有关逗号表达式的问题

[liguomail_7]

我在看书的时候姓潭的写的遇到两个问题①关于赋值，前面他说不能写成a=b=1这样的，后面又说可以，到底能不能这样写啊？②就是逗号表达试的问题，一个逗号表达式x=a=3,6*a整个表达式的值是多少啊？我认为是18，大家能帮我吗？

[ecstar]

我在vc和dev-cpp中测试过竟然都是3
???

[maddevil]

就是12和16啊 难道你不是这样吗?!

#include <stdio.h>
#include <conio.h>
int main(void)
{

int i = 10;
int j,k;

j = (i++, i++, i++);
k = (++i, ++i, ++i);


printf("j=%d\n",j);
printf("k=%d\n",k);

getch();

}

[killme2008]

赞成一楼的

[maddevil]

to wingfiring(别逗了):

关于逗号运算符标准是有规定的
以下是标准里关于运算符的描述: (我英文不好,难道是我没看懂??还是别的什么情况,请指教)
ISO/IEC 9899:1999 (E) ©ISO/IEC

6.5.17 Comma operator

Syntax

1 expression:
assignment-expression
expression , assignment-expression
Semantics

2 The left operand of a comma operator is evaluated as a void expression; there is a
sequence point after its evaluation. Then the right operand is evaluated; the result has its
type and value.94) If an attempt is made to modify the result of a comma operator or to
access it after the next sequence point, the behavior is undefined.

3 EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot
appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists
of initializers). On the other hand, it can be used within a parenthesized expression or within the second
expression of a conditional operator in such contexts. In the function call

f(a, (t=3, t+2), c)

the function has three arguments, the second of which has the value 5.

[maddevil]

到底是怎么一个样子?谁出来说清楚.
我真的好想知道到底是谁错了
我觉得我是对的,可还是很不放心,所以很想搞清楚

[foreversq]

第二个问题是18

[wingfiring]

to maddevil()
If an attempt is made to modify the result of a comma operator or to
access it after the next sequence point, the behavior is undefined.

这不是说得很清楚了吗？the behavior is undefined！
如果在逗号表达式中试图修改后继的表达式，结果都是未定义的。

作为一个原则，除了&&和||,任何复合表达式的求值顺序都是未定义的，任何关于计算顺序的假设都是错误的。错误的根本原因在于副作用，=,++,--都是有副作用的，如果表达式的最终结果可能受到副作用的影响，则表达式的值就是无意义的。

举例来说:
i++;
i++,j, j+k
表达式虽然也有副作用，但是，它没有影响到表达式的值,所以是确定的结果。
i=3, j*i;

这个有问题，j*i依赖于i,但是i的发生了副作用。

[TianGuangZao]

b=(a=2,a+1) 绝对是定义了的。

On the other hand, it can be used within a parenthesized expression or within the second
expression of a conditional operator in such contexts. In the function call

f(a, (t=3, t+2), c)

the function has three arguments, the second of which has the value 5.

写的很清楚。

to:wingfiring(别逗了)
If an attempt is made to modify the result of a comma operator or to
access it after the next sequence point, the behavior is undefined.
有断章取义嫌疑。

”如果在逗号表达式中试图修改后继的表达式，结果都是未定义的。“
这样的翻译可不敢恭维。

[killme2008]

dev-cpp:

int main(int argc, char* argv[])
{
int x,a;
x=a=3,6*a;
printf("x=%d\n",x);
printf("a=%d\n",a);
return 0;
}

测试结果
x=3
a=3

[maddevil]

你这样是不对的
应该这样:
int main(int argc, char* argv[])
{
int x,a,k;
k=(x=a=3,6*a);
printf("k=%d\n",k);
return 0;
}

[killme2008]

楼上，呵呵，抱歉，我是抄上面的试了一下，没注意看
答案确实是18

[ceran]

1.完全可以!老谭先说不行可能是想说明一些编程规范,谁知道呢?
2.答案18,应该没问题.

[lilililll]

如果是X=(A=3,6*A),则X的值是18；
若是X=A=3,6*A
此时X的值是3

[lilililll]

谭的书上说的是x的值，不是整个逗号表达式的值，
X=(A=3,6*A)是一个赋值表达式，将一个逗号表达式的值赋给X

[_码农一个_]

哈，逗号运算符的问题。

我想题主应该是在学标准C时，遇到的吧？

如果是标准C，那么第二个问题的答案就一定是18了。

[sakurar]

不同编译器会有不同

[haosjt]

结果一定是18

[fierygnu]

to wingfiring(别逗了)：
逗号表达式的运算顺序也是标准定义的。

[deeply]

测试环境：vc++6.0

int main(int argc, char* argv[])
{
int x,a;
x=a=3,6*a;
printf("x=%d\n",x);
printf("a=%d\n",a);
return 0;
}

测试结果
x=3
a=3

[maddevil]

to:wingfiring(别逗了)

我同意你关于副作用的说法 但逗号运算符应该不受副作用的影响?
以下这断话是The C++ Programming Language (The Third Edtion)的6.2.2里说的

The operators , (comma), && (logical and),and || (logical or)
guarantee that their left-hand operand is evaluated before their right-hand operand. For example,b=(a=2,a+1) assigns 3 to b.

你对这断话有什么看法