On Jan 2, 2008 12:06 AM, Adarsh Srivastava
<[EMAIL PROTECTED]> wrote:
>
> Hello,
>
> Thanks a lot Chas, Jenda , Venkat...the solution (eval)works fine, now
> that I have implemented it. There seem to be some issues though:
>
> 1. Perl doest seem to catch errors like divide-by-zero error. Eg: for an
> input expression like 99 / 0, it simply displays nothing as output. (no
> errors thrown).
snip
Not true. If you aren't seeing the errors then you aren't checking $@
like I did in my example. Exceptions (die, croak, divide-by-zero,
etc) thrown by a chunk of code in an eval (string or block) do not
cause the program to halt, but it do set $@ to the error message that
would have been sent. Anytime you do an eval you should check the
status of this variable:
eval $expr;
die "got error [EMAIL PROTECTED] eval'ing [$expr]" if $@;
snip
> 2. Typecast variables are considered to be erroneous inputs. Eg: an
> input like ' (int)var1' in the expression is not evaluated as a typecast
> variable.
>
> Is designing a separate parser for this the only solution that I have?
snip
As I said before, if the expression you want to execute is not a Perl
expression you can't eval it. Perl doesn't do typecasting (since it
has no types*), so the language is definitely not Perl. You should sit
down and figure out what language is being used for your expressions.
After you know all of the valid inputs you can determine if that
language can be easily transformed into Perl. If it can't be
transformed into Perl with a couple of simple regexes you are better
off writing a parser. If it comes to that, take a look at
Parse::RecDescent**. It might also be possible that the equations
were meant for a program you can call from Perl. For instance, here
is how Perl can wrap the standard UNIX command bc:
#!/usr/bin/perl
use strict;
use warnings;
use IPC::Open3;
my ($in, $out, $err);
my $pid = open3($in, $out, $err, "bc");
while (my $line = <>) {
print $in $line; #send the expression to bc
chomp(my $result = <$out>); #get bc's answer
print "$result\n";
}
snip
> P.S- If at all some kind of disclaimer is coming is coming with these
> mails of mine, please excuse me for that. I am trying to get them
> removed but they keep attaching automatically.
snip
You should consider using a different address for use with mailing
lists then. As you can probably tell, I am partial to Gmail***.
* at least not like ANSI C has types
** http://search.cpan.org/dist/Parse-RecDescent/lib/Parse/RecDescent.pm
*** https://www.google.com/accounts/NewAccount?service=mail
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