[英]Different results when using increment operator (arr[i++] vs arr[i]; i++;)
[英]inner increment i++ output unchanged result
我正在阅读《 C Primer Plus》这本书,并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
发生这种情况的原因是,在while
循环范围内,每次执行将变量x
重新定义为等于100,这将x
定义在外部,因此您不会将其递增3倍,而是将其递增3倍。 在每个新的迭代中, 新的 x都会增加1,您会在输出中看到101。
每次进入while循环时,将内部x值重置为100,您可以添加一个外部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;
}
这里有很多解释,所以
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);
}
我会说一些关于static
变量和一般storage classes
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
隐藏原始x
(在while
循环表达式中用作计数器)。 在while
循环的每次迭代中while
您都将获得内部x
相同值101
,因为while
循环的每次迭代都会创建x
新实例(内部x
),并使用值100
初始化,并在循环主体的末尾销毁' x
。
我如何使其输出?
如果希望内部x
的最后一个值在while
循环的每次迭代中都保持不变,则可以简单地将其设为静态:
while (x++ < 33) //original x
{
static int x = 100;
x++;
printf("x in while loop: %d at %p\n", x, &x);
}
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