在Clojure中实施中断 [英] Implementing break in Clojure

问题描述

如果条件匹配返回最后一个值，是否有办法打破Clojure中的循环？大多数算法从发现结果时返回结果并避免完成整个执行过程中受益。

Is there a way to break out of a loop in Clojure if a condition is matched returning the last value? Most algorithms benefit from returning a result when they find it and avoiding completing the whole execution.

比方说，我有一个从0到100的100个数字向量，我想找到数字10。一旦找到10，我想停止执行。

Let's say that I have a vector of 100 numbers ranging from 0 to 100 and I want to find the number 10. Once 10 is found I want the execution to stop.

比我的示例更简单的情况如下：

An even simpler case than my example is the following:

(defn MySearch
    [y]
    (when (< y 10)

;;Corrected. Thanks to dsm who pointed it out. Previously was (< y 5).

        (if (= (nth [1 2 3 4 5 6 7 8 9 10] y) 10)
            (println 
                "I found it! Now I want to stop executing!"
            )
        )
        (recur
            (inc y)
        )
    )
)

(MySearch 0)

找到5个时如何停止？

我已经搜索了足够多的东西，但找不到任何实现方法。我还在这里找到了一个答案，指出我要问的内容在Clojure中不存在，但我觉得有些牵强。即使是这种情况，我也可以自己执行类似的事情吗？

I have searched enough and I can't find any way of implementing this. I have also found an answer here that states that what I am asking doesn't exist in Clojure but I find it a little far-fetched. Even if this is the case can I implement something like that myself?

（我是Clojure的新手。）

(I am new to Clojure.)

推荐答案

您几乎完全正确。重新格式化您的代码，我们会得到

You nearly got it right. Reformatting your code, we get

(defn MySearch [y]
  (when (< y 10)
    (if (= (nth [1 2 3 4 5 6 7 8 9 10] y) 10)
      "I found it! Now I want to stop executing!")
    (recur (inc y))))

...其中-为简单起见-我摆脱了 println ，并希望该函数返回您的消息。

... where - for simplicity - I've got rid of the println, and had the function hopefully return your message.

但是，正如您所注意到的，它不是：

But, as you've noticed, it doesn't:

(MySearch 0)
;nil

为什么？

问题在于（重复...）在外部中， if 。

• 如果（< y 10）条件满足满足时，（如果...）和
  满足 （递归...）依次执行，并返回后者
  的结果。


• 最终， y 是 10 ，因此当条件失败时，因此当
  时返回 nil 。

• If the (< y 10) condition for the when is met, the (if ...) and the (recur ...) are executed in turn, and the result of the latter returned.
• Eventually, y is 10, so the when condition fails, so the when returns nil.

让内部如果 ：

Let's move the recur inside the if:

(defn MySearch [y]
  (when (< y 10)
    (if (= (nth [1 2 3 4 5 6 7 8 9 10] y) 10)
      "I found it! Now I want to stop executing!"
      (recur (inc y)))))

现在，最低点：

(MySearch 0)
;"I found it! Now I want to stop executing!"

由于我们返回了消息，因此我们知道该函数确实停止了执行。否则它将继续并返回 nil 。

Because we returned the message, we know that the function did stop executing. Otherwise it would have gone on and returned nil.

在已安装 println 的情况下，该函数将输出消息并返回 nil 立即执行，因为它会继续执行。所以-关于它是否停止执行，您再也不明智了。

With the println in place, the function would output the message and return nil immediately, as it would do had it gone on executing. So - as to whether it stopped executing, you're none the wiser.

顺便说一下，作为找到牵强的 的答案，让我再试一次：

By the way, as the author of the answer you find far-fetched, let me try again:

• Clojure中没有中断声明。



• 反之亦然：

• There is no break statement in Clojure.

• It's the other way round:

• 默认情况下，您跳出循环。


• 您必须使用 recurr 才能继续。

• You break out of a loop by default.
• You have to use recur to continue it.

recur 是对该函数的特殊递归调用（或循环），您正在执行：

recur is a special recursive call to the function (or loop) you are executing:

• 其中一个是返回值。


• 据说处于尾巴位置。

• one where it is the returned value.
• It is said to be in tail position.

大多数Lisp系统会自动检测到此类呼叫-所谓的 tail呼叫。
因此，他们没有或需要像 recur 这样的构造。

Most Lisp systems detect such calls - so called tail calls - automatically. So they don't have or need a construct like recur.

话虽如此，Clojure 1.5引入了 reduced ：a <类似code> break 的减少结构。您可以在此处中阅读。

Having said that, Clojure 1.5 introduced reduced: a break-like construct for reduce. You can read about it here.

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