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author HackBot
date Wed, 23 Mar 2016 02:40:16 +0000
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1 # -----------------------------------------------------------------------------
2 # calc.py
3 #
4 # A simple calculator with variables. This is from O'Reilly's
5 # "Lex and Yacc", p. 63.
6 #
7 # This example uses unicode strings for tokens, docstrings, and input.
8 # -----------------------------------------------------------------------------
9
10 import sys
11 sys.path.insert(0,"../..")
12
13 tokens = (
14 'NAME','NUMBER',
15 'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
16 'LPAREN','RPAREN',
17 )
18
19 # Tokens
20
21 t_PLUS = ur'\+'
22 t_MINUS = ur'-'
23 t_TIMES = ur'\*'
24 t_DIVIDE = ur'/'
25 t_EQUALS = ur'='
26 t_LPAREN = ur'\('
27 t_RPAREN = ur'\)'
28 t_NAME = ur'[a-zA-Z_][a-zA-Z0-9_]*'
29
30 def t_NUMBER(t):
31 ur'\d+'
32 try:
33 t.value = int(t.value)
34 except ValueError:
35 print "Integer value too large", t.value
36 t.value = 0
37 return t
38
39 t_ignore = u" \t"
40
41 def t_newline(t):
42 ur'\n+'
43 t.lexer.lineno += t.value.count("\n")
44
45 def t_error(t):
46 print "Illegal character '%s'" % t.value[0]
47 t.lexer.skip(1)
48
49 # Build the lexer
50 import ply.lex as lex
51 lex.lex()
52
53 # Parsing rules
54
55 precedence = (
56 ('left','PLUS','MINUS'),
57 ('left','TIMES','DIVIDE'),
58 ('right','UMINUS'),
59 )
60
61 # dictionary of names
62 names = { }
63
64 def p_statement_assign(p):
65 'statement : NAME EQUALS expression'
66 names[p[1]] = p[3]
67
68 def p_statement_expr(p):
69 'statement : expression'
70 print p[1]
71
72 def p_expression_binop(p):
73 '''expression : expression PLUS expression
74 | expression MINUS expression
75 | expression TIMES expression
76 | expression DIVIDE expression'''
77 if p[2] == u'+' : p[0] = p[1] + p[3]
78 elif p[2] == u'-': p[0] = p[1] - p[3]
79 elif p[2] == u'*': p[0] = p[1] * p[3]
80 elif p[2] == u'/': p[0] = p[1] / p[3]
81
82 def p_expression_uminus(p):
83 'expression : MINUS expression %prec UMINUS'
84 p[0] = -p[2]
85
86 def p_expression_group(p):
87 'expression : LPAREN expression RPAREN'
88 p[0] = p[2]
89
90 def p_expression_number(p):
91 'expression : NUMBER'
92 p[0] = p[1]
93
94 def p_expression_name(p):
95 'expression : NAME'
96 try:
97 p[0] = names[p[1]]
98 except LookupError:
99 print "Undefined name '%s'" % p[1]
100 p[0] = 0
101
102 def p_error(p):
103 if p:
104 print "Syntax error at '%s'" % p.value
105 else:
106 print "Syntax error at EOF"
107
108 import ply.yacc as yacc
109 yacc.yacc()
110
111 while 1:
112 try:
113 s = raw_input('calc > ')
114 except EOFError:
115 break
116 if not s: continue
117 yacc.parse(unicode(s))