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Convert to Hexadecimal

by Anonymous Monk
on May 17, 2012 at 17:07 UTC ( #971125=perlquestion: print w/replies, xml ) Need Help??
Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Currently in Perl 5.10.1 I succeeded in denting the wall with my head trying to perform a rather simple task. Starting with a simple ASCII string I need to

- find its length
- convert the length into 2 hex bytes.

my $string = "39860000NO00"; my $length = length($string);
I need to convert the length (12) to the string "\x00\x0c"
any help will be greatly appreciated.

Replies are listed 'Best First'.
Re: Convert to Hexadecimal
by davido (Archbishop) on May 17, 2012 at 18:03 UTC

    unpack or sprintf to get the hex digits. The substitution operator to match the hex digits, prepending each pair with the literal '\x'.

    my $string = "39860000N000"; my $hexlen = sprintf( '%4.4x', length $string ) =~ s/ ( \p{Hex}{2} ) / +\\x$1/grx; print "Length of ($string) in hex: ($hexlen)\n";

    In older versions of Perl that didn't have the /r flag you would probably want to break that into a couple of statements (and may still wish to do so for clarity's sake):

    my $string = '39860000N000'; my $hexlen = sprintf '%4.4x', length $string; $hexlen =~ s/ ( \p{Hex}{2} ) /\\x$1/gx; print "Length of ($string) in hex: ($hexlen)\n";


      $hexlen = printf("\\x%02x\\x%02x", unpack("C2", pack("n", length($string)));

      (Explanation in reply to OP.)

      "My two cents aren't worth a dime.";
      Forever grateful!

      Works like a charm. It will take me some time to understand it though.

Re: Convert to Hexadecimal
by sauoq (Abbot) on May 17, 2012 at 19:00 UTC

    As davido points out, sprintf() would be almost good enough on its own.

    $ perl -le 'printf( "%04x\n", 12)' 000c

    And, as he points out, you can get the rest of the way with s///. That's pretty ugly though. You could also get there a few other ways... but they are all pretty ugly.

    It would be nice if you could have something like

    perl -le 'printf( "\\x%02x\\x%02x\n", 12 )' # WRONG \x0c\x00

    Unfortunately, "12" is one argument, not two. But that's where unpack comes in handy. You want to unpack a number into two (char sized) values. That leads you to

    perl -le 'printf( "\\x%02x\\x%02x\n", unpack( "C2", 12 ))' WRONG \x31\x32

    But that's wrong too as 12 is a scalar and gets unpacked (by that) as "1", "2" (ASCII 0x31 and 0x32 respectively.) And this is where pack is useful. You want to pack that 12 into a single value before unpacking it into two. Two characters is a short...

    perl -le 'printf( "\\x%02x\\x%02x\n", unpack( "C2", pack("S", 12)))' \x0c\x00

    Oops. Still not quite right. My architecture is little-endian (x86). But that's okay, we can force pack to give us a big-endian short.

    perl -le 'printf( "\\x%02x\\x%02x\n", unpack( "C2", pack("n", 12)))' \x00\x0c # Another way to say that . . . perl -le 'printf( "\\x%02x\\x%02x\n", unpack( "C2", pack("S>", 12)))' \x00\x0c

    It might not be beautiful, but I think it's better than futzing about with string operations after the conversion to hex. Of course, if you read perlpacktut as suggested, I'm sure you've come to this solution already! ;-)

    "My two cents aren't worth a dime.";
Re: Convert to Hexadecimal
by afoken (Abbot) on May 17, 2012 at 17:11 UTC

    See perlpacktut


    Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)
Re: Convert to Hexadecimal
by ikegami (Pope) on May 17, 2012 at 19:12 UTC
    Hex is text. «2 hex bytes» makes no sense. You actually want «2 bytes equal to "\x00\x0C"», and that's obtained using
    pack('n', length($string))

    You might also be interested in the following:

    pack('n/a*', $string)

    It's short for

    pack('n', length($string)) . $string

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