assignment operator return values

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

Dear monks,

I have a question about the '=' operator :-)

I caught myself doing the following:

sub path_to_root {
    my $self = shift;
    my $path = $self->get_branch_length;
    while ( $self ) {
        if ( $self->get_parent ) {
            $self = $self->get_parent;
            $path += $self->get_branch_length;
        }
        else {
            return $path;
        }
    }
}
[download]

That is to say, I make the $self->get_parent method call twice, once to check if there really is a parent, the second time to step to that parent during my traversal (here we're traversing a tree from initial node $self to the root, but this issue is more broadly applicable, obviously).

So if we do an assignment, what value is returned that I can test? Is it the left hand value - i.e. would the following be equivalent, but more efficient?

sub path_to_root {
    my $self = shift;
    my $path = $self->get_branch_length;
    while ( $self ) {
        if ( $self = $self->get_parent ) {
            $path += $self->get_branch_length;
        }
        else {
            return $path;
        }
    }
}
[download]

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Re: assignment operator return values by Aristotle (Chancellor) on Feb 22, 2006 at 01:30 UTC
Yes, that works as you expect. Assignments read from right to left, ie. `$x = $y = $z = 10` assigns 10 to `$z`, then assigns the value of `$z` to `$y`, then the value of `$y` to `$x` – so all three variables end up with the value 10. However, the code you want this for can be written cleaner without inline assignment: `sub path_to_root { my $self = shift; my $path = $self->get_branch_length; my $parent = $self->get_parent; while ( $parent ) { $path += $parent->get_branch_length; $parent = $parent->get_parent; } return $path; }` [download] Makeshifts last the longest.	[reply] [d/l] [select]
Re: assignment operator return values by GrandFather (Saint) on Feb 22, 2006 at 01:29 UTC
perlop - Assignment Operators may help. The assignment operator returns the object assigned to as an lvalue so your assignement and test is fine. Update: I'd be inclined to rewrite that as: `while ( $self ) { $path += $self->get_branch_length if $self = $self->get_parent; } return $path;` [download] DWIM is Perl's answer to Gödel	[reply] [d/l]
Re: assignment operator return values by QM (Parson) on Feb 22, 2006 at 01:34 UTC
TIAS: Try It And See. Not to be too onerous, but you've given enough info to test it yourself. Why not just do this? C:\>perl -demo Loading DB routines from perl5db.pl version 1.28 Editor support available. Enter h or `h h' for help, or `perldoc perldebug' for more help. main::(-e:1): mo DB<1> for (-1..1) { print "$_"; print "\tTrue" if ($x = $_); print " +\n" } -1 True 0 1 True DB<2> [download] And there you have it. But what happens when `get_parent` returns a false value? Here it seems OK, as `$self` is lexical to this sub. In other situations, you might not want to overwrite `$self`. Yes, the assignment is done, and you can then test for it. -QM -- Quantum Mechanics: The dreams stuff is made of	[reply] [d/l] [select]