Using scalar values as conditionals

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

This isn't a question about how something works, but how/why it's used. In "Learning Perl" (my new favorite book), this appears in the section about the "if" control structure:

You may actually use any scalar value as the conditional of the if control structure. That's handy if you want to store a true or false value into a variable, like this:
$is_bigger=$name gt 'fred';
if ($is_bigger) {...}
[download]

I understand how that works, but I'm curious as to how people use it, and why it would be used instead of

if ($name gt 'fred') {...}
[download]

I want to train myself to use good coding practices, but also to understand them, so I'd appreciate any thoughts on using this particular one.

__________
"Abby-somebody. Abby-normal."
—Young Frankenstein

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Re: Using scalar values as conditionals by Abigail-II (Bishop) on May 28, 2002 at 16:50 UTC
You are focussing to much on the details of the example. Don't forget this example comes from a book. Books are small. Code fragments in books need to be even smaller. In a real program, the `$name gt 'fred'` could be a long, expensive expression, whose value is needed more than once. Or the comparison between `$name` and `'fred'` and `if` statement has some code between it, code that may, or may not modify `$name`. Abigail	[reply] [d/l] [select]
Thanks, Re: Using scalar values as conditionals by amarceluk (Beadle) on May 28, 2002 at 17:02 UTC
Yes, very good points. In particular, I hadn't thought about there being code that might modify $name. Thank you! I guess I do focus too much on the details because I'm still at an early stage of learning and I try very hard to understand exactly how everything works, but don't have enough knowledge/imagination to see how the example being demonstrated might be used in real code. Hopefully as I write more real code it'll be easier to understand these things! __________ "Abby-somebody. Abby-normal." —Young Frankenstein	[reply]
Re: Using scalar values as conditionals by samtregar (Abbot) on May 28, 2002 at 18:43 UTC
It's unusual to save the value of a comparison, but it is very common to use a variable in an if statement. Consider this code: `# get office_id for this office my $office_id = get_office_id(); if ($office_id) { # found an office } else { # no office found }` [download] The hypothetical `get_office_id()` function could be written in such a way that the office id returned will either be a positive integer or undef if the office id cannot be found. Then the `$office_id` can be used directly in an `if` to test to result. Of course, this will be wrong if 0 is a vaild office id. Then you need to do: `# get office_id for this office my $office_id = get_office_id(); if (defined $office_id) { # found an office } else { # no office found }` [download] Although this is a common pattern it is actually not the best way to write functions in Perl. A better way is to separate the status code from the return value: `# get office_id for this office my ($ok, $office_id) = get_office_id(); if ($ok) { # found an office } else { # no office found }` [download] That way there can be no confusion between the office id itself and the success of the call. -sam	[reply] [d/l] [select]
Re: Using scalar values as conditionals by Biker (Priest) on May 29, 2002 at 07:39 UTC
"...and why it would be used..." Consider the situation where it's expensive to get to the value of `$is_bigger`. Like for instance getting the information from a database, from an external file or prompting the user. If you then need to test it a number of times, it makes sense to 'remember' the value of `$is_bigger`. Everything went worng, just as foreseen.	[reply] [d/l] [select]