fix error with unit fields

Author: PhilippImhof

renderer.php

@@ -365,16 +365,16 @@ protected function create_input_box(qtype_formulas_part $part, $answerindex,
         /** @var qype_formulas_question $question */
         $question = $qa->get_question();
 
-        $variablename = $part->partindex;
-        if ($part->has_combined_unit_field()) {
-            $variablename .= '_';
+        // The variable name will be N_M for answer #M in part #N or N_ for the (single) combined
+        // unit field of part N, or N_u for the (single) unit field of part N.
+        // FIXME: N_xxx for unit field with xxx = numbox
+        $variablename = $part->partindex . '_';
+        if ($answerindex === self::UNIT_FIELD) {
+            $variablename .= $part->numbox;
         } else {
-            $variablename .= ($answerindex == $part->numbox ? '_u' : "_{$answerindex}");
+            $variablename .= ($answerindex === self::COMBINED_FIELD ? '' : $answerindex);
         }
 
-        // The variable name will be N_M for answer #M in part #N or N_ for the (single) combined
-        // unit field of part N, or N_u for the (single) unit field of part N.
-        $variablename = "{$part->partindex}_{$answerindex}";
         $currentanswer = $qa->get_last_qt_var($variablename);
         $inputname = $qa->get_qt_field_name($variablename);
 
@@ -392,9 +392,12 @@ protected function create_input_box(qtype_formulas_part $part, $answerindex,
             default:
                 $titlestring = 'number';
         }
-        if ($part->postunit !== '') {
+        if ($answerindex === self::COMBINED_FIELD) {
             $titlestring .= '_unit';
         }
+        if ($answerindex === self::UNIT_FIELD) {
+            $titlestring = 'unit';
+        }
         $title = get_string($titlestring, 'qtype_formulas');
 
         $inputattributes = [