krahets/hello-algo · error

堆疊為空

Error message

堆疊為空

What it means

Unrecoverable Zig @panic from peek() on the linked-list stack. Guard `if (self.size() == 0) @panic("堆疊為空")` runs before `self.stack_top.?.val`, preventing a null optional unwrap. Peeking an empty stack is treated as a caller logic error.

Source

Thrown at zh-hant/codes/zig/chapter_stack_and_queue/linkedlist_stack.zig:46

        // 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();
        }

        // 獲取堆疊的長度
        pub fn size(self: *Self) usize {
            return self.stk_size;
        }

        // 判斷堆疊是否為空
        pub fn isEmpty(self: *Self) bool {
            return self.size() == 0;
        }

        // 訪問堆疊頂元素
        pub fn peek(self: *Self) T {
            if (self.size() == 0) @panic("堆疊為空");
            return self.stack_top.?.val;
        }  

        // 入堆疊
        pub fn push(self: *Self, num: T) !void {
            var node = try self.mem_allocator.create(inc.ListNode(T));
            node.init(num);
            node.next = self.stack_top;
            self.stack_top = node;
            self.stk_size += 1;
        } 

        // 出堆疊
        pub fn pop(self: *Self) T {
            var num = self.peek();
            self.stack_top = self.stack_top.?.next;
            self.stk_size -= 1;
            return num;

View on GitHub (pinned to 69932aed18)

Solutions

1. Guard with `if (!stk.isEmpty())` before every peek().
2. Restructure so peek() runs only inside a branch that already established size() > 0.
3. Add a ?T-returning wrapper if empty-top inspection is a legitimate case.

Example fix

// before
const top = stk.peek();
// after
const top = if (stk.isEmpty()) null else stk.peek();

// call BEFORE peek()
if (stk.isEmpty()) return null;
const top = stk.peek();

When it happens

Trigger: Call peek() on a LinkedListStack never pushed, or popped down to a null top. Also triggered by nested evaluators/parsers that inspect the top before any push or after the final pop.

Common situations: Expression evaluators peeking for a pending operator on an empty operand stack; bracket matchers peeking before pushing; DFS-emulation peeking an empty work stack; tests asserting on the top of a freshly built stack.

Related errors

• стек пуст
• 堆疊為空
• 栈为空
• очередь пуста
• 佇列為空

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/c907050133407829. Report an issue: GitHub.