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calc1.py
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calc1.py
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class Tokens:
INTEGER = 'INTEGER'
PLUS = 'PLUS'
MINUS = 'MINUS'
EOF = 'EOF'
MUL = 'MUL'
DIV = 'DIV'
LPAREN = '('
RPAREN = ')'
OP_LIST = (PLUS, MINUS, MUL, DIV)
class Token:
def __init__(self, type, value):
self.type = type
self.value = value
def __str__(self):
return 'Token({}, {})'.format(self.type, repr(self.value))
def __repr__(self):
return self.__str__()
class Lexer:
def __init__(self, text):
self.text = text
self.pos = 0
self.current_char = self.text[self.pos]
def error(self):
raise Exception('Error parsing input')
def advance(self):
self.pos += 1
if self.pos > len(self.text) - 1:
self.current_char = None
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)
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():
return Token(Tokens.INTEGER, self.integer())
if self.current_char == '+':
self.advance()
return Token(Tokens.PLUS, '+')
if self.current_char == '-':
self.advance()
return Token(Tokens.MINUS, '-')
if self.current_char == '*':
self.advance()
return Token(Tokens.MUL, '*')
if self.current_char == '/':
self.advance()
return Token(Tokens.DIV, '/')
if self.current_char == '(':
self.advance()
return Token(Tokens.LPAREN, '(')
if self.current_char == ')':
self.advance()
return Token(Tokens.RPAREN, ')')
self.error()
return Token(Tokens.EOF, None)
class Interpreter:
def __init__(self, lexer):
self.lexer = lexer
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
if token.type == Tokens.INTEGER:
self.eat(Tokens.INTEGER)
return token.value
elif token.type == Tokens.LPAREN:
self.eat(Tokens.LPAREN)
result = self.expr()
self.eat(Tokens.RPAREN)
return result
def term(self):
result = self.factor()
while self.current_token.type in (Tokens.MUL, Tokens.DIV):
token = self.current_token
if token.type == Tokens.MUL:
self.eat(Tokens.MUL)
result = result * self.factor()
elif token.type == Tokens.DIV:
self.eat(Tokens.DIV)
result = result / self.factor()
return result
def expr(self):
result = self.term()
while self.current_token.type in (Tokens.PLUS, Tokens.MINUS):
token = self.current_token
if token.type == Tokens.PLUS:
self.eat(Tokens.PLUS)
result = result + self.term()
elif token.type == Tokens.MINUS:
self.eat(Tokens.MINUS)
result = result - self.term()
return result
def main():
while 1:
try:
text = input('calc> ')
except EOFError:
break
if not text:
continue
lexer = Lexer(text)
interpreter = Interpreter(lexer)
result = interpreter.expr()
print(result)
if __name__ == '__main__':
main()