词汇范围与动态范围 [英] Lexical scoping vs dynamic scoping

问题描述

所以我有这个问题，我必须使用两个不同的作用域规则来找出输出.我知道使用词法作用域的输出是a=3和b=1，但是我很难用动态作用域来确定输出. _{注意:下面的代码示例使用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\n%d\n",a,b);
}

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

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

这就是我想出的. 使用动态作用域，可以更改对a和b的非本地引用.所以我有a=2(从p()返回)，然后是b=4(在q()内部). 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

我们知道，C没有动态作用域，但假设它具有动态作用域，则程序将输出3 4.

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

总的来说，a和b是全局的.将a设置为2，因为我们将看到p将返回.

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中的局部的.本地a设置为0，但很快就会消失.全局b设置为1.局部p设置为2，并且将返回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1，而b是q中的局部1.在这里，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是全局值1(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).

在函数print的最后一步中，我们看到了与静态作用域不同的地方.在静态范围内，a和b将是全局范围.使用动态作用域，我们必须查看调用函数的链，在q中我们找到一个变量b，它将是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.

这篇关于词汇范围与动态范围的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！