Logic Tests Series (2) - DECR 逻辑测试系列之二 - DECR

@justyy 's series of Logits Tests:

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++, 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.

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@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）

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++, 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。

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