130242014066+王伟华+第2次实验

#定义加、减、乘号、除、跟结束符
INTEGER, PLUS, MINUS, MUL, DIV, EOF = (
    ‘INTEGER‘, ‘PLUS‘, ‘MINUS‘, ‘MUL‘, ‘DIV‘, ‘EOF‘
)

class Token(object):
    def __init__(self, type, value):
        # 标记的类型: INTEGER, PLUS, MINUS, MUL, DIV, or EOF
        self.type = type
        # 标记的值: 非负整数的值, ‘+‘, ‘-‘, ‘*‘, ‘/‘, 或者无
        self.value = value

    def __str__(self):
        return ‘Token({type}, {value})‘.format(
            type=self.type,
            value=repr(self.value)
        )

    def __repr__(self):
        return self.__str__()

class Lexer(object):
    def __init__(self, text):
        self.text = text
        # self.pos是self.text的索引值
        self.pos = 0
        self.current_char = self.text[self.pos]

    def error(self):
        raise Exception(‘Invalid character‘)

    def advance(self):
        self.pos += 1
        if self.pos > len(self.text) - 1:
            self.current_char = None  # 输入结束
        else:
            self.current_char = self.text[self.pos]

    def skip_whitespace(self):
        while self.current_char is not None and self.current_char.isspace():
            self.advance()

    def integer(self):
        result = ‘‘
        while self.current_char is not None and self.current_char.isdigit():
            result += self.current_char
            self.advance()
        return int(result)
　　

　　#判断输入的是什么

def get_next_token(self):
        while self.current_char is not None:
            if self.current_char.isspace():
                self.skip_whitespace()
                continue
            if self.current_char.isdigit():   #如果current_char是数字
                return Token(INTEGER, self.integer())
            if self.current_char == ‘+‘:     #如果current_char是+
                self.advance()
                return Token(PLUS, ‘+‘)
            if self.current_char == ‘-‘:     #如果current_char是-
                self.advance()
                return Token(MINUS, ‘-‘)
            if self.current_char == ‘*‘:     #如果current_char是*
                self.advance()
                return Token(MUL, ‘*‘)
            if self.current_char == ‘/‘:     #如果current_char是/
                self.advance()
                return Token(DIV, ‘/‘)
            self.error()                     #如果都不符合就返回错误
        return Token(EOF, None)

class Interpreter(object):
    def __init__(self, lexer):
        self.lexer = lexer
        # 将当前标记设置为输入的第一次标记
        self.current_token = self.lexer.get_next_token()

    def error(self):
        raise Exception(‘Invalid syntax‘)

    def eat(self, token_type):
        if self.current_token.type == token_type:
            self.current_token = self.lexer.get_next_token()
        else:
            self.error()

    def factor(self):
        token = self.current_token
        self.eat(INTEGER)
        return token.value

    def term(self):
        result = self.factor()
        while self.current_token.type in (MUL, DIV):
            token = self.current_token
            if token.type == MUL:
                self.eat(MUL)
                result = result * self.factor()
            elif token.type == DIV:
                self.eat(DIV)
                result = result / self.factor()
        return result

    def expr(self):
        result = self.term()
        while self.current_token.type in (PLUS, MINUS):
            token = self.current_token
            if token.type == PLUS:
                self.eat(PLUS)
                result = result + self.term()
            elif token.type == MINUS:
                self.eat(MINUS)
                result = result - self.term()
        return result

def main():
    while True:
        try:
            text = raw_input(‘calc> ‘)
        except EOFError:
            break
        if not text:
            continue
        lexer = Lexer(text)
        interpreter = Interpreter(lexer)
        result = interpreter.expr()
        print(result)

if __name__ == ‘__main__‘:
    main()