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#######################################################################
# Name: calc.py
# Purpose: Simple expression evaluator example
# Author: Igor R. Dejanovic <igor DOT dejanovic AT gmail DOT com>
# Copyright: (c) 2009-2014 Igor R. Dejanovic <igor DOT dejanovic AT gmail DOT com>
# License: MIT License
#
# This example demonstrates grammar definition using python constructs as
# well as using semantic actions to evaluate simple expression in infix
# notation.
#######################################################################
from __future__ import unicode_literals, print_function
try:
text=unicode
except:
text=str
from arpeggio import Optional, ZeroOrMore, OneOrMore, EOF, \
ParserPython, PTNodeVisitor, visit_parse_tree
from arpeggio import RegExMatch as _
def number(): return _(r'\d*\.\d*|\d+')
def factor(): return Optional(["+","-"]), [number,
("(", expression, ")")]
def term(): return factor, ZeroOrMore(["*","/"], factor)
def expression(): return term, ZeroOrMore(["+", "-"], term)
def calc(): return OneOrMore(expression), EOF
class CalcVisitor(PTNodeVisitor):
def visit_number(self, node, children):
"""
Converts node value to float.
"""
if self.debug:
print("Converting {}.".format(node.value))
return float(node.value)
def visit_factor(self, node, children):
"""
Removes parenthesis if exists and returns what was contained inside.
"""
if self.debug:
print("Factor {}".format(children))
if len(children) == 1:
return children[0]
sign = -1 if children[0] == '-' else 1
return sign * children[-1]
def visit_term(self, node, children):
"""
Divides or multiplies factors.
Factor nodes will be already evaluated.
"""
if self.debug:
print("Term {}".format(children))
term = children[0]
for i in range(2, len(children), 2):
if children[i-1] == "*":
term *= children[i]
else:
term /= children[i]
if self.debug:
print("Term = {}".format(term))
return term
def visit_expression(self, node, children):
"""
Adds or substracts terms.
Term nodes will be already evaluated.
"""
if self.debug:
print("Expression {}".format(children))
expr = 0
start = 0
# Check for unary + or - operator
if text(children[0]) in "+-":
start = 1
for i in range(start, len(children), 2):
if i and children[i - 1] == "-":
expr -= children[i]
else:
expr += children[i]
if self.debug:
print("Expression = {}".format(expr))
return expr
def main(debug=False):
# First we will make a parser - an instance of the calc parser model.
# Parser model is given in the form of python constructs therefore we
# are using ParserPython class.
parser = ParserPython(calc, debug=debug)
# An expression we want to evaluate
input_expr = "-(4-1)*5+(2+4.67)+5.89/(.2+7)"
# We create a parse tree out of textual input_expr
parse_tree = parser.parse(input_expr)
result = visit_parse_tree(parse_tree, CalcVisitor(debug=debug))
# visit_parse_tree will start semantic analysis.
# In this case semantic analysis will evaluate expression and
# returned value will be evaluated result of the input_expr expression.
print("{} = {}".format(input_expr, result))
if __name__ == "__main__":
# In debug mode dot (graphviz) files for parser model
# and parse tree will be created for visualization.
# Checkout current folder for .dot files.
main(debug=True)