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

I am using unpack to unpack an element that consists of a 32 byte hex string plus an optional unsigned integer. If the integer exists, I want to join the 2 with an underscore.

I tried: 
join("_", unpack("H32V*", $string));

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but it always performs the join even if there is no second element. Adding an intermediate assignment, made it work: 
my @temp = unpack("H32V*", $string);
join("_", @temp);

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But that seems kludgy and I would like to avoid it. So, is there any way to properly cast/assign the result of unpack so that an undefined second element is ignored by join without using an intermediate variable?

Replies are listed 'Best First'.
Re: Casting return value of unpack properly
by jethro (Monsignor) on Feb 12, 2013 at 16:00 UTC

    I got the following results with perl v5.14.2:

    
my $string="1234567812345678";

print join("_", unpack("H32V*", $string)),"\n";

my @temp = unpack("H32V*", $string); print join("_", @temp),"\n"; 


$string="12345678123456781111";

print join("_", unpack("H32V*", $string)),"\n";

@temp = unpack("H32V*", $string); print join("_", @temp),"\n"; 

#prints:

31323348355037383132333435363738
31323348355037383132333435363738
31323334353637383132333435363738_825307441
31323334353637383132333435363738_825307441

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    As you can see, no difference between the two forms

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[d/l]
      You are correct... I'm not sure what changed but I could have sworn it wasn't working before....
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      Edit: *whistle*
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Re: Casting return value of unpack properly
by AnomalousMonk (Archbishop) on Feb 12, 2013 at 16:33 UTC
    ... it always performs the join even if there is no second element.

    By this, I think you mean that there is a  '_' present in all cases, even if it the last (and dangling) character in the unpack-ed string.

    I tried essentially the same code as jethro under ActiveState 5.8.9 and Strawberries 5.10.1.5, 5.12.3.0 and 5.14.2.1 and get no extraneous underscore. 

    >perl -wMstrict -le
"my $p = 'ABCDabcdABCDabcd' . pack 'V', 0x3039;
 print qq{'$p'};
 ;;
 my $unp = join '_', unpack 'H32V*', $p;
 print qq{'$unp'};
 ;;
 $p = 'ABCDabcdABCDabcd';
 print qq{'$p'};
 ;;
 $unp = join '_', unpack 'H32V*', $p;
 print qq{'$p'};
"
'ABCDabcdABCDabcd90  '
'41424344616263644142434461626364_12345'
'ABCDabcdABCDabcd'
'ABCDabcdABCDabcd'

    [download]

    Can you supply a short, stand-alone code example that demonstrates the effect you seek to avoid?

[reply]
[d/l]
[select]
      I can't replicate it now. Perhaps before I was using 'B*' or 'A*' instead of 'V*'... but not sure what happened. I apologize for taking your time on what seems to be a non-issue.
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        Oh, well... Just one of those days... sigh

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Re: Casting return value of unpack properly
by Anonymous Monk on Feb 12, 2013 at 16:00 UTC

    You could always throw in a quick grep {defined $_} to filter them out.

[reply]
[d/l]

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