在迭代期間導航修改陣列的陷阱

修改數組時遍歷時會導致問題，因為元素的刪除或插入會改變索引結構，而循環變量或迭代器未同步更新，導致跳過元素或異常；例如JavaScript中從前向後遍歷並刪除元素時，後續元素前移但索引遞增會跳過下一個元素；Python中直接修改列表可能引發RuntimeError或行為異常；避免該問題的方法包括：1. 反向遍歷，刪除元素不影響未處理的低索引項；2. 先收集待修改的索引或元素，迭代結束後再統一處理，且刪除時需逆序操作；3. 使用filter、map等函數式方法生成新數組，避免原數組變異；還需注意forEach、for...in等隱式迭代中修改數組同樣危險，Java中未用iterator.remove()則拋出ConcurrentModificationException；因此應優先採用反向遍歷、延遲修改或函數式編程來確保代碼安全可靠。

Modifying an array while iterating over it is a common source of bugs and unexpected behavior in programming. Whether you're working with JavaScript, Python, Java, or another language, altering the structure of an array—such as adding, removing, or splicing elements—during a loop can lead to skipped elements, infinite loops, or crashes. Understanding why this happens and how to avoid it is crucial for writing reliable code.

Why Modifying Arrays During Iteration Causes Problems

When you iterate through an array using a standard loop (like a for loop or forEach ), the underlying mechanism often relies on indices or internal pointers. If you modify the array during iteration—especially by removing or inserting elements—the indices shift, but the loop counter or iterator may not account for this change.

For example, consider this JavaScript snippet:

 let arr = ['a', 'b', 'c', 'd'];
for (let i = 0; i < arr.length; i ) {
  if (arr[i] === &#39;b&#39;) {
    arr.splice(i, 1); // Remove element at index i
  }
}

After removing 'b' at index 1, 'c' moves to index 1, and 'd' moves to index 2. But the loop increments i to 2 on the next iteration, so 'c' (now at index 1) gets skipped entirely.

In languages like Python, using a for loop on a list while modifying it raises a RuntimeError if using iterators, or leads to unpredictable behavior if modifying in place.

Safe Strategies to Avoid These Pitfalls

Instead of modifying an array during iteration, use one of these safer approaches:

1. Iterate Backwards (When Removing Elements)

If you must modify the array directly, iterate from the end to the beginning. This way, removing an element doesn't affect the indices of items you haven't processed yet.

 for (let i = arr.length - 1; i >= 0; i--) {
  if (arr[i] === 'b') {
    arr.splice(i, 1);
  }
}

This works because removing an element at a higher index doesn't shift the positions of elements with lower indices.

2. Collect Indices or Items to Modify, Then Apply Changes After

Build a list of items or indices to remove or update during iteration, then perform the modifications afterward.

 const indicesToRemove = [];
arr.forEach((item, index) => {
  if (item === 'b') {
    indicesToRemove.push(index);
  }
});

// Remove in reverse order to avoid index shifting
for (let i = indicesToRemove.length - 1; i >= 0; i--) {
  arr.splice(indicesToRemove[i], 1);
}

3. Use Functional Methods: Filter, Map, and Reduce

Instead of mutating the original array, create a new one using non-destructive methods.

To remove elements:

 arr = arr.filter(item => item !== 'b');

To transform elements:

 arr = arr.map(item => item === 'b' ? 'replaced' : item);

This approach is cleaner, easier to reason about, and avoids mutation-related bugs entirely.

Watch Out for Hidden Iteration

Some methods may look safe but still involve iteration under the hood. For example:

• Using forEach , map , or filter while modifying the original array inside the callback is still dangerous.
• In Python, modifying a list while using a for item in list: loop can lead to skipped items.
• In Java, modifying an ArrayList while using an Iterator without using iterator.remove() throws a ConcurrentModificationException .

Always check the documentation for the language and method you're using.

Summary

Modifying arrays during iteration is risky because it disrupts the expected flow of indices or iterator state. To avoid issues:

• Iterate backwards when removing elements by index.
• Defer modifications until after iteration.
• Prefer functional methods like filter() and map() that return new arrays.
• Be cautious with built-in iteration methods that may not support mutation.

By following these practices, you'll write safer, more predictable code—especially when dealing with dynamic data collections.

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