How to pass optional parameters to a Perl subroutine? - perl

Question

I wanted to pass two arguments to my subroutine. The first parameter is required, the second parameter I want to be optional. Actually it works for non-array parameter in this script:
deftest ("string value test",9);
sub deftest{
my ($strvalue, $num) = #_;
if (defined $num){
print "\nCHECK 1 - Defined value $num";
} else {
$num //= 99;
}
print "\nstrvalue: $strvalue num: $num\n";
}
When I invoke this subroutine with ("string value test",9); then on the screen:
CHECK 1 - Defined value 9
strvalue: string value test num: 9
When I don't enter the second argument, then the value is 99. So this works fine.
The problem is, when I use the array in the first argument:
my #arrSwap = (1,2,3);
deftest2 (#arrSwap,5);
sub deftest2{
my (#arrSwap, $num) = #_;
if (defined $num){
print "\n\nCHECK 2 - Defined val $num";
} else {
$num //= 55;
}
print "\narrSwap $arrSwap[1] num: $num\n";
}
After invoking this subroutine with deftest2 (#arrSwap,5); then on the screen I have only: arrSwap 2 num: 55
(but 5 would be expected as it is defined).
Why it doesn't work with the array?

Answer 1

Because perl passes a single list as arguments, and assigning my ( #arrSwap, $num ) = #_ will always mean an empty $num, because #arrSwap will consume the entirety of the input.
If you want to do this, you need to pass a reference
my #arrSwap = ( 1,2,3 );
deftest2 ( \#arrSwap, 5);
sub deftest2 {
my ( $arrSwap_ref, $num ) = #_;
$num //= 55;
print "\narrSwap", $arrSwap -> [1], " num: $num\n";
}
Otherwise perl simply has no way to tell whether the last number is part of the list you sent, or an optional parameter.

Answer 2

The arguments to subroutines are lists in Perl. When you assign the arguments to an array, Perl doesn't know how long that array is supposed to be, so there is no way to do this:
sub frobnicate {
my (#foo, $bar) = #_;
}
In that case, $bar will always be undef, because it just assigns the rest of the list to #foo.
Instead, you need to make the array-part of your arguments a reference, and derefence it correctly.
sub frobnicate {
my ($foo, $bar) = #_;
foreach my $element (#{ $foo }) {
# ...
}
}
And then call it with a reference.
frobnicate(\#foo, 123);
Because a reference is a scalar value, you now know how many elements will be in the argument list, and you can have optional arguments at the end of the list.

Answer 3

The array is expanded in the parameter list and forms the first N parameters, with $num appearing at the end
If you pass the array that way then there is no way to tell from within the subroutine how many parameters came from the array, so it is always best to pass an array by reference
Your code would look like this
my #arrSwap = ( 1, 2, 3 );
deftest2( \#arrSwap, 5 );
deftest2( \#arrSwap );
sub deftest2 {
my ( $arrSwap, $num ) = #_;
if ( defined $num ) {
print "\n\nCHECK 2 - Defined val $num";
}
else {
$num = 55;
}
print "\narrSwap $arrSwap->[1] num: $num\n";
}
output
CHECK 2 - Defined val 5
arrSwap 2 num: 5
arrSwap 2 num: 55