diff --git a/src/main.rs b/src/main.rs
index a150264..36d3599 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -95,6 +95,16 @@ impl TodoList {
         self.next_id += 1;
         Ok(())
     }
+
+    fn complete_task(&mut self, id: usize) -> Result<(), TodoError> {
+        match self.tasks.iter_mut().find(|task| task.id == id) {
+            Some(task) => {
+                task.completed = true;
+                Ok(())
+            }
+            None => Err(TodoError::TaskNotFound(id)),
+        }
+    }
 }
 
 fn main() {