Using dynamic operators in an if() statement

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

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Re: Using dynamic operators in an if() statement by sgifford (Prior) on Aug 22, 2006 at 04:57 UTC
I think what you're looking for is a string `eval`, which would be something like this: `if (eval "\$consumption $operator")` [download] But I wouldn't recommend you do that. It means any errors in `$operator` will be runtime instead of compile-time errors, and that code will be re-compiled every time through your loop, which is bad for performance. Instead, consider customizing your condition with a small sub, something like: `sub process_match { my($consumption)=@_; return $consumption > 98.0; } if (process_match($consumption)) { do something; }` [download] This gives you the same flexibility without any of the downsides of string `eval`. Good luck! Update: `$consumption` should be escaped in `eval`. Thanks, ikegami! -- sgifford's Web page	[reply] [d/l] [select]
Re^2: Using dynamic operators in an if() statement by VSarkiss (Monsignor) on Aug 22, 2006 at 15:06 UTC
Yes, this is a very good approach. To expand on it, you could even dispatch to multiple selection functions based on other criteria (the OP doesn't make clear what the criteria are, but hints at them). Something like this (untested) example: `my %match = ( high => sub { $_[0] > 98.0 }, low => sub { $_[0] < 55.0 }, exact => sub { $_[0] = 77.0 }, ); # ...later... my $test = decide_somehow(); if ($match{$test}->($consumption)) { print "Test $test succeeded\n"; do_something(); };` [download] There are other ways to generalize the approach; this is just an example that may or may not be appropriate, depending on other program constraints. Do not rebuke them with harsh words ... but rather lead them gently - with URLs - so that they may learn wisdom.	[reply] [d/l]
Re: Using dynamic operators in an if() statement by GrandFather (Saint) on Aug 22, 2006 at 04:37 UTC
Why not simply: `... use constant THRESHOLD => 98.0; ... if ($consumption > THRESHOLD) { # do something }` [download] DWIM is Perl's answer to Gödel	[reply] [d/l]
Re: Using dynamic operators in an if() statement by Zaxo (Archbishop) on Aug 22, 2006 at 04:55 UTC
I think you're trying to remember, `my $operator = '> 98.0'; if ( eval $consumption . $operator ) { }` [download] I think that absorbing the operator into a string to be eval'd doesn't get you anything you want. I'd prefer, `sub LIMIT () { 98.0 } if ($consumption > LIMIT) { }` [download] After Compline, Zaxo	[reply] [d/l] [select]
Re^2: Using dynamic operators in an if() statement by ikegami (Patriarch) on Aug 22, 2006 at 05:31 UTC
Shouldn't that be `if ( eval "\$consumption $operator" )` [download] I doubt the existance of a need to stringify and evaluate the contents of `$consumption`.	[reply] [d/l] [select]
Re: Using dynamic operators in an if() statement by aquarium (Curate) on Aug 22, 2006 at 12:57 UTC
reason why it doesn't work is that '> 98.0' is a string...not an operator and a value, unless you eval it. Therefore the "if" statement sees a list of two strings, and evaluates the list, which returns number of elements in list in scalar contenxt, which is equal to "2". "2" means TRUE in perl so the "if" always calls the sub. if the operator is always ">" then why on earth would you try to codify it into a variable? There's no need to be fancy, so just code it simple...and we can all go home :) `if($consumption > $threshold) {do something;}` [download] the hardest line to type correctly is: stty erase ^H	[reply] [d/l]


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