ll1文法分析器 c语言

LL(1)文法是一种上下文无关文法，可以用于描述程序语言的语法结构。在编译原理中，LL(1)文法分析器是一种自顶向下的语法分析器，它可以根据输入的符号串，判断是否符合给定文法的规则，并构建出语法树。

在C语言中，可以使用LL(1)文法分析器来解析C源代码，以便进行语法检查、代码生成等操作。下面是一个简单的LL(1)文法分析器的实现，可以用来解析C语言的基本语法结构：

1. 定义文法规则

首先，需要定义C语言的文法规则。例如，可以定义如下的文法规则：

<program> ::= <declaration-list>
<declaration-list> ::= <declaration> | <declaration> <declaration-list>
<declaration> ::= <type-specifier> <identifier> ";" | <type-specifier> <identifier> "(" <parameter-list> ")" <compound-statement>
<type-specifier> ::= "void" | "int" | "float" | "double" | "char" | "short" | "long"
<identifier> ::= <letter> | <identifier> <letter> | <identifier> <digit>
<letter> ::= "a" | "b" | "c" | ... | "z" | "A" | "B" | "C" | ... | "Z"
<digit> ::= "0" | "1" | "2" | ... | "9"
<parameter-list> ::= <parameter-declaration> | <parameter-declaration> "," <parameter-list>
<parameter-declaration> ::= <type-specifier> <identifier>
<compound-statement> ::= "{" <declaration-list> <statement-list> "}"
<statement-list> ::= <statement> | <statement> <statement-list>
<statement> ::= <expression-statement> | <compound-statement> | <selection-statement> | <iteration-statement> | <jump-statement>
<expression-statement> ::= <expression> ";"
<expression> ::= <assignment-expression> | <expression> "," <assignment-expression>
<assignment-expression> ::= <conditional-expression> | <unary-expression> <assignment-operator> <assignment-expression>
<conditional-expression> ::= <logical-or-expression> | <logical-or-expression> "?" <expression> ":" <conditional-expression>
<logical-or-expression> ::= <logical-and-expression> | <logical-or-expression> "||" <logical-and-expression>
<logical-and-expression> ::= <equality-expression> | <logical-and-expression> "&&" <equality-expression>
<equality-expression> ::= <relational-expression> | <equality-expression> "==" <relational-expression> | <equality-expression> "!=" <relational-expression>
<relational-expression> ::= <additive-expression> | <relational-expression> "<" <additive-expression> | <relational-expression> "<=" <additive-expression> | <relational-expression> ">" <additive-expression> | <relational-expression> ">=" <additive-expression>
<additive-expression> ::= <multiplicative-expression> | <additive-expression> "+" <multiplicative-expression> | <additive-expression> "-" <multiplicative-expression>
<multiplicative-expression> ::= <unary-expression> | <multiplicative-expression> "*" <unary-expression> | <multiplicative-expression> "/" <unary-expression>
<unary-expression> ::= <primary-expression> | <unary-operator> <unary-expression>
<primary-expression> ::= <identifier> | <constant> | "(" <expression> ")"
<constant> ::= <integer-constant> | <floating-constant> | <character-constant>
<integer-constant> ::= <digit> | <integer-constant> <digit>
<floating-constant> ::= <digit> "." <digit> | <digit> "." <digit> <exponent-part> | "." <digit> <exponent-part> | <digit> <exponent-part>
<exponent-part> ::= "e" <sign> <digit> | "E" <sign> <digit> | <empty>
<sign> ::= "+" | "-"
<character-constant> ::= "'" <character> "'"
<character> ::= <letter> | <digit> | <special-character>
<special-character> ::= "~" | "!" | "@" | "#" | "$" | "%" | "^" | "&" | "*" | "(" | ")" | "-" | "+" | "=" | "{" | "}" | "[" | "]" | ":" | ";" | "<" | ">" | "," | "." | "?" | "/"
<unary-operator> ::= "+" | "-" | "!" | "~"
<assignment-operator> ::= "=" | "+=" | "-=" | "*=" | "/="
<selection-statement> ::= "if" "(" <expression> ")" <statement> | "if" "(" <expression> ")" <statement> "else" <statement> | "switch" "(" <expression> ")" <compound-statement>
<iteration-statement> ::= "while" "(" <expression> ")" <statement> | "do" <statement> "while" "(" <expression> ")" ";" | "for" "(" <expression> ";" <expression> ";" <expression> ")" <statement> | "for" "(" <declaration> <expression> ";" <expression> ")" <statement>
<jump-statement> ::= "goto" <identifier> ";" | "continue" ";" | "break" ";" | "return" <expression> ";" | "return" ";"

2. 实现LL(1)文法分析器

接下来，可以根据上述文法规则，实现LL(1)文法分析器。具体实现步骤如下：

（1）将输入的符号串转换为Token序列，即将源代码中的每个单词转换为对应的Token类型。

（2）定义LL(1)分析表，该表包含所有的非终结符和终结符，以及它们之间的推导规则。根据文法规则，可以逐步构建出LL(1)分析表。

（3）根据LL(1)分析表和Token序列，进行语法分析。具体步骤如下：

- 从Token序列中读入下一个Token，记为lookahead。
- 根据当前的非终结符和lookahead，查找LL(1)分析表中对应的推导规则。
- 如果推导规则中存在非终结符，则将该非终结符入栈，继续进行分析。
- 如果推导规则中存在终结符，则将该终结符与lookahead进行匹配，如果匹配成功，则弹出栈顶的非终结符，继续进行分析；否则，分析失败。
- 如果Token序列已经分析完毕，并且栈中只剩下一个非终结符（即开始符号），则分析成功；否则，分析失败。

下面是一个简单的LL(1)文法分析器的伪代码实现：

function LL1Parser(input):
    initialize stack with start symbol
    initialize lookahead with first token
    while stack is not empty:
        if top of stack is non-terminal:
            if LL(1) table contains (top of stack, lookahead):
                replace top of stack with corresponding production rule
                continue parsing
            else:
                report syntax error
                return
        else if top of stack is terminal:
            if top of stack matches lookahead:
                pop top of stack
                read next token from input and set it as lookahead
                continue parsing
            else:
                report syntax error
                return
    if input is fully parsed and stack is empty:
        report success
    else:
        report syntax error

以上就是一个简单的LL(1)文法分析器的实现方法，可以用来解析C语言的基本语法结构。