#
# simpleArith.py
#
# Example of defining an arithmetic expression parser using
# the operatorPrecedence helper method in pyparsing.
#
# Copyright 2006, by Paul McGuire
#
from pyparsing import *
integer = Word(nums).setParseAction(lambda t:int(t[0]))
variable = Word(alphas,exact=1)
operand = integer | variable
expop = Literal('^')
signop = oneOf('+ -')
multop = oneOf('* /')
plusop = oneOf('+ -')
factop = Literal('!')
# To use the operatorPrecedence helper:
# 1. Define the "atom" operand term of the grammar.
# For this simple grammar, the smallest operand is either
# and integer or a variable. This will be the first argument
# to the operatorPrecedence method.
# 2. Define a list of tuples for each level of operator
# precendence. Each tuple is of the form
# (opExpr, numTerms, rightLeftAssoc, parseAction), where
# - opExpr is the pyparsing expression for the operator;
# may also be a string, which will be converted to a Literal
# - numTerms is the number of terms for this operator (must
# be 1 or 2)
# - rightLeftAssoc is the indicator whether the operator is
# right or left associative, using the pyparsing-defined
# constants opAssoc.RIGHT and opAssoc.LEFT.
# - parseAction is the parse action to be associated with
# expressions matching this operator expression (the
# parse action tuple member may be omitted)
# 3. Call operatorPrecedence passing the operand expression and
# the operator precedence list, and save the returned value
# as the generated pyparsing expression. You can then use
# this expression to parse input strings, or incorporate it
# into a larger, more complex grammar.
#
expr = operatorPrecedence( operand,
[("!", 1, opAssoc.LEFT),
("^", 2, opAssoc.RIGHT),
(signop, 1, opAssoc.RIGHT),
(multop, 2, opAssoc.LEFT),
(plusop, 2, opAssoc.LEFT),]
)
test = ["9 + 2 + 3",
"9 + 2 * 3",
"(9 + 2) * 3",
"(9 + -2) * 3",
"(9 + -2) * 3^2^2",
"(9! + -2) * 3^2^2",
"M*X + B",
"M*(X + B)",
"1+2*-3^4*5+-+-6",]
for t in test:
print t
print expr.parseString(t)
print
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