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