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  1. import re
  2. from collections import namedtuple
  3. from fractions import Fraction
  4.  
  5. Token = namedtuple('Token', ['type', 'value'])
  6.  
  7. NUM_RE = r'(?P<NUM>\d+(\.\d*)?)'
  8. WS_RE = r'(?P<WS>\s+)'
  9. MUL_RE = r'(?P<MUL>\*)'
  10. DIV_RE = r'(?P<DIV>/)'
  11. PLUS_RE = r'(?P<PLUS>\+)'
  12. MINUS_RE = r'(?P<MINUS>-)'
  13. LPAREN = r'(?P<LPAREN>\()'
  14. RPAREN = r'(?P<RPAREN>\))'
  15.  
  16. REGEXP = re.compile('|'.join([NUM_RE, WS_RE, MUL_RE, DIV_RE, PLUS_RE, MINUS_RE,
  17.                              LPAREN, RPAREN]))
  18.  
  19.  
  20. class Evaluator(object):
  21.     def tokenize(self, s):
  22.         scanner = REGEXP.scanner(s)
  23.         for tok in iter(scanner.match, None):
  24.             if tok.lastgroup != 'WS':  # skip whitespaces
  25.                 yield Token(tok.lastgroup, tok.group())
  26.  
  27.     def eval(self, s):
  28.         self.tokens = self.tokenize(s)
  29.         self.current_t = None
  30.         self.next_t = None
  31.         self.advance()
  32.         return self.expr()
  33.  
  34.     def advance(self):
  35.         self.current_t, self.next_t = self.next_t, next(self.tokens, None)
  36.  
  37.     def accept(self, tp):
  38.         if self.next_t and self.next_t.type == tp:
  39.             self.advance()
  40.             return True
  41.         else:
  42.             return False
  43.  
  44.     def expect(self, tp):
  45.         if not self.accept(tp):
  46.             error_msg = "Expected {}, got {}".format(tp, self.next_t and self.next_t.value)
  47.             raise SyntaxError(error_msg)
  48.  
  49.     def expr(self):
  50.         left = self.term()
  51.         while self.accept('PLUS') or self.accept('MINUS'):
  52.             if self.current_t.type == 'PLUS':
  53.                 right = self.term()
  54.                 left += right
  55.             else:
  56.                 right = self.term()
  57.                 left -= right
  58.         return left
  59.  
  60.     def term(self):
  61.         left = self.factor()
  62.         while self.accept('MUL') or self.accept('DIV'):
  63.             if self.current_t.type == 'MUL':
  64.                 left *= self.factor()
  65.             else:
  66.                 left /= self.factor()
  67.         return left
  68.  
  69.     def factor(self):
  70.         if self.accept('MINUS') and self.accept('NUM'):
  71.             return -1 * Fraction(self.current_t.value)
  72.         elif self.accept('NUM'):
  73.             return Fraction(self.current_t.value)
  74.         else:
  75.             self.expect('LPAREN')
  76.             exp = self.expr()
  77.             self.expect('RPAREN')
  78.             return exp
  79.  
  80.  
  81. # tests
  82. def test():
  83.     suit = {'2 + 2 * 2': 6,
  84.             '(5 * 2) / 4': Fraction("5/2"),
  85.             '1 + 1 + 1': 3,
  86.             '3 * 4 / 2': 6,
  87.             '-3 * (5 + 0)': -15,
  88.             '3 * (5 + )': 15,
  89.             '2.5 / -5': Fraction("-1/2")}
  90.     ev = Evaluator()
  91.     for expr, res in suit.iteritems():
  92.         try:
  93.             print "{} -> {}, expected {}".format(expr, ev.eval(expr), res)
  94.         except SyntaxError as e:
  95.             print "{} failed with: {}".format(expr, e)
  96.  
  97. if __name__ == '__main__':
  98.     test()
Success #stdin #stdout 0.08s 12488KB
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stdin
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Standard input is empty
stdout
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3 * 4 / 2 -> 6, expected 6
2 + 2 * 2 -> 6, expected 6
-3 * (5 + 0) -> -15, expected -15
2.5 / -5 -> -1/2, expected -1/2
3 * (5 + ) failed with: Expected LPAREN, got )
1 + 1 + 1 -> 3, expected 3
(5 * 2) / 4 -> 5/2, expected 5/2
https://ideone.com/YzGnNP
language:
Python (cpython 2.7.16)
created:
11 years ago
visibility:
public

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