Fix to issue #1762 varmatrix with EMPTYANSWER is broken.

doc/en/Authoring/Inputs/Matrix_input.md

@@ -24,4 +24,6 @@ The matrix of variable size input is a textarea into which students type in thei
 
 Students must separate their matrix elements by spaces, and newline characters.
 
-Input box size is used to determine the starting width of the input.
+Input box size is used to determine the starting width of the input.
+
+If you use the `allowempty` option then an empty answer is indicated by the `EMPTYANSWER` tag.  This is a different _type_ than a matrix.  (We could have chosen `matrix()` as the empty matrix, but `EMPTYANSWER` is more in keeping with other inputs.)

stack/input/varmatrix/varmatrix.class.php

@@ -185,6 +185,9 @@ protected function response_to_contents($response) {
         $contents = [];
         if (array_key_exists($this->name, $response)) {
             $sans = $response[$this->name];
+            if (trim($sans) === '' && $this->get_extra_option('allowempty')) {
+                return(['EMPTYANSWER']);
+            }
             $rowsin = explode("\n", $sans);
             foreach ($rowsin as $key => $row) {
                 $cleanrow = trim($row);
@@ -200,7 +203,6 @@ protected function response_to_contents($response) {
             $maxlen = max(count($entries), $maxlen);
             $contents[$key] = $entries;
         }
-
         foreach ($contents as $key => $row) {
             // Pad out short rows.
             $padrow = [];
@@ -209,9 +211,6 @@ protected function response_to_contents($response) {
             }
             $contents[$key] = array_merge($row, $padrow);
         }
-        if ($contents == [] && $this->get_extra_option('allowempty')) {
-            $contents = ['EMPTYANSWER'];
-        }
         return $contents;
     }
 
@@ -241,7 +240,7 @@ protected function caslines_to_answer($caslines, $secrules = false) {
      */
     public function contents_to_maxima($contents) {
         if ($contents == ['EMPTYANSWER']) {
-            return 'matrix(EMPTYCHAR)';
+            return 'EMPTYANSWER';
         }
         $matrix = [];
         foreach ($contents as $row) {

tests/input_varmatrix_test.php

@@ -346,7 +346,7 @@ public function test_validate_student_response_blank_allowempty(): void {
         );
         $this->assertEquals(stack_input::SCORE, $state->status);
         $this->assertEquals('', $state->note);
-        $this->assertEquals('matrix(EMPTYCHAR)', $state->contentsmodified);
+        $this->assertEquals('EMPTYANSWER', $state->contentsmodified);
         $this->assertEquals(
             '',
             $state->contentsdisplayed