it works if i use join and ""

It works as in how? It parses? Yes, of course it will parse, what it will not do is give you a valid answer. I could join( "", 2, $eq, 1 ) all day long, but that doesn't mean 2 equals 1! The only reason that is "true" is because the value is "2 eq 1" (the string), which is not undefined or "", so it evaluates to true.

Depending on what you want to do, you could just make a sub that contains your operator. That way, you can avoid an 'eval'.

while (<>) {
    $num ="2";
    $eq = sub { $_[0] == $_[1] };
    /(\d+).*/;
    if ($eq->( $num, $1 ) ) {
        print "match\n";
    }
}
[download]

my $eq = sub { $_[0] == $_[1] };
my $lt = sub { $_[0] <  $_[1] };
# ...

while (<>) {
    $num ="2";
    if ( ...some logic... ) {
         $op = $eq;
    }
    else {
         $op = $lt;
    }
    $eq = sub { $_[0] == $_[1] };
    /(\d+).*/;
    if ($op->( $num, $1 ) ) {
        print "match\n";
    }
}
[download]

    -Bryan

thanks for that, however, could you explain the line

 
sub { $_[0] == $_[1] }
[download]

what is this doing?

CODE tags added by Arunbear

Sure, I could do that for you.

Read more... (2 kB)

Also, check out perlsub for a better explanation.

    -Bryan

Alternatively, you can use a dispatch table. It's very similar to the solution already provided, except that $op can now be displayed and serialized.

my %ops = (
   '==' => sub { $_[0] == $_[1] },
   '<'  => sub { $_[0] <  $_[1] },
);

...

if (...) {
   $op = '==';
} else {
   $op = '<';
}

...

if ($ops{$op}->($num1, $num2)) {
   print("$num1 $op $num2\n";
} else {
   print("$num1 not $op $num2\n";
}
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sub _eq($$){ $_[0] == $_[1] ? 1 : 0 }
sub _ne($$){ $_[0] == $_[1] ? 0 : 1 }
my $op = \&_eq; 
print $op->( 1, 2 ) ? 'EQ' : 'NEQ'";
[download]

You shouldn't use prototypes unless necessary, because of their side-effects. In this case, the prototype isn't even being checked because you're using a reference to the function.

Read more... (382 Bytes)

And why bother with ?1:0 and ?0:1 since == already returns a boolean value.

You are correct that the code isn't using the prototypes or requires the 1:0. I used them both, however, to be very clear to our anonymous reader what it was the code was doing. Also, I used 1:0 because I don't like using undef as a boolean false; it's not relevant here, but I find that 0 as false makes debugging easier which has led to this habit.

For those curious, this is how I might do it using tie.
Read more... (2 kB)

if ( eval ( "$num $eq $1") ) {
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It is worth mentioning that one shouldn't eval user-supplied strings until they have been checked for safety (untainted). If a user is supplying the value for $eq, they could supply Perl code that would be executed: obviously, that's potentially dangerous.

<-radiant.matrix->
Larry Wall is Yoda: there is no try{} (ok, except in Perl6; way to ruin a joke, Larry! ;P)
The Code that can be seen is not the true Code

$num = 2;
while (<>) {
    if ( /(\A|\D)$num(\D|\Z)/ ) {
        print "match\n";
    }
}
[download]

If you have rules for what surrounds your $num value in the input string, you may better limit your search.