C语法归约之表达式（3）

太过爱你忘了你带给我的痛 2022-08-12 07:59 160阅读 0赞

### 1、概要 ###

前三篇博文（[0][]，[1][]，[2][]）主要讨论是声明和定义的相关语法归约。从这篇文字开始，将开始进行有效的计算，涉及表达式和语句。

首先，表达式和语句这两个概念是不同的，表达式是没有分号(;)的，而表达式+；构成语句。但这不是语句的全部，语句还可以是跳转，迭代，标签语句等等（[见图片C][0]语句文法图）！打个比喻来说吧——就是蛋和鸡的关系！当然不是那个悖论，而是表达式是蛋，孵化是分号，鸡是语句。蛋孵化成鸡。鸡肚子里可以有蛋！嘎嘎！二者区别必须明白。不能混为一谈！

在C语言中，如果单纯就表达式计算而言，表达式就是一个人，而分号；好似一张毕业证，有它你就是正规的劳动人员，哪怕你什么都不干什么都不会（比如单个分号；）而没它，你就是没户口的黑孩，不能参与计算劳动，编译器抱怨的不得了，就是不让你上岗运行！

其次，说到表达式，运算符的优先级问题就不得不提一提了。C中的优先级是由表达式文法决定的！当初学C的时候都是死记硬背的，那时候觉得这个优先级是C它爹里奇严格要求，每次运算的时候都要检测后面的优先级，看它是不是比当前运算符优先级高。而且一直觉得编译器也是这样做的。而事实上不是，广义上讲是严格要求，因为它已经写入了文法，归约顺序是固定的，就像法律条文一样，是严格约束的；但是从狭义上而论。这个优先级是很自然的事情，不需要摇头晃脑的背，也不用比较优先级高低来实现，而是语法归约过程很自然的东西。一级一级的归约，一级一级的蹦哒，到了哪个阶段，那个阶段归约，优先级自然就隐含在里面了！它们的归约顺序决定了它们优先级！

![Center][]

通过表达式对归约过程的理解，把它画成一个顺时钟的表圈更直观！如上图所示！(图片较大，另存为下载可正常查阅)

在中心处写了一段：运算符的结合只能从高到低而不能逆向进行（归约箭头方向决定了），较低运算符优先结合的方法是使用（）在零点方向。（）是相当复位运算符，使得较低运算符能够回到最高优先级处改变运算符结合顺序！这样较低运算符能够先运算之后再和较高运算符结合；注意归约过程不会跳跃！（下文示例可以得知这一点）即向‘上游’运算符结合，必须顺时钟走一圈，并且使用了()，否则达到零点前expression处就归约停止！

举个简单例子帮助理解这一段话：a+1<<8和a+(1<<8)

a+1<<8归约过程
    a是id->primary-expr->postfix-expr->unary-expr->cast-expr->mul-expre->additive-expr(暂停)
    +
    1是constant->primary-expr->postfix-expr->unary-expr->cast-expr->mul-expr(暂停)
    然后additive-expr+mul-expr->additive-expr->shift-expr
    <<
    8是constant->primary-expr->postfix-expr->unary-expr->cast-expr->mul-expr->additive-expre(暂停)
    shift-expr<<additive-expr->shift-expr->relational-expr->equality-expr->...->expr(暂停)
    a+(1<<8)归约过程
    a是id->primary-expr->postfix-expr->unary-expr->cast-expr->mul-expr->additive-expr(暂停)
    +
    (
    1是constant->primary-expr->postfix-expr->unary-expr->cast-expr->mul-expr->additive-expr->shift-expr(暂停)
    <<
    8是constant->primary-expr->postfix-expr->unary-expr->cast-expr->mul-expr->additive-expr(暂停)
    shift-expr<<additive-expr->shift-expr->relational-expr->equality-expr->...->expr(暂停)
    )
    (expr)->primary-expr->postfix-expr->unary-expr->cast-expr->mul-expr
    additive-expr+mul-expr->additive-expr->shift-expr->...->expr

注意区分它们的不同！或许你会问如何知道到什么时候需要暂停，什么时候继续归约？答案就在非终结符的follow基上！什么是follow集以及first基，请参与《编译原理》第4章语法分析部分！这里就不重述了！

在这里你只需要知道：当下一个运算符是当前非终结符要归约到的非终结符的follow基时，则继续归约，否则暂停。比如上面这个例子1的归约shift-expression的follow基包含<<，那么additive-expression就需要归约到shift-expression，再往下，relational-expression的follow基不包含<<，所以shift-expression就到此暂停，不能再归约下去！而等待其他部分（<< 和additive-expression）到来，再一起归约！

如果上文看不懂，不要紧，在下面的示例中会给出很多组合范例，可以多看几个就理解了，当然你要结合C99文法或者上图来理解！

最后，为了更好描述原理，这里要相对前面的声明定义而言做些调整！单个示例进行提示！表达式示例分两大类，基本常用和高级复合！在此文将给出基本常用表达式归约。下一篇给出复合表达式归约。

