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

I have a 4-element array named arr. Each element contains 1 byte of binary data. I need to output these elements in sequence into a file. Currently I am doing this and it works:

print FILE pack ("C", $arr [0]). pack ("C", $arr [1]) . pack ("C", $arr [2]) . pack ("C", $arr [3]);

(I have to use pack(), otherwise what is output to the file is the decimal value of the byte in ASCII.)

My question is: is there a better way of doing this?

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Re: Writing the elements of an array to a file
by choroba (Cardinal) on Aug 31, 2015 at 15:20 UTC
    The shortest and most effective way is to use a quantifier in pack: 
    print {$FILE} pack 'C4', @arr;

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    You should have thought of a loop at least: 

    print {$FILE} pack 'C', $_ for @arr;

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    Update: If you need pack, the array doesn't contain binary data. It contains the numbers.

    لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
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      Thanks!

      Using a quantifier did the trick; I didn't know that using pack() this way was possible.

      The documentation suggests that the quantifier should be surrounded by square brackets, as in pack ("C[4]", @arr)? I guess it doesn't matter.

      Not sure I understand your remark about the binary data. The elements are the result of some bit-wise operations on data. Perl keeps them in its internal representation of numbers; I just want them output as binary data (bytes). If I don't use pack(), Perl just outputs them as it would output any number by default - as a decimal number in ASCII format, which isn't what I want.

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        Hello bontchev,

        The documentation suggests that the quantifier should be surrounded by square brackets...

        Not really. From pack (emphasis added):

        The following rules apply:
        • Each letter may optionally be followed by a number indicating the repeat count. A numeric repeat count may optionally be enclosed in brackets, ...

        Hope that helps,

        Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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        It's also possible to use  '*' as a quantifier to pack/unpack whatever number of the specified data type happens to be present: 

        c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $p = pack 'C*', (65, 66, 0x43, ord('D'), 0b01000101);
 print qq{'$p'};
 ;;
 my @u = unpack 'C*', $p;
 dd \@u;
"
'ABCDE'
[65 .. 69]

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        Give a man a fish:  <%-{-{-{-<

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        Binary data versus numbers: 
        print "\x45" | "\x59"; # Prints \x5d.
print 'E'    | 'Y';    # Same as the above, prints ].
print 69     | 89;     # Prints 93 (5d in hex).

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        | is clever, it knows whether it operates on bytes (strings) or numbers.

        Update: fixed typo (s/78/89/), thanks poj.

        لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
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