krahets/hello-algo · error

インデックスが範囲外です

Error message

インデックスが範囲外です

What it means

This panic is raised by get() in a hand-written dynamic array / MyList (Japanese localization). It fires when the supplied index is outside [0, items.len). Note: because index is usize (unsigned), the `index < 0` half of the guard is dead code — only the `index >= self.items.len` check can ever trigger. @panic aborts the process and is not catchable.

Source

Thrown at ja/codes/zig/chapter_array_and_linkedlist/my_list.zig:58

        const new_item_ptr = &self.items[self.items.len - 1];
        new_item_ptr.* = item;
    }

    // リストの長さを取得（現在の要素数）
    pub fn getSize(self: *Self) usize {
        return self.items.len;
    }

    // リスト容量を取得する
    pub fn getCapacity(self: *Self) usize {
        return self.capacity;
    }

    // 要素にアクセス
    pub fn get(self: *Self, index: usize) i32 {
        // インデックスが範囲外なら例外を送出する。以下同様
        if (index < 0 or index >= self.items.len) {
            @panic("インデックスが範囲外です");
        }
        return self.items[index];
    }

    // 要素を更新
    pub fn set(self: *Self, index: usize, num: i32) void {
        // インデックスが範囲外なら例外を送出する。以下同様
        if (index < 0 or index >= self.items.len) {
            @panic("インデックスが範囲外です");
        }
        self.items[index] = num;
    }

    // 中間に要素を挿入
    pub fn insert(self: *Self, index: usize, item: i32) !void {
        if (index < 0 or index >= self.items.len) {
            @panic("インデックスが範囲外です");
        }

View on GitHub (pinned to 69932aed18)

Solutions

1. Bounds-check before access: if (idx >= list.getSize()) return early or handle.
2. Drive loops with list.getSize() captured once, iterating idx < size.
3. Re-fetch the size after any insert/remove mutation before re-indexing.
4. Remove the dead `index < 0` branch — it misleads readers about usize semantics.

Example fix

// before
var v = list.get(idx); // @panics when idx >= len

// after
if (idx >= list.getSize()) {
    return; // handle out-of-range
}
var v = list.get(idx);

// Bounds-check before indexed read
if (idx < list.getSize()) {
    var v = list.get(idx);
}

When it happens

Trigger: Calling list.get(idx) with idx >= current element count, or with idx on a freshly-cleared list.

Common situations: Off-by-one loops (using <= len instead of < len); stale cached length after removes; reading from a list that was shrunk; negative-index expectations ported from Python.

Related errors

• индекс выходит за границы
• スタックが空です
• キューが空です
• 両端キューが空です
• キューが空です

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/3d6418154bbee3c2. Report an issue: GitHub.