输出依旧老风格，整个归约栈，其实是一颗树，只不过是栈的形式描述的。每次归约都是从孩子到父亲的建树过程，因为这是LR(1)，自底向上！（LL正好相反，自顶向下）整个归约纳入一个函数当中，每次栈的输出开头“declaration-specifiers declarator \{ block-item-list“它属于函数归约的部分，压在栈中而已，可以忽略不看，重点在表达式的归约上！另外，表达式之间过度部分如下，也可以忽略不看！

declaration-specifiers declarator { expression-statement
    declaration-specifiers declarator { statement
    declaration-specifiers declarator { block-item
    declaration-specifiers declarator { block-item-list

### 2、表达式归约 ###

#### 2.1、基本表达式 ####

##### 示例代码 #####

void basic_exp()
    {
    	left = a[i]; 
    	left = i++; 
    	left = i--; 
    	left = fun(i); 
    	left = girl.age;
    	left = girl->wight;
    	left = ++i;
    	left = --i;
    	left = *a;
    	left = &a;
    	left = !a;
    	left = ~a;
    	left = +a;
    	left = -a;
    	left = sizeof(int);
    	left = (int *)a;
    	left = a * b;
    	left = a / b;
    	left = a % b;
    	left = a + b;
    	left = a - b;
    	left = a << 1;
    	left = b >> 1;
    	left = a > b;
    	left = a < b;
    	left = a <= b;
    	left = a >= b;
    	left = a != b;
    	left = a == b;
    	left = a & b;
    	left = a ^ b;
    	left = a | b;
    	left = a && b;
    	left = a || b;
    	left = a > b ? a : b;
    	left = a = 1;
    	left = a >>= 1;
    	left = a, b;
    	left = (a + b) * c;
    }

##### 归约过程 #####

void basic\_exp()

curr: void  next: basic_exp  
    type-specifier 
    declaration-specifiers 
    
    curr: ID  next: (  
    declaration-specifiers direct-declarator 
    
    curr: (  next: )  
    
    curr: )  next: {  
    declaration-specifiers direct-declarator 
    declaration-specifiers declarator 
    
    curr: {  next: left

left = a\[i\];

curr: ID  next: =  
    declaration-specifiers declarator { primary-expression 
    declaration-specifiers declarator { postfix-expression 
    declaration-specifiers declarator { unary-expression 
    
    curr: =  next: a  
    declaration-specifiers declarator { unary-expression assignment-operator 
    
    curr: ID  next: [  
    declaration-specifiers declarator { unary-expression assignment-operator primary-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression 
    
    curr: [  next: i  
    
    curr: ID  next: ]  
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ primary-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ postfix-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ unary-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ cast-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ multiplicative-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ additive-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ shift-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ relational-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ equality-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ AND-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ exclusive-OR-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ inclusive-OR-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ logical-AND-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ logical-OR-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ conditional-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ assignment-expression 
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression [ expression 
    
    curr: ]  next: ;  
    declaration-specifiers declarator { unary-expression assignment-operator postfix-expression 
    declaration-specifiers declarator { unary-expression assignment-operator unary-expression 
    declaration-specifiers declarator { unary-expression assignment-operator cast-expression 
    declaration-specifiers declarator { unary-expression assignment-operator multiplicative-expression 
    declaration-specifiers declarator { unary-expression assignment-operator additive-expression 
    declaration-specifiers declarator { unary-expression assignment-operator shift-expression 
    declaration-specifiers declarator { unary-expression assignment-operator relational-expression 
    declaration-specifiers declarator { unary-expression assignment-operator equality-expression 
    declaration-specifiers declarator { unary-expression assignment-operator AND-expression 
    declaration-specifiers declarator { unary-expression assignment-operator exclusive-OR-expression 
    declaration-specifiers declarator { unary-expression assignment-operator inclusive-OR-expression 
    declaration-specifiers declarator { unary-expression assignment-operator logical-AND-expression 
    declaration-specifiers declarator { unary-expression assignment-operator logical-OR-expression 
    declaration-specifiers declarator { unary-expression assignment-operator conditional-expression 
    declaration-specifiers declarator { unary-expression assignment-operator assignment-expression 
    declaration-specifiers declarator { assignment-expression 
    declaration-specifiers declarator { expression 
    
    curr: ;  next: left  
    declaration-specifiers declarator { expression-statement 
    declaration-specifiers declarator { statement 
    declaration-specifiers declarator { block-item 
    declaration-specifiers declarator { block-item-list

left = i--;

left = fun(i);

left = girl.age;

left = girl->wight;

left = ++i;

left = --i;

left = \*a;

left = &a;

left = !a;

left = ~a;

left = +a;

