词法范围与动态范围 [英] Lexical scoping vs dynamic scoping

问题描述

所以我遇到了这个问题，我必须使用两种不同的范围规则来计算输出.我知道使用词法范围的输出是 a=3 和 b=1，但我很难使用动态范围计算输出.
_{注意:下面的代码示例使用了 C 语法，但让我们将其视为伪代码.}

So I have this problem where I have to figure out the output using two different scoping rules. I know the output using lexical scoping is a=3 and b=1, but I am having hard time figure out the output using dynamic scoping.
_{Note:the code example that follows uses C syntax, but let's just treat it as pseudo-code.}

int a,b;

int p() {
    int a, p;
    a = 0; b = 1; p = 2;
    return p;
}

void print() {
    printf("%d
%d
",a,b);
}

void q () {
    int b;
    a = 3; b = 4;
    print();
}

main() {
    a = p();
    q();
}

这是我想出来的.使用动态范围，对 a 和 b 的非本地引用可以改变.所以我有 a=2 (从 p() 返回)，然后 b=4 (在 q() 内).所以输出是2 4?

Here is what I come up with. Using Dynamic scoping, the nonlocal references to a and b can change. So I have a=2 ( return from p() ), then b=4 ( inside q() ). So the output is 2 4?

推荐答案

正如我们所知，C 没有动态范围，但假设它有，程序将打印 3 4.

As we know, C doesn't have dynamic scoping, but assuming it did, the program would print 3 4.

在 main 中，a 和 b 是全局变量.a 将设置为 2，因为我们将看到这是 p 将返回的内容.

In main, a and b are the global ones. a will be set to 2, as we will see that this is what p will return.

在 p 中，从 main 调用，b 仍然是全局的，但 a 是 p 中的局部.本地a设置为0，但很快就会消失.全局 b 设置为 1.本地 p 设置为 2，将返回 2.现在全局 b 是 1.

In p, called from main, b is still the global one, but a is the one local in p. The local a is set to 0, but will soon disappear. The global b is set to 1. The local p is set to 2, and 2 will be returned. Now the global b is 1.

在q中，从main调用，a是全局的，而b是q中的局部.这里全局a设置为3，局部b设置为4.

In q, called from main, a is the global one, but b is the one local in q. Here the global a is set to 3, and the local b is set to 4.

在打印中，从 q 调用，a 是全局的(值为 3)，b 是 q 中的局部值(值为 4).

In print, called from q, a is the global one (which has the value 3), and b is the one local in q (which has the value 4).

在最后一步中，在函数 print 中，我们看到了与静态作用域的区别.使用静态范围 a 和 b 将是全局范围.使用动态范围，我们必须查看调用函数链，在 q 中我们找到一个变量 b，它将是在 print 中使用的 b.

It is in this last step, inside the function print, that we see a difference from static scoping. With static scoping a and b would be the global ones. With dynamic scoping, we have to look at the chain of calling functions, and in q we find a variable b, which will be the b used inside print.

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