替代 for 循环语法 [英] Alternative for-loop syntax

问题描述

以下是 C 标准的片段(的第 6.8.5 节)n1256 TC3 C99).

Below is a snippet of the C standard(section 6.8.5 of the n1256 TC3 C99).

迭代语句:
   while ( 表达式 ) 语句
   do 语句 while ( 表达式 ) ;
   for ( expression_opt ; expression_opt ; expression_opt ) 语句
   for ( 声明 表达式_opt ; 表达式_opt ) 语句

iteration-statement:
    while ( expression ) statement
    do statement while ( expression ) ;
    for ( expression_opt ; expression_opt ; expression_opt ) statement
    for ( declaration expression_opt ; expression_opt ) statement

引起我兴趣的是最后一个语句:for ( declaration expression ; expression ) statement.6.8.5.1 解释了 for 循环，但只提到了 for ( clause-1 ; expression-2 ; expression-3 ) 语句 语法.

What piques my interest is the last statement: for ( declaration expression ; expression ) statement. 6.8.5.1 explains the for loop, but only mentions the for ( clause-1 ; expression-2 ; expression-3 ) statement syntax.

我曾尝试根据此语法编写代码，但它们都给了我语法错误.示例:

I did a few attempts at writing code according to this syntax, but they all gave me syntax errors. Examples:

for (int i = 0, i; i++) { /* ... */ }
for (int i = 0; !(i++)) { /* ... */ }

当使用 GCC(v4.9.2) 编译时，所有这些都会导致类似于 error: expected ‘;’ before ‘)’ token 的错误.

Which all results in errors similar to error: expected ‘;’ before ‘)’ token when compiled using GCC(v4.9.2).

我不确定我是否以正确的方式解释了标准.可以以某种有用的方式使用此语法，还是我忽略了什么?

I'm not sure if I'm interpreting the standard in the right way. Can this syntax be used in some useful way, or have I overlooked something?

推荐答案

如果你看到，语法是，

 for ( declaration expression1opt ; expression2opt ) statement

让我们将其与一般陈述进行比较

Let's compare it with a general statement

 for (int i = 0; i < 10; i++) printf("%d 	", i);

这里，

• int i = 0; 表示declaration [包括;]
• i <10 表示 expression1opt [可选]
• ; 符合;的语法要求[必须，如语法所述]
• i++ 是 expression2opt [可选]
• printf("%d ", i); 是语句

• int i = 0; denotes declaration [includes the ;]
• i < 10 denotes expression1opt [optional]
• ; is as per the syntax requirement of ; [must, as described in syntax]
• i++ is the expression2opt [optional]
• printf("%d ", i); is the statement

现在，就你而言，

for (int i = 0, i; i++) { /* ... */ }

• int i = 0, i; 表示声明
• i++ 表示 expression1opt
• ; 缺失 .....
• int i = 0, i; denotes declaration
• i++ denotes expression1opt
• ; is missing .....

这里的最后一点产生了错误.您需要有 ; 才能通过语法检查.

The last point here produces the error. You need to have the ; to pass the syntax check.

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