left = -a;

left = sizeof(int);

left = (int \*)a;

left = a \* b;

left = a / b;

left = a % b;

left = a + b;

left = a - b;

left = a << 1;

left = b >> 1;

left = a > b;

left = a < b;

left = a <= b;

left = a >= b;

left = a != b;

left = a == b;

left = a & b;

left = a ^ b;

left = a | b;

left = a && b;

left = a || b;

left = a > b ? a : b;

curr: ID  next: =  
    declaration-specifiers declarator { block-item-list primary-expression 
    declaration-specifiers declarator { block-item-list postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression 
    
    curr: =  next: a  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator 
    
    curr: ID  next: >  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator multiplicative-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator additive-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator shift-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression 
    
    curr: >  next: b  
    
    curr: ID  next: ?  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression > primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression > postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression > unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression > cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression > multiplicative-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression > additive-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression > shift-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator equality-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator exclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator inclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression 
    
    curr: ?  next: a  
    
    curr: ID  next: :  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? multiplicative-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? additive-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? shift-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? relational-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? equality-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? exclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? inclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? logical-AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? logical-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? conditional-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? assignment-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression 
    
    curr: :  next: b  
    
    curr: ID  next: ;  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : multiplicative-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : additive-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : shift-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : relational-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : equality-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : exclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : inclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : logical-AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : logical-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression ? expression : conditional-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator conditional-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator assignment-expression 
    declaration-specifiers declarator { block-item-list assignment-expression 
    declaration-specifiers declarator { block-item-list expression 
    
    curr: ;  next: left  
    declaration-specifiers declarator { block-item-list expression-statement 
    declaration-specifiers declarator { block-item-list statement 
    declaration-specifiers declarator { block-item-list block-item 
    declaration-specifiers declarator { block-item-list

left = a >>= 1;//其余赋值运算符都一样过程

left = a, b;

left = (a + b) \* c;

curr: ID  next: =  
    declaration-specifiers declarator { block-item-list primary-expression 
    declaration-specifiers declarator { block-item-list postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression 
    
    curr: =  next: (  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator 
    
    curr: (  next: a  
    
    curr: ID  next: +  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( multiplicative-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( additive-expression 
    
    curr: +  next: b  
    
    curr: ID  next: )  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( additive-expression + primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( additive-expression + postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( additive-expression + unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( additive-expression + cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( additive-expression + multiplicative-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( additive-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( shift-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( relational-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( equality-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( exclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( inclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( logical-AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( logical-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( conditional-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( assignment-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator ( expression 
    
    curr: )  next: *  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator multiplicative-expression 
    
    curr: *  next: c  
    
    curr: ID  next: ;  
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator multiplicative-expression * primary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator multiplicative-expression * postfix-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator multiplicative-expression * unary-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator multiplicative-expression * cast-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator multiplicative-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator additive-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator shift-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator relational-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator equality-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator exclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator inclusive-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-AND-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator logical-OR-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator conditional-expression 
    declaration-specifiers declarator { block-item-list unary-expression assignment-operator assignment-expression 
    declaration-specifiers declarator { block-item-list assignment-expression 
    declaration-specifiers declarator { block-item-list expression 
    
    curr: ;  next: }  
    declaration-specifiers declarator { block-item-list expression-statement 
    declaration-specifiers declarator { block-item-list statement 
    declaration-specifiers declarator { block-item-list block-item 
    declaration-specifiers declarator { block-item-list 
    
    curr: }  next: NULL  
    declaration-specifiers declarator compound-statement 
    function-definition 
    function-definition-list 
    translation-unit

##### 总结 #####

你会发现相同优先级的运算符，它们归约过程完全一样，比如\*/%，再比如< > <= >=，而不同优先级的运算符归约过程略有区别！总体都相似，都是围绕表盘顺时钟转一圈！在下文当中，你将会看到复合表达式中，它将非常喜欢转圈圈！^!^！

声明：写了前三篇，发现已经被爬虫爬走了，爬走就爬走呗，这没啥，关键是没有注明来源，实在是不尊重我的劳动！别人还因为我抄他的呢！！！

特此声明：

21世纪什么最重要，开源！知识无贵贱，知识无国界！知识需要传播，知识需要应用！不要版权，但要尊重劳动，转载请注明出处！

作者：my\_live\_123

[下篇：高级复合表达式][Link 1]

#####     #####

[0]: http://blog.csdn.net/cwcmcw/article/details/39378607
[1]: http://blog.csdn.net/cwcmcw/article/details/39430145
[2]: http://blog.csdn.net/cwcmcw/article/details/39434417
[Center]: /images/20220810/7957e30b82114f30936a2345d947ea81.png
[Link 1]: http://blog.csdn.net/cwcmcw/article/details/39478903