Question

我正在阅读《 C Primer Plus》这本书，并且遇到了如下代码片段：

int main()
{
    int x = 30;

    printf("x in outer block: %d at %p\n", x, &x);
    {
        int x = 77; // new x , hides first x
        printf("x in inner block: %d at %p\n", x, &x);
    }
    printf("x in outer block: %d at %p\n", x, &x);
    while (x++ < 33) //original x
    {
        int x = 100;
        x++;
        printf("x in while loop: %d at %p\n", x, &x);
    }
    printf("x in outer block: %d at %p\n", x, &x);

    return 0;
}

它输出：

In [23]: !./a.out
x in outer block: 30 at 0x7ffee7243788
x in inner block: 77 at 0x7ffee7243784
x in outer block: 30 at 0x7ffee7243788
x in while loop: 101 at 0x7ffee7243780
x in while loop: 101 at 0x7ffee7243780
x in while loop: 101 at 0x7ffee7243780
x in outer block: 34 at 0x7ffee7243788

这让我很困惑

x in while loop: 101 at 0x7ffee7243780
x in while loop: 101 at 0x7ffee7243780
x in while loop: 101 at 0x7ffee7243780

我如何使其输出？

x in while loop: 101 at 0x7ffee7243780
x in while loop: 102 at 0x7ffee7243780
x in while loop: 103 at 0x7ffee7243780

Answer 1

之所以会这样，是因为在while循环范围内，每次执行将变量x重新定义为等于100，这遮盖了在外部定义的x，因此您不必递增一三遍，但内心一遍。在每个新迭代中， new x都增加1，在输出中看到101。

Answer 2

每次进入while循环时，将内部x值重置为100，就可以添加一个外部var，然后在while循环中将其递增，将其值添加到100。

int main()
{
    int x = 30;
    int y = 0;
    printf("x in outer block: %d at \n", x);
    {
        int x = 77; /* new x , hides first x*/
        printf("x in inner block: %d at \n", x);
    }
    printf("x in outer block: %d at\n", x);

    while (x++ < 33) /*original x*/
    {
        int x = 100;/*value resets on every entry to 100*/
        x++;
        x +=y;
        ++y;/*value is incremented on every entry */

        printf("x in while loop: %d at\n", x);
    }
    printf("x in outer block: %d at \n", x);
    return 0;
}

Answer 3

这里有很多解释

让我直接回答您的问题“ 如何输出？”

while (x++ < 33) //original x
{
    //int x = 100;
    static int x = 100;
    x++;
    printf("x in while loop: %d at %p\n", x, &x);
}

我想说的是读关于static变量和一般storage classes

的内容

Answer 4

在此while循环中：

while (x++ < 33) //original x
{
    int x = 100; // this x hides the original x
    x++;
    printf("x in while loop: %d at %p\n", x, &x);
}

在循环体中声明的x隐藏了原始的x（在while循环表达式中用作计数器）。在while循环的每次迭代中，您都将获得内部101的相同值x，因为while循环的每次迭代都会创建x的新实例（内部{{ 1}}）初始化，其值为x，并在循环主体的末尾销毁它。

我如何使其输出？

如果希望内部100的最后一个值在x循环的每次迭代中保持不变，则只需将内部while设为静态：

while (x++ < 33) //original x
{
    static int x = 100;
    x++;
    printf("x in while loop: %d at %p\n", x, &x);
}

内部增量i ++输出不变的结果

4 个答案: