- ...

class/LogicFunction.php

@@ -203,6 +203,9 @@ class LogicFunction implements JsonSerializable
     public static function isCorrectDNF(array $input_vars, string $exp): bool
     {
         $exp = trim($exp); // trim spaces
+        if ($exp === "0") {
+            return true;
+        }
         $minterms = explode("|", $exp); // break up the expression into minterms
         $minterms = array_map(fn($mt) => trim($mt, " ()\t"), $minterms); // strip the parentheses off the minterms
         $minterms = array_map(fn($mt) => str_replace(" ", "", $mt), $minterms); // remove spaces
@@ -250,7 +253,12 @@ class LogicFunction implements JsonSerializable
                 $minterms[] = $term;
             }
         }
-        return join(" | ", $minterms);
+
+        $dnf = join(" | ", $minterms);
+        if ($dnf === "") {
+            $dnf = "0";
+        }
+        return $dnf;
     }
 
     public function drawNetwork(string $outvar = "f"): string

class/Tasks/LogicFunctionTask.php

@@ -15,9 +15,6 @@ class LogicFunctionTask extends LogicTaskBase
             $this->setCorrectAnswer($this->getLogicFunction()->getExpression());
         } else {
             $dnf = $this->getLogicFunction()->toDNF();
-            if ($dnf === "") {
-                $dnf = "0";
-            }
             $this->setCorrectAnswer($dnf);
         }
     }