Meaning of //

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

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Re: Meaning of // by huck (Prior) on Oct 17, 2019 at 17:10 UTC
EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. https://perldoc.perl.org/perlop.html#Logical-Defined-Or	[reply]
Re^2: Meaning of // by hanspr (Sexton) on Oct 17, 2019 at 17:14 UTC
Thanks a lot	[reply]
Re: Meaning of // by Discipulus (Canon) on Oct 17, 2019 at 18:19 UTC
Hola hanspr, this useful feature was added in 5.10 and it is used also very conveniently directly in the assignements: `$val //= 42;` [download] see also defined or: // and //= L* There are no rules, there are no thumbs.. Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.	[reply] [d/l]
Re: Meaning of // by stevieb (Canon) on Oct 17, 2019 at 20:31 UTC
It's the only reason I switched my more complex software to 5.10 from 5.8 minimum requirement. It alleviates doing stuff like: `sub func { my ($thing) = @_; $thing = defined $thing ? $thing : 'non-thing'; # or if (defined $thing){ $thing = $thing; } else { $thing = 'non-thing'; } }` [download] Much cleaner to use the previously-mentioned defined-or: `sub func { my ($thing) = @_; $thing //= 'non-thing'; }` [download] That might not look like much of a difference, but add some complexity and other rules surrounding incoming variables and you can significantly reduce the keystrokes, code complexity and most important, readability, especially if in loop-type scenarios.	[reply] [d/l] [select]
Re^2: Meaning of // by AnomalousMonk (Archbishop) on Oct 17, 2019 at 21:30 UTC
... cleaner ... Cleaner when used to do something reasonable as in your examples, but I hope hanspr never actually saw some of the examples given in the OP. As pointed out by pryrt, `return 1 unless ($flag // 0);` is exactly equivalent to `return 1 unless $flag;` (except more confusing), and `... if $HAS_FOCUS // '';` likewise. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: Meaning of // by ikegami (Patriarch) on Oct 19, 2019 at 13:40 UTC
`undef` is false, so `return 1 unless ($flag // 0); ... if $HAS_FOCUS // '';` [download] are better written as `return 1 unless $flag; ... if $HAS_FOCUS;` [download]	[reply] [d/l] [select]
Re: Meaning of // by rsFalse (Chaplain) on Oct 20, 2019 at 11:33 UTC
I use '//' sometimes to avoid warnings, e.g.: `perl -wle 'my $number; print 1 if 0 + ( $number // 0 )'` [download] ..e.g. in bigger expressions. Otherwise I get: "Use of uninitialized value $number in addition (+) at -e line 1."	[reply] [d/l]
Re: Meaning of // by LanX (Saint) on Oct 17, 2019 at 19:43 UTC
Other told you what it does, though the author attempted to do strange things. Something `or 1` is always true, so this `return 1 unless ($flag // 1)` will never happen. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l] [select]
Re^2: Meaning of // by pryrt (Abbot) on Oct 17, 2019 at 19:54 UTC
Except when `$flag` is false. C:\usr\local\share\PassThru\perl>perl -le "print +1//55" 1 C:\usr\local\share\PassThru\perl>perl -le "print +undef//55" 55 C:\usr\local\share\PassThru\perl>perl -le "print +0//55" 0	[reply] [d/l]
Re^3: Meaning of // by LanX (Saint) on Oct 17, 2019 at 23:28 UTC
> Except when $flag is false. You mean defined but false, but you are right, I was wrong. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]
Re: Meaning of // by hanspr (Sexton) on Nov 28, 2019 at 15:53 UTC
Thanks for all your comments and examples, I did understand its use.	[reply]