C 编程：+= vs =+
我和我的教授正在就 C 语言中的 += 运算符进行一些辩论。他说 += 或 =+ 可以工作，但他不确定为什么 =+ 可以工作。
int main()
{
int i = 0, myArray[5] = {1,1,1,1,1};
while(i < 5)
{
myArray[i] += 3 + i;
printf("%d\n", myArray[i]);
i++;
}
system("pause");
}
输出将产生 4, 5, 6, 7, 8。将 += 运算符更改为 =+ 会产生相同的结果。然而 -= 的作用与 =- 不同（这很明显，因为它将 3 视为 3）。
那么 C 专家：
- 为什么这可以与 =+ 一起使用？
- C 编译器如何处理 =+ 与 +=？
My professor and I are engaging in a bit of a debate about the += operator in C. He says that += or =+ will work, but he is not certain why =+ works.
int main()
{
int i = 0, myArray[5] = {1,1,1,1,1};
while(i < 5)
{
myArray[i] += 3 + i;
printf("%d\n", myArray[i]);
i++;
}
system("pause");
}
The output will yield 4, 5, 6, 7, 8. Changing the += operator to =+ yields the same results. However -= does not do the same as =- (which is obvious as it treats the 3 as a 3).
So C gurus:
- Why does this work with =+?
- How does a C compiler treat =+ versus +=?
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评论（4）
他错了；
+=与=+完全不同。表达式
x =+ 3被解析为x = (+3)。在这里，
+成为（相当无用的）一元+运算符。 （与否定相反）使用一元否定运算符将表达式
x =- 3解析为x = (-3)。He is wrong;
+=is completely different from=+.The expression
x =+ 3is parsed asx = (+3).Here,
+becomes the (rather useless) unary+operator. (the opposite of negation)The expression
x =- 3is parsed asx = (-3), using the unary negation operator.您的教授正在记住 C 的古代版本，其中
=+、=-、=*等实际上是这样做的与+=、-=、*=等含义相同（我们所说的版本比通常称为“ K&R”在这里。版本 6 UNIX，如果没记错的话。）在 C 的当前版本中，它们并不意味着同一件事；相反，它们的含义不同。首先带有等号的版本将被解析，就好像等号和后面的内容之间有空格一样。这恰好会为
=-和=+生成一个有效的程序（尽管不是执行您期望的操作的程序），因为-和+可以用作一元运算符。=*或=/可以用来解决争论。a *= 3会将a乘以三，a /= 3会将其除以三，但a =* 3是一个语义错误（因为一元*只能应用于指针），而a =/ 3是一个语法错误（因为/ 不能用作一元运算符）。Your professor is remembering ancient versions of C in which
=+,=-,=*etc did in fact mean the same thing as+=,-=,*=etc. (We're talking older than the version generally referred to as "K&R" here. Version 6 UNIX, if memory serves.)In current versions of C, they do not mean the same thing; the versions with the equals sign first will be parsed as if there was a space in between the equals and whatever comes after. This happens to produce a valid program (albeit not a program that does what you expect) for
=-and=+because-and+can be used as unary operators.=*or=/could be used to settle the argument.a *= 3will multiplyaby three, anda /= 3will divide it by three, buta =* 3is a semantic error (because unary*can only be applied to pointers) anda =/ 3is a syntax error (because/can not be used as an unary operator).代码
将产生
myArray[i] = myArray[i] + 3 + i;而
产生
myArray[i] = 3 + i这就是我得到的。
Code
will yield
myArray[i] = myArray[i] + 3 + i;whereas
yields
myArray[i] = 3 + ithat's what I got.
+和-一样也是一元运算符。+is also a unary operator as is-.