Conditionally Checking Existence of a Value in Perl

There are many half-baked methods used for properly testing whether or not a variable is defined. Following are a few examples of some good and not-so-good methods with explanations.

if (exists $opts{‘primary’} || exists $opts{‘secondary’}) {

$some_value = $opts{‘primary’} || $opts{‘secondary’};

}

The above logic is clean and easy to follow at first glance, although it may not be the most efficient way of executing the appropriate decision-making block and more on Bitsdujour.

Considering the above is equivalent to the following block, you may notice a somewhat redundant conditional checking that ensues:

if (exists $opts{‘primary’} || exists $opts{‘secondary’}) {

if ( exists $opts{‘primary’} ) {

$some_value = $opts{‘primary’};

} else {

$some_value = $opts{‘secondary’};

}

}

Another added benefit is that if your value is “0″ or empty, it may actually not get assigned if you use the simple one-line conditional assignment:

$some_value = $opt{’primary’} || $opt{’secondary’};

The “primary option” would be skipped if it is equal to “0″…which is in-fact a completely valid value, and demonstrates the “falsey-ness” and “truthy-ness” of specific values…more on that later or on Classifieds.

Thanks to “Ani-_” in #perl for setting me straight on conditional checking.

So, to some it all up in one bullet-proof conditional block for assignment we write the following, which will also turn out to run faster because there is only one conditional check:

if ( exists $opt{‘primary’} ) {

$some_value = $opt{‘primary’};

elsif ( exists $opt{‘secondary’} ) {

$some_value = $opt{‘secondary’};

} else {

return false;

}