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

My test code is like this:

#!/usr/bin/perl
$code = 'if($a>$b){';
$code .= '$result = 1;}';

$a =1;
$b = 0;
$result =0;

eval $code;
print $@ if ($@);
print $result;

[download]
There are no information displayed on the screen. What's wrong with the code ?
thanks!

Update

To bmann:

yes,it is right.But I have another question:

#!/usr/bin/perl
$code  = 'if($port_a $cmp_op $port_b){$result=1;}';
$code .= 'else{$result=0;}';
$port_a =7777;
$port_b = 6666;
$cmp_op = ">";
eval $code;
print $@ if($@);
print $result; 
$perl eval
Scalar found where operator expected at (eval 1) line 1, near "$port_a
+ $cmp_op"
        (Missing operator before $cmp_op?)

[download]

So I can't use eval like that which I must specify operator not scalar,is it true ? Are there any solution to do this ? thanks!

Replies are listed 'Best First'.
Re: How to use eval?
by matija (Priest) on Apr 09, 2004 at 07:30 UTC
    There is nothing wrong with the code.

    When I try it, it prints "1" to the screen, just as it should.

    Now, if you had said print "$result\n"; at the end, that 1 would be on a line of it's own, and more easily visible - which is what I think your problem was.

[reply]
[d/l]
      $ more eval
#!/usr/bin/perl
$code = 'if($a>$b){ $result =1} ';

$a =1;
$b = 0;
$result =0;

eval $code;
print "eval error\n" if($@);
print $result;
$ eval 
$ perl -v

This is perl, v5.8.2 built for sun4-solaris

Copyright 1987-2003, Larry Wall

      [download]
      You can see what's happened on my computer,there are nothing .
      Which perl version do you use ?Maybe it is a bug?
      thanks!
[reply]
[d/l]
        eval is a shell builtin. Just rename your script or run it with perl eval.
[reply]
[d/l]
[select]
Re: How to use eval?
by BUU (Prior) on Apr 09, 2004 at 10:26 UTC
    (Addressing the update) You are litterally trying to execute the code if($port_a $cmp_op $port_B) and that isn't valid perl syntax. A quick fix would probably be to just define $port_a,$cmp_op and $port_b before you initialize $code and then just do $code = "if($port_a $cmp_op $port_b)" which will work.
[reply]
[d/l]
[select]

      It didn't work for me. Despite a number of variations I always got this error message:

      Scalar found where operator expected at (eval 1) line 1, near "$port_a
+ $cmp_op"
        (Missing operator before $cmp_op?)
Scalar found where operator expected at (eval 1) line 1, near "$cmp_op
+ $port_b"
        (Missing operator before $port_b?)

      [download]

      If I understand correctly, the eval is having trouble with the '>' but so far I haven't figured out how to fix it.

      xenchu

      That's about all there is to it, except for everything else. -<b>Programming Perl</b> (p.346)
[reply]
[d/l]
[select]
        Comment out the line with 'eval', and print it instead 

        #eval $code;
print $code;

        [download]

        You might be surprised at the value of $code.

        I'm sure you already know this, but I'll repeat it anyway - double quotes interpolate contained variables, single quotes do not. perl is trying to eval if ($port_a $cmp_op $port_b)... which is destined to fail - it won't even compile. In order to evaluate it properly, $code needs to to contain if ($port_a < $port_b)....

        I'll leave it for you to construct the string properly ;) Once you do, comment the print and uncomment the eval.

        BTW, what are you trying to accomplish? Could there be a better way?

        Update: BTW, I didn't see your update until after posting this node. I assume most would see it easier if you posted a response instead the update.

        Update #2 Update #1 is directed at the OP, not xenchu

[reply]
[d/l]
[select]
Re: How to use eval?
by Anonymous Monk on Apr 10, 2004 at 03:42 UTC
    Please don't use updates to ask new questions, you're not carrying on a conversation.
[reply]

Back to Seekers of Perl Wisdom