using eval

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Ok, I’m an extreme newb, I don’t use my Perl projects on a server for anyone other then myself and I honestly usually just get by with my "while" and "if" statements

However, I’ve run into a problem that if and while statements would be very messy at.

say I have a variable like $foobar and $foobar holds x^4-3 or even x+3 I’m wondering if there’s a way I can have Perl evaluate this and give me the result... I think it has to do with the eval function but I tried this simple code and it didn’t work...

$question="4+3";

eval {$answer=$question;};
print "$question=$answer";
[download]

the result is 4+3=4+3... not exactly what I need...

Any and all help would be greatly appreciated, also I need it to do it using bedmas, I don’t know if it does by default or not

Edit by castaway added code tags

Comment on using eval Download Code

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Re: using eval by Somni (Friar) on May 26, 2004 at 02:09 UTC
The eval you're using, `eval { ... };` is called a block eval. It catches exceptions (die statements), it does not evaluate the contents of the variables as Perl code. The eval you want is a string eval, `$answer = eval $question;`. You should also realize that these must be valid Perl expressions, and they use the Perl operators. `x^4-3` xors the string "x" with 4, then subtracts 3; you probably meant `$x ** 4 - 3`, but that requires $x be defined. For further details please see eval and perlop. I also have a script that evaluates code entered in on the command line called math.	[reply] [d/l] [select]
Re: using eval by mifflin (Curate) on May 26, 2004 at 00:11 UTC
this program `$y = 4; $x = '$z = $y**3+3'; eval $x; print $z;` [download] will print out 67. Is this an example of what you are talking about?	[reply] [d/l]
Re: using eval by eclark (Scribe) on May 26, 2004 at 02:15 UTC
Without getting into why this is bad, I'll just fix your sample so it works `$question="4+3"; $answer = eval $question; print "$question=$answer";` [download]	[reply] [d/l]
Re: using eval by Trag (Monk) on May 26, 2004 at 01:13 UTC
I'm not sure how to solve your problem, but = is the assignment operater. You are telling Perl that $answer should be the same as $question. == is the "equal to" operater. Also, your code calls eval but then does nothing with it. You should have $foo = eval {$answer=$question}; print $foo;. BTW, you only need one ; in the second line. `our @item = reverse (114, 101, 107, 99, 97, 104, 32, 108, 114, 101, 80, 32, 114, 101, 104, 116, 111, 110, 97, 32, 116, 115, 117, + 74); local $my = reverse ")meti@\ ,rhc (pam tnirp";eval $my;` [download]	[reply] [d/l]
Re: using eval by sleepingsquirrel (Chaplain) on May 26, 2004 at 15:05 UTC
`$question='$x**4-3'; $s="\$answer=$question"; $x=2; eval $s; print "\$answer=$answer\n";` [download]	[reply] [d/l]
Re: using eval by Anonymous Monk on May 28, 2004 at 18:59 UTC
Try this indeed my $question = "4+3"; eval "\$answer = $question";	[reply]


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