JavaScript中递归和执行流程的更清晰的解释？ [英] A clearer explanation for recursion and flow of execution in JavaScript?

问题描述

我正在阅读Eloquent JavaScript，我遇到了这个难题的例子：

I was reading Eloquent JavaScript and I came across this example for the puzzle:

考虑这个难题：从$ b $开始b数字1并重复
加5或乘以3，
无限数量的新数字可以产生
。你会如何编写一个
函数，给定一个数字，尝试
来查找一系列加法和
乘法产生
数？

Consider this puzzle: By starting from the number 1 and repeatedly either adding 5 or multiplying by 3, an infinite amount of new numbers can be produced. How would you write a function that, given a number, tries to find a sequence of additions and multiplications that produce that number?

以下是解决方案的代码：

Here's the code for the solution:

function findSequence(goal) {
  function find(start, history) {
    if (start == goal)
      return history;
    else if (start > goal)
      return null;
    else
      return find(start + 5, "(" + history + " + 5)") ||
             find(start * 3, "(" + history + " * 3)");
  }
  return find(1, "1");
}

print(findSequence(24));

如果没有参数的值，有人可以清理dod find如何执行和目标？递归是怎么发生的？

Could someone clear up how dod find get executed if it didn't have a value for the arguments start and goal? Also how did the recursion happen?

推荐答案

但是找到 没有 start 和目标的值，没有执行。它首先使用 1 的值执行 start ，并且目标的唯一值 24 。

But find didn't get executed without a value for start and goal. It was first executed with the value 1 for start, and the only value for goal was 24.

也许您对操作顺序感到困惑。在那里我们看到函数的声明， findSequence 。在声明期间，不执行任何代码。 findSequence 函数仅在最后一行执行，执行函数的结果将被打印出来。

Perhaps you're confused about the order of operations. There we see the declaration of a function, findSequence. During the declaration, no code is executed. The findSequence function only gets executed later, on the last line, where the result of executing the function gets printed out.

在 findSequence 的声明中，有另一个函数的声明， find 。再一次，直到后来它才会被执行。 findSequence 函数只有一个可执行代码行，一个调用 find（1，1）。执行该一行会以递归的方式触发 find 的执行次数。 find 函数引用目标;当Javascript解释器执行代码时， goal 总是引用 findSequence 的参数，因为在此示例中 findSequence 仅调用一次，目标始终具有相同的值， 24 。

Within the declaration of findSequence, there's a declaration of another function, find. Once again, it doesn't get executed until later. The findSequence function has just one executable line of code, the one that calls find(1, "1"). Execution of that one line triggers the execution of find some number of times, recursively. The find function makes reference to goal; when the Javascript interpreter executes the code, goal always refers to the parameter of findSequence, and since in this example findSequence is only called once, goal always has the same value, 24.

您应该能够看到递归发生的位置。如果 start 等于 goal ，则函数停止;它返回它到达该数字的历史。如果 start 大于 goal ，则返回 null ，表示该路径不是目标号码的路径。如果 start 仍然小于 goal ，那么该函数会尝试调用自身 value plus 5.如果返回非null值，那么返回的是。否则，它会尝试乘以3并返回该历史值。

You should be able to see where the recursion happened. If start was equal to goal, then the function stops; it returns the history of how it arrived at that number. If start is greater than goal, then it returns null, indicating that that path was not a path to the target number. If start is still less than goal, then the function tries calling itself with its start value plus 5. If that returns a non-null value, then that's what gets returned. Otherwise, it tries multiplying by 3 and returning that history value instead.

请注意，尽管此代码可以返回许多数字，但它无法返回 all 数字。如果目标是 2 ，例如， findSequence 将返回 null 因为无法从 1 开始，并通过添加 5获得 2 或乘以 3 。

Note that although this code can return many numbers, it cannot return all numbers. If the goal is 2, for example, findSequence will return null because there is no way to start at 1 and get to 2 by adding 5 or multiplying by 3.

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