@justyy 's series of Logits Tests:
- [Introduction to Logic Tests Series](https://helloacm.com/introduction-to-logic-tests-series/)

What you can do with this tiny programming language with only 4 instructions? Today, we are going to implement the DECR function which decrements the variable by one, for example, DECR(X) where X=5 will make X=4.

```
DECR(X) {

}
```

The only 4 instructions are: INCR, LOOP, ZERO and ASGN. And all variables hold non-negative integers. To make X decremented by 1, we can increment a temp variable by (X-1) times..

```
DECR(X) {
   ZERO(V1)
   LOOP(X) {
     ASGN(X, V1)
     INCR(V1)
  }
}
```

We translate this to [C++](https://helloacm.com/c-coding-exercise-use-hashtablepriority-queue-to-compute-the-relative-ranks/), which might be a bit easy to understand.

```
void decr(unsigned int &x) {
   int v1 = 0;
   int xx = x;
   for (; x > 0; -- x) {
      xx = v1;
      v1 ++;
   }
   x = xx;
}
```

If you change the value of the loop variable x, the loop iteration count may be altered. Therefore, we declare a temp variable xx and assign the xx to X after loop when it gets incremented (x-1) times.

![](https://cdn.pixabay.com/photo/2016/11/19/14/00/code-1839406_960_720.jpg)
*Image Credit: pixabay.com*

@justyy 的逻辑测试系列:
- [逻辑测试系列 - 一种只有4种语句的编程语言 - （1）](https://justyy.com/archives/5320)

这种只有4条语句的语言能做什么呢？今天我们来定义一个DECR函数，该函数就是把 变量 X 减一。

```
DECR(X) {

}
```

要求填写函数体，使用 INCR、LOOP、ZERO、和 ASGN 仅有的4个语句。我们不妨想一下，已知变量 X 是非负整数，那么我们只需要 循环 X-1次，每次把X从0加1即可。

```
DECR(X) {
   ZERO(V1)
   LOOP(X) {
     ASGN(X, V1)
     INCR(V1)
  }
}
```

我们翻译成 [C++](https://justyy.com/archives/2257), 可能比较好懂一些:

```
void decr(unsigned int &x) {
   int v1 = 0;
   int xx = x;
   for (; x > 0; -- x) {
      xx = v1;
      v1 ++;
   }
   x = xx;
}
```

由于C++的 for 里改变 x 值会改变循环次数，所以我们用了一个临时变量 xx。

![](https://justyy.com/wp-content/uploads/2017/07/justyy-steemit.png)

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// Later, it may be reposted to my blogs: [justyy.com](https://justyy.com), [helloacm.com](https://helloacm.com) and [codingforspeed.com](https://codingforspeed.com)  稍后同步到我的[中文博客](https://justyy.com)和英文[计算机](https://helloacm.com)[博客](https://codingforspeed.com)。
- [逻辑测试系列之二 – DECR](https://justyy.com/archives/5339)
- [Logic Tests Series (2) – DECR](https://helloacm.com/logic-tests-series-2-decr/)
- [记一次通过手机TeamViewer远程登陆家里服务器再远程登陆VPS敲命令在STEEMIT上发贴的经历](https://justyy.com/archives/5322)
- [The experience of using Teamviewer on iPhone SE, connecting to desktop, and SSH to VPS, in order to make a Robot Python script up running.](https://helloacm.com/the-experience-of-using-teamviewer-on-iphone-se-connecting-to-desktop-and-ssh-to-vps-in-order-to-make-a-robot-python-script-up-running/)

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原文首发于 https://Steemit.com 首发。感谢阅读，如有可能，欢迎Follow, Upvote, Reply, ReSteem (repost) @justyy 激励我创作更多更好的内容。