java在递归函数中保留信息 [英] java retain information in recursive function

问题描述

是否可以通过java的辅助函数保留信息，而不使用静态变量。

Is it possible to retain information via a helper function with java, without using static variables.

例如，

public void foo(){
    int v = 0;
    fooHelper(2);
}

public void fooHelper(int depth){
    v++;
    fooHelper(depth-1)
}

即我想更新变量v不会丢失每个递归情况的信息，而不必访问函数外的变量。

Namely I want to update variable v without loosing the information for each recursive case, without having to access a variable outside the function.

推荐答案

忘记所有答案告诉你声明属性，或者在每次递归调用中更新可变对象。在一个真正的函数式递归样式中，你通过将它作为参数和/或返回类型传递来保留信息。

Forget about all the answers that tell you to declare attributes, or to update mutable objects in each recursive call. In a true functional, recursive style you "retain" information by passing it as parameters and/or return types.

让我用一个简单的例子来说明，让我们说你想要递归计算 int [] 中元素的总和。这里， state （需要在递归调用之间保留的信息）是数组中的当前索引和到目前为止的总和。以下是如何做到这一点：

Let me illustrate with a simple example, let's say that you want to recursively calculate the sum of the elements in an int[]. Here, the state (the information that needs to be retained between recursive calls) is the current index in the array and the sum so far. Here's how to do it:

public int sum(int[] array) {
    return sum(array, 0, 0); 
}

private int sum(int[] array, int idx, int acc) {
    if (idx == array.length)
        return acc;
    return sum(array, idx+1, acc+array[idx]);
}

这样称呼：

int[] array = {1, 2, 3};
System.out.println(sum(array));

如您所见，无需声明（静态或实例）属性，也无需传递和修改可变对象（列表，映射） - 我甚至不使用局部变量，因为解决问题所需的所有必需信息都作为方法参数出现。

As you can see, there's no need to declare (static or instance) attributes, and no need to pass and modify mutable objects (lists, maps) - I'm not even using local variables, because all the required information needed to solve the problem is present as method parameters.

在您的问题中的代码中， v 变量应该执行 acc 参数在我的答案中所做的事情，即：每次调用递归时修改累加值。最后，你只需要从辅助函数（它不能有 void 返回类型）返回累计值，这就是你将如何得到 foo（）。

In the code in your question the v variable is supposed to do what the acc parameter is doing in my answer, namely: modifying an accumulated value each time the recursion is called. In the end, you just need to return the accumulated value from the helper function (which must not have a void return type) and that's how you'll get the value in foo().

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