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Re: 3-byte representation

by ikegami (Pope)
on Oct 12, 2011 at 17:50 UTC ( #931057=note: print w/ replies, xml ) Need Help??


in reply to 3-byte representation

If you want big-endian byte order:

pack: $s = substr(pack('l>', $n), 1); unpack: $n = unpack('l>', "$s\0")/256;

If you want little-endian byte order:

pack: $s = substr(pack('l<', $n), 0, 3); unpack: $n = unpack('l<', "\0$s")/256;

PS — Going from 3 to 4 increases the space needed by 33%, not 25%.

Update: Slightly simpler solutions.
Update: The /256 got dropped when I posted my solution. Fixed.


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Re^2: 3-byte representation
by BrowserUk (Pope) on Oct 12, 2011 at 18:32 UTC

    Hm?

    Big-endian:

    sub pack24{ substr( pack('l>', $_[0]), 1) };; sub unpack24{ unpack('l>', "$_[0]\0") };; print "$_: ", unpack24( pack24( $_ ) ) for ( -8388608, -8388607, -2, -1, 0, 1, 2, 8388606, 8388607 );; -8388608: -2147483648 -8388607: -2147483392 -2: -512 -1: -256 0: 0 1: 256 2: 512 8388606: 2147483136 8388607: 2147483392

    Little-endian:

    sub pack24{ substr( pack('l<', $_[0]), 1) };; sub unpack24{ unpack('l<', "$_[0]\0") };; print "$_: ", unpack24( pack24( $_ ) ) for ( -8388608, -8388607, -2, -1, 0, 1, 2, 8388606, 8388607 );; -8388608: 16744448 -8388607: 16744448 -2: 16777215 -1: 16777215 0: 0 1: 0 2: 0 8388606: 32767 8388607: 32767

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

      Thanks, a "/256" got accidentally dropped when I posted my code. Fixed.

Re^2: 3-byte representation
by BrowserUk (Pope) on Oct 12, 2011 at 19:10 UTC
    I tested my solutions before posting them.

    Really? Really, really? Hm?

    #! perl -slw use strict; sub pack24be{ substr( pack('l>', $_[0]), 1) } sub unpack24be{ unpack('l>', "$_[0]\0") >> 8 } sub pack24le{ substr( pack('l<', $_[0] ), 0, 3); } sub unpack24le{ unpack( 'l<', "\0$_[0]" ) >> 8 } print( "be: $_: ", unpack24be( pack24be( $_ ) ) ), print( "le: $_: ", unpack24le( pack24le( $_ ) ) ) for ( -8388608, -8388607, -2, -1, 0, 1, 2, 8388606, 8388607 ) __END__ C:\test>junk101 (version 2 ) be: -8388608: -2147483648 le: -8388608: -2147483648 be: -8388607: -2147483392 le: -8388607: -2147483392 be: -2: -512 le: -2: -512 be: -1: -256 le: -1: -256 be: 0: 0 le: 0: 0 be: 1: 256 le: 1: 256 be: 2: 512 le: 2: 512 be: 8388606: 2147483136 le: 8388606: 2147483136 be: 8388607: 2147483392 le: 8388607: 2147483392 C:\test>junk101 (version 3) be: -8388608: 72057594029539328 le: -8388608: 72057594029539328 be: -8388607: 72057594029539329 le: -8388607: 72057594029539329 be: -2: 72057594037927934 le: -2: 72057594037927934 be: -1: 72057594037927935 le: -1: 72057594037927935 be: 0: 0 le: 0: 0 be: 1: 1 le: 1: 1 be: 2: 2 le: 2: 2 be: 8388606: 8388606 le: 8388606: 8388606 be: 8388607: 8388607 le: 8388607: 8388607

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
Re^2: 3-byte representation
by BrowserUk (Pope) on Oct 12, 2011 at 19:12 UTC
    Update: The >> 8 got dropped when I posted my solution. Fixed.

    Silently becomes

    Update: The /256 got dropped when I posted my solution. Fixed.

    Hm. "Got dropped" huh!

      Thank you for all your gentle answers, but if I run this:
      #!/usr/bin/perl print "Content-type: text/html\n\n"; srand(); $fil="ch"; open(OUT, '>>'.$fil); for($i=1;$i<10001;$i++){ $j=int(rand(20000))+440000; $k=substr(pack('l>',$j),1); print OUT $k; } close(OUT); print "DONE !";
      it produces one file with a variable length, but always greater than 30000 characters (for exemple 30045) ! So I suppose something is wrong in the pack line coding...

        Add binmode OUT after the open.

        Also,

        1. you should be using a mode of '>' not '>>', unless your intention is to append to an existing file.
        2. for($i=1;$i<10001;$i++){ is more easily and clearly written as for my $i ( 1 .. 10000 ) {
        3. int(rand(20000))+440000 produces always positive integer.

          If you don't need signed integers, your question would have been much simpler to answer.

        4. $k=substr(pack('l>',$j),1); print OUT $k;

          Packing and writing one value at a time is hugely expensive compared to packing all 10000 values in a single pass and then writing them once.

        5. For goodness sake, use a little horizontal whitespace.

          Don't you find this infinitely more readable?

          #!/usr/bin/perl use strict; print "Content-type: text/html\n\n"; srand(); my $fil = "ch"; open( OUT, '>', $fil ) or die $!; binmode OUT; for my $i ( 1 .. 10_000 ) { my $j = int( rand( 20_000 ) ) + 440_000; my $k = substr( pack( 'l>', $j ), 1 ); print OUT $k; } close( OUT ); print "DONE !";

          Remember that your source code is the equivalent of the professional chef's plated up meal.

          His presentation is what makes the difference between a paid for cover and a decent home cooked meal. It is both his edge and his profit margin.


        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
      It was /256. Just misremembered cause I changed comps in the middle.

        I actually downloaded the >>8 version before it changed.

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