Re^2: Can't understand function returning undefs

Thanks. Not that I like this behavior but I think I'll have to live with it.

But if we are at it. My problem is that I have a function that returns two values when data is available to it and when no data remained it returns undef.

sub recv_data{
    if(read_some_data){
        return($stuff1, $stuff2);
    } else {
        return;
    }
}
[download]

What I would like to do is to call this function in a while until all data is consumed. I tried to do this first by

while(($x, $y) = recv_data){
    ...
}
[download]

which failed due to the above reasons (i.e., the while block always executed since ($x, $y) = recv_data always evaluates to true even if the function returns undef).

I still have the option to write

while(1){
    my($x, $y) = recv_data;
    last unless defined $x;
    ...
}
[download]

but I fail to see this as elegant.

How to organize this code so that I obtain the required behavior in an elegant and readable way?

rg0now

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Re^3: Can't understand function returning undefs by AnomalousMonk (Archbishop) on Sep 24, 2010 at 21:24 UTC
`sub recv_data{ if(read_some_data){ return($stuff1, $stuff2); } else { return; } }` [download] The function in your example returns an empty list, not `undef`, in list context when there is no more data available. If your real function actually returns undef in list context (which is 'supplied' to the function by the `($x, $y)` sub-expression in your example code), then the list will always have at least a single item (i.e., `undef`) and will always evaluate as true; only the empty list or array is false. See the code below for the difference between `return;` and `return undef;` `>perl -le "use warnings FATAL => 'all'; use strict; ; my @pairs1 = qw(1 2 3 4); ; sub S1 { return if not @pairs1; return pop @pairs1, pop @pairs1; } ; my ($x, $y); while (($x, $y) = S1()) { print qq{$x $y}; } ; my @pairs2 = qw(5 6 7 8); ; sub S2 { return undef if not @pairs2; return pop @pairs2, pop @pairs2; } ; while (($x, $y) = S2()) { print qq{$x $y}; } " 4 3 2 1 8 7 6 5 Use of uninitialized value $x in concatenation (.) or string at ...` [download]	[reply] [d/l] [select]
Re^4: Can't understand function returning undefs by rg0now (Chaplain) on Sep 24, 2010 at 21:32 UTC
The function in your example returns an empty list, not undef, in list context when there is no more data available. Thanks for this update. I never actually thought these two cases are different. And I thought I knew Perl... rg0now	[reply]
Re^3: Can't understand function returning undefs (explicit) by tye (Sage) on Sep 24, 2010 at 21:01 UTC
`while( defined( ( ($x,$y)= recv_data() )[0] ) )` [download] But I'd rather do that like: `my( $x, $y )= recv_data(); while( defined $x ) { ... ( $x, $y )= recv_data(); }` [download] - tye	[reply] [d/l] [select]
Re^3: Can't understand function returning undefs by ikegami (Patriarch) on Sep 24, 2010 at 21:55 UTC
But if we are at it. My problem is that I have a function that returns two values when data is available to it and when no data remained it returns undef. It should return an empty list when no data remains. Compare `my @a = qw( a b c d ); my $i = sub { @a ? splice(@a, 0, 2) : () }; while (my ($x,$y) = $i->()) { print("$x:$y\n"); }` [download] with `my @a = qw( a b c d ); my $i = sub { @a ? splice(@a, 0, 2) : (undef,undef) }; while (my ($x,$y) = $i->()) { print("$x:$y\n"); }` [download] Update: Added missing semicolons.	[reply] [d/l] [select]