calc.py revision 6498 
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
10import sys
11sys.path.insert(0,"../..")
12
13tokens = (
14    'NAME','NUMBER',
15    'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
16    'LPAREN','RPAREN',
17    )
18
19# Tokens
20
21t_PLUS    = ur'\+'
22t_MINUS   = ur'-'
23t_TIMES   = ur'\*'
24t_DIVIDE  = ur'/'
25t_EQUALS  = ur'='
26t_LPAREN  = ur'\('
27t_RPAREN  = ur'\)'
28t_NAME    = ur'[a-zA-Z_][a-zA-Z0-9_]*'
29
30def 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
39t_ignore = u" \t"
40
41def t_newline(t):
42    ur'\n+'
43    t.lexer.lineno += t.value.count("\n")
44
45def t_error(t):
46    print "Illegal character '%s'" % t.value[0]
47    t.lexer.skip(1)
48
49# Build the lexer
50import ply.lex as lex
51lex.lex()
52
53# Parsing rules
54
55precedence = (
56    ('left','PLUS','MINUS'),
57    ('left','TIMES','DIVIDE'),
58    ('right','UMINUS'),
59    )
60
61# dictionary of names
62names = { }
63
64def p_statement_assign(p):
65    'statement : NAME EQUALS expression'
66    names[p[1]] = p[3]
67
68def p_statement_expr(p):
69    'statement : expression'
70    print p[1]
71
72def 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
82def p_expression_uminus(p):
83    'expression : MINUS expression %prec UMINUS'
84    p[0] = -p[2]
85
86def p_expression_group(p):
87    'expression : LPAREN expression RPAREN'
88    p[0] = p[2]
89
90def p_expression_number(p):
91    'expression : NUMBER'
92    p[0] = p[1]
93
94def 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
102def p_error(p):
103    if p:
104        print "Syntax error at '%s'" % p.value
105    else:
106        print "Syntax error at EOF"
107
108import ply.yacc as yacc
109yacc.yacc()
110
111while 1:
112    try:
113        s = raw_input('calc > ')
114    except EOFError:
115        break
116    if not s: continue
117    yacc.parse(unicode(s))
118