109 lines
2.2 KiB
Python
109 lines
2.2 KiB
Python
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# -----------------------------------------------------------------------------
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# calc.py
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#
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# A simple calculator with variables. This is from O'Reilly's
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# "Lex and Yacc", p. 63.
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# -----------------------------------------------------------------------------
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tokens = (
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'NAME','NUMBER',
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'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
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'LPAREN','RPAREN',
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)
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# Tokens
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t_PLUS = r'\+'
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t_MINUS = r'-'
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t_TIMES = r'\*'
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t_DIVIDE = r'/'
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t_EQUALS = r'='
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t_LPAREN = r'\('
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t_RPAREN = r'\)'
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t_NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
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def t_NUMBER(t):
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r'\d+'
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try:
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t.value = int(t.value)
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except ValueError:
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print "Integer value too large", t.value
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t.value = 0
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return t
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t_ignore = " \t"
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def t_newline(t):
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r'\n+'
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t.lineno += t.value.count("\n")
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def t_error(t):
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print "Illegal character '%s'" % t.value[0]
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t.skip(1)
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# Build the lexer
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import lex
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lex.lex()
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# Parsing rules
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precedence = (
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('left','PLUS','MINUS'),
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('left','TIMES','DIVIDE'),
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('right','UMINUS'),
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)
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# dictionary of names
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names = { }
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def p_statement_assign(t):
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'statement : NAME EQUALS expression'
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names[t[1]] = t[3]
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def p_statement_expr(t):
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'statement : expression'
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print t[1]
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def p_expression_binop(t):
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'''expression : expression PLUS expression
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| expression MINUS expression
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| expression TIMES expression
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| expression DIVIDE expression'''
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if t[2] == '+' : t[0] = t[1] + t[3]
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elif t[2] == '-': t[0] = t[1] - t[3]
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elif t[2] == '*': t[0] = t[1] * t[3]
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elif t[2] == '/': t[0] = t[1] / t[3]
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def p_expression_uminus(t):
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'expression : MINUS expression %prec UMINUS'
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t[0] = -t[2]
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def p_expression_group(t):
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'expression : LPAREN expression RPAREN'
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t[0] = t[2]
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def p_expression_number(t):
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'expression : NUMBER'
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t[0] = t[1]
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def p_expression_name(t):
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'expression : NAME'
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try:
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t[0] = names[t[1]]
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except LookupError:
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print "Undefined name '%s'" % t[1]
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t[0] = 0
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def p_error(t):
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print "Syntax error at '%s'" % t.value
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import yacc
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yacc.yacc()
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while 1:
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try:
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s = raw_input('calc > ')
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except EOFError:
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break
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yacc.parse(s)
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