Beefy Boxes and Bandwidth Generously Provided by pair Networks
No such thing as a small change
 
PerlMonks  

Re: eval question

by tsee (Curate)
on Apr 17, 2007 at 15:46 UTC ( #610562=note: print w/ replies, xml ) Need Help??


in reply to eval question

Others have suggested using Parse::RecDescent to create a parser for arithmetic expressions. That's all good advice, but you may want something less involved that writing your own parser. Maybe you should have a look at the Math::Symbolic module on CPAN. (Caveat: I'm the author.)

You could do this: (untested)

use Math::Symbolic qw/parse_from_string/; my $ms; eval { $ms = parse_from_string($expression) }; if ($@) {...} # catch parse errors my $value; eval { $value = $ms->value() }; if ($@) {...} # catch execution errors

The above code might break because Math::Symbolic supports variables - which have no value by default. So you can check that the user didn't use any variables in his expression before calling value():

# parse... unless ( $ms->is_constant() ) { # complain about variables in the tree } my $value; eval { $value = $ms->value() }; if ($@) {...} # catch execution errors

If you have to evaluate the expressions very often, you will find that calling ->value() is slow. (It walks the expression tree.) In that case, you can compile the tree to Perl code. You'd do that with ->to_sub():

# parse... # note the parens for list context my ($coderef) = $ms->to_sub(); my $value = $coderef->();

The module can do much more than that, of course. For something fancy, you can check out an experiment of mine, which is unfortunately rather undocumented: Math::SymbolicX::Calculator::Interface::Web. It's a feature-starved AJAX-enabled symbolic calculator with a worksheet-ish appearance. That's just a reference for your entertainment, though. Don't ever think of using that.

Admittedly, if I was sticking to my promise of this being less complex, I should have stopped after the second code snipped. :)

Cheers,
Steffen


Comment on Re: eval question
Select or Download Code

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://610562]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others about the Monastery: (7)
As of 2015-07-08 00:07 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









    Results (93 votes), past polls