Can I safely mutate an array I am iterating over, if it returns to its original state after each iteration?

java, loops

Solution

In a word, yes.

You don't specify the type of `board.legalMoves`. You say that it's array, but it can't be, since you're calling `isEmpty()` on it. I therefore suspect that you mean `ArrayList`. If that's the case, the documentation is pretty clear:

The iterators returned by this class's `iterator` and `listIterator` methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own `remove` or `add` methods, the iterator will throw a `ConcurrentModificationException`. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

I see two ways around this:

1) Avoid structural modifications. In other words, it's OK to change the values of the elements, but it's not OK to add/remove elements.

2) Iterate over the `ArrayList` using indices:

for (int i = 0; i < board.legalMoves.size(); i++) {
    Tuple move = board.get(i);
    ...
}

Problem

I am writing a minimax algorithm for a game in Java, and, for speed purposes, mutating the game state as I recursively work through the decision tree. However, this involves modifying the list of moves I am iterating over. ``` public int minimax(int currentDepth) { if (currentDepth == depth || board.legalMoves.isEmpty()) { int eval = board.eval(); board.takeBack(1); return eval; } int x = Integer.MIN_VALUE; for (Tuple move : board.legalMoves) { board.move(move); x = max(x, -1*minimax(currentDepth+1)); board.takeBack(1); } return x } ``` The `board.move()` method mutates the `ArrayList legalMoves`, but `takeBack(1)` brings it back to its original state. Could this cause any problems?

Original source