krahets/hello-algo · error · 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

Thrown by MyList.get (Japanese: 'インデックスが範囲外です' = 'index out of range') when index is outside [0, size). The list uses a fixed backing array with a logical size separate from capacity, so get must refuse reads beyond the logical length to avoid returning uninitialized slots.

Source

Thrown at ja/codes/javascript/chapter_array_and_linkedlist/my_list.js:32

    /* コンストラクタ */
    constructor() {
        this.#arr = new Array(this.#capacity);
    }

    /* リストの長さを取得（現在の要素数） */
    size() {
        return this.#size;
    }

    /* リスト容量を取得する */
    capacity() {
        return this.#capacity;
    }

    /* 要素にアクセス */
    get(index) {
        // 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 || index >= this.#size) throw new Error('インデックスが範囲外です');
        return this.#arr[index];
    }

    /* 要素を更新 */
    set(index, num) {
        if (index < 0 || index >= this.#size) throw new Error('インデックスが範囲外です');
        this.#arr[index] = num;
    }

    /* 末尾に要素を追加 */
    add(num) {
        // 長さが容量に等しい場合は拡張が必要
        if (this.#size === this.#capacity) {
            this.extendCapacity();
        }
        // 新しい要素をリストの末尾に追加する
        this.#arr[this.#size] = num;
        this.#size++;

View on GitHub (pinned to 69932aed18)

Solutions

1. Bounds-check 0 <= index < list.size() before get.
2. Iterate using size(), never capacity().
3. After inserts/removes, recompute the bound before indexing.

Example fix

// before
const v = list.get(i); // throws if i >= size

// after
const v = (i >= 0 && i < list.size()) ? list.get(i) : undefined;

const v = (i >= 0 && i < list.size()) ? list.get(i) : undefined;

function isValidReadIndex(list, i) {
  return Number.isInteger(i) && i >= 0 && i < list.size();
}

try { return list.get(i); }
catch (e) { if (!/範囲外/.test(e.message)) throw e; return undefined; }

When it happens

Trigger: Calling get with an index >= current size; passing a negative index; using capacity() as the upper bound instead of size().

Common situations: Confusing size with capacity; stale loop bounds after add/remove/insert; off-by-one in for loops.

Related errors

• Index Out Of Bounds Exception
• Index out of bounds
• Illegal Argument Exception
• Illegal Argument Exception
• Heap is empty.

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/c0f1d0996d6ae9be. Report an issue: GitHub.