What causes "佇列為空" in krahets/hello-algo?

Gate with `if (!q.isEmpty())` before peek().

佇列為空

Error message

佇列為空

What it means

Unrecoverable Zig @panic from peek() on the singly-linked-list queue. Guard `if (self.size() == 0) @panic("佇列為空")` runs before `self.front.?.val`, preventing a null optional unwrap. Peeking an empty queue is defined as a logic error.

Source

Thrown at zh-hant/codes/zig/chapter_stack_and_queue/linkedlist_queue.zig:48

        // 析構函式（釋放記憶體）
        pub fn deinit(self: *Self) void {
            if (self.mem_arena == null) return;
            self.mem_arena.?.deinit();
        }

        // 獲取佇列的長度
        pub fn size(self: *Self) usize {
            return self.que_size;
        }

        // 判斷佇列是否為空
        pub fn isEmpty(self: *Self) bool {
            return self.size() == 0;
        }

        // 訪問佇列首元素
        pub fn peek(self: *Self) T {
            if (self.size() == 0) @panic("佇列為空");
            return self.front.?.val;
        }  

        // 入列
        pub fn push(self: *Self, num: T) !void {
            // 在尾節點後新增 num
            var node = try self.mem_allocator.create(inc.ListNode(T));
            node.init(num);
            // 如果佇列為空，則令頭、尾節點都指向該節點
            if (self.front == null) {
                self.front = node;
                self.rear = node;
            // 如果佇列不為空，則將該節點新增到尾節點後
            } else {
                self.rear.?.next = node;
                self.rear = node;
            }
            self.que_size += 1;

View on GitHub (pinned to 69932aed18)

Solutions

Gate with `if (!q.isEmpty())` before peek().
Loop `while (!q.isEmpty())` for consumption so no peek runs at size 0.
Wrap with a ?T-returning helper for callers that legitimately face an empty queue.

Example fix

// before
const head = q.peek();
// after
const head = if (q.isEmpty()) null else q.peek();

Defensive patterns

Strategy: validation

Validate before calling

// call BEFORE peek()
if (q.isEmpty()) return null;
const head = q.peek();

Prevention

Check isEmpty()/size() before peek().
Consume with `while (!q.isEmpty())` so peek never runs at size 0.
Build a ?T-returning wrapper if empty-peek is legitimate.

When it happens

Trigger: Call peek() on a LinkedListQueue with zero elements (never enqueued, or fully dequeued). Reached by helper code that peeks before the first push or after the final pop.

Common situations: BFS/level-order traversal peeking after draining the last node; polling a queue before the producer enqueues; UI/log code displaying the queue head without a size check.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/0d25ecb74824c846. Report an issue: GitHub.