math syntax checker written in python

All I need is to check, using python, if a string is a valid math expression or not.

For simplicity let's say I just need + - * / operators (+ - as unary too) with numbers and nested parenthesis. I add also simple variable names for completeness.

So I can test this way:

test("-3 * (2 + 1)") #valid
test("-3 * ")        #NOT valid

test("v1 + v2")      #valid
test("v2 - 2v")      #NOT valid ("2v" not a valid variable name)

I tried pyparsing but just trying the example: "simple algebraic expression parser, that performs +,-,*,/ and ^ arithmetic operations" I get passed invalid code and also trying to fix it I always get wrong syntaxes being parsed without raising Exceptions

just try:

>>>test('9', 9)
9 qwerty = 9.0 ['9'] => ['9']
>>>test('9 qwerty', 9)
9 qwerty = 9.0 ['9'] => ['9']

both test pass... o_O

Any advice?

python math syntax-checking

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You could try building a simple parser yourself to tokenize the string of the arithmetic expression and then build an expression tree, if the tree is valid (the leaves are all operands and the internal nodes are all operators) then you can say that the expression is valid.

The basic concept is to make a few helper functions to create your parser.

def extract() will get the next character from the expression
def peek() similar to extract but used if there is no whitespace to check the next character
get_expression()
get_next_token()

Alternatively if you can guarantee whitespace between characters you could use split() to do all the tokenizing.

Then you build your tree and evaluate if its structured correctly

Try this for more info: http://effbot.org/zone/simple-top-down-parsing.htm

answered 2011-02-03T15:21:58.857

math syntax checker written in python

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