Closes #1396. Auto index from v1.9.4 doesn't error on v1.9.6.

README.md

@@ -65,6 +65,8 @@
 
   3. Fixed tests for `fread` 1378.2 and 1378.3 with `showProgress = FALSE`, closes [#1397](https://github.com/Rdatatable/data.table/issues/1397). Thanks to @JanGorecki for the PR.
 
+  4. Worked around auto index error in `v1.9.6` to account for indices created with `v1.9.4`, [#1396](https://github.com/Rdatatable/data.table/issues/1396). Thanks @GRandom.
+
 ### Changes in v1.9.6  (on CRAN 19 Sep 2015)
 
 #### NEW FEATURES

inst/tests/tests.Rraw

@@ -7068,6 +7068,13 @@ Y = data.table(A=2:4, B=5:7)
 test(1575.1, X[Y, on=c(A="a")], error="not found in x")
 test(1575.2, X[Y, on=c(a="a")], error="not found in i")
 
+# work around for issue introduced in v1.9.4, #1396
+X = data.table(x=5:1, y=6:10)
+setattr(X, 'index', integer(0))
+setattr(attr(X, 'index'), 'x', 5:1) # auto indexed attribute as created from v1.9.4
+ans = capture.output(X[, z := 1:5, verbose=TRUE])
+test(1576, ans[4], "Dropping index 'x' as it doesn't have '__' at the beginning of index name. It is very likely created using v1.9.4 of data.table.")
+
 ##########################
 
 

src/assign.c

@@ -619,7 +619,15 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values, SEXP v
             for (i=0; i<LENGTH(cols); i++) {
                 tc2 = c2 = CHAR(STRING_ELT(names, INTEGER(cols)[i]-1));  // the column name being updated; e.g. "col1"
                 while (*tc1) {
-                    if (*tc1!='_' || *(tc1+1)!='_') error("Internal error: __ not found in index name");
+                    if (*tc1!='_' || *(tc1+1)!='_') {
+                    	// fix for #1396
+                    	if (verbose) {
+                    		Rprintf("Dropping index '%s' as it doesn't have '__' at the beginning of index name. It is very likely created using v1.9.4 of data.table.\n", c1);
+                    	}
+                    	setAttrib(index, a, R_NilValue);
+                    	i = LENGTH(cols);
+                    	break;
+                    }
                     tc1 += 2;
                     if (*tc1=='\0') error("Internal error: index name ends with trailing __");
                     while (*tc1 && *tc2 && *tc1 == *tc2) { tc1++; tc2++; }