Turning a string into an programatic mathematical expression

John Hunter jdhunter at ace.bsd.uchicago.edu
Wed Oct 20 04:17:20 CEST 2004

>>>>> "Daniel" == Daniel Bickett <dbickett at gmail.com> writes:

    Daniel> The title really says it all. I'm trying to take input
    Daniel> from a user (intended to be a mathematical expression),
    Daniel> from a text box for example, and evaluate it
    Daniel> mathematically within the program. For clarification: the
    Daniel> user inputs the string "4*5(3-3)", I would be interested
    Daniel> in a straight-forward way to find the result of that,
    Daniel> based only on a string. The follow-up question would be
    Daniel> how to incorporate variables into the mix, however I'll
    Daniel> leave it at that for now. Thanks for your time :)

Depending on what sort of grammar you want to require for input
strings, it can be as simple as

  >>> s = "4*5*(3-3)"
  >>> eval(s)

Although it may not appear so at first glance, I think you are
describing a proposed solution to your problem and not the problem you
are really trying to solve.  If you restate the problem you are trying
to solve, you may get better answers.  From my read of your question,
you're on the road to writing a mathematical expression parser.  Note
that python has an expression parser built in, and it gives you access
to that parser in www.python.org/doc/current/lib/module-parser.html;,
by reusing the python parser you may save yourself some labor.

If you are interested in numeric rather than symbolic processing of
the mathematical expressions, the weave module, via blitz expression
templates, does something quite close.  You give it strings like
'a*(b+c)' where a, b and c are numeric arrays, and it will use blitz
expression templates to process this in a single loop without the
creation of temporaries and multiple loops that such expressions
generally entail - www.scipy.org/documentation/weave .

The book "Scientific and Engineering C++" by Barton and Nackman
describes how to implement a mathematical expression parser that
supports variables, binary operations, etc.


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