(duplicated(int_vec, FALSE));
   int n_unique_cols = 0;
   for (int i=0; i<length(int_vec); ++i) {
     int col_number = INTEGER(int_vec)[i];
@@ -193,7 +193,7 @@ SEXP uniq_diff(SEXP int_or_list, int ncol, bool is_measure) {
       }
     } else if (!LOGICAL(is_duplicated)[i]) n_unique_cols++;
   }
-  SEXP unique_col_numbers = PROTECT(allocVector(INTSXP, n_unique_cols)); 
+  SEXP unique_col_numbers = PROTECT(allocVector(INTSXP, n_unique_cols));
   int unique_i = 0;
   for (int i=0; i<length(is_duplicated); ++i) {
     if (!LOGICAL(is_duplicated)[i]) {