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

Hello,

Please is it possible to check if a signed integer packed in 4 bytes is within a certain range, without unpacking it at first ?

I've tried to pack both the lower and higher boundary signed integers with the 'l' canevas, but using '<' and '>' operators doesn't work...

The reason of my question: I've one huge file containing packed signed integers and I need to compare then to a range of two signed integers, but the unpack operation for each value in the file is too much time-consuming !

Maybe I need to use an other canevas for such purpose ?

Thanks in advance for your miraculous help, Germain

Replies are listed 'Best First'.
Re: comparison of packed signed integers
by BrowserUk (Patriarch) on Oct 30, 2011 at 11:24 UTC

    Updated: To reflect the reality check from johngg below.

    Comparison of packed integer will only work if they are unsigned (or all positive or all negative) and packed in big-endian (Network) order: 

    $belo = pack 'N', 10000; $behi = pack 'N', 20000; $ben = pack 'N', 123
+45;;
$belo lt $ben && $behi gt $ben and print 'Works';;
Works

    [download]

    Not if they are packed in little-endian (VAX) order:

    $lelo = pack 'V', 10000; $lehi = pack 'V', 20000; $len = pack 'V', 123
+45;;
$lelo lt $len && $lehi gt $len and print 'Works';;

    [download]

    The same thing for packed signed integers (but only works if the numbers are all positive or all negative):

    $bello = pack 'l>', 10000; $belhi = pack 'l>', 20000; $beln = pack 'l>
+', 12345;;
$bello lt $beln && $belhi gt $beln and print 'Works';;
Works

$lello = pack 'l<', 10000; $lelhi = pack 'l<', 20000; $leln = pack 'l<
+', 12345;;
$lello lt $leln && $lelhi gt $leln and print 'Works';;

    [download]
    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    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.
[reply]
[d/l]
[select]

      I'm not sure that will work if negative numbers come into it.

      knoppix@Microknoppix:~$ perl -E '
> $pos = pack q{N}, 10000;
> say unpack q{B*}, $pos;
> $neg = pack q{N}, -10000;
> say unpack q{B*}, $neg;
> say $pos gt $neg ? q{Good} : q{Bad};'
00000000000000000010011100010000
11111111111111111101100011110000
Bad
knoppix@Microknoppix:~$

      [download]

      You'd probably have to expand the tests to cater for that.

      Cheers,

      JohnGG

[reply]
[d/l]

        pack template 'N' is for unsigned integers. If you need to cater for signed integers, you need template 'l>'.

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        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.
[reply]

      Hi BrowserUK and thank you for your fast answer :-)

      Here is my code for the file creation.

      open(OUT,'>'.$fil) or die $!;
binmode OUT;
for $i(1 .. 10000){
 $lat=int(rand(20000))+440000;
 $lon=int(rand(10000))+20000;
 $typ=chr(31);
 $cod=pack('l>l>C',$lat,$lon,$typ);
 print OUT $cod;
}
close(OUT);

      [download]
      And here is my code for the read and comparison:

      $lah=pack('l>',$lah);
$lab=pack('l>',$lab);
$loh=pack('l>',$loh);
$lob=pack('l>',$lob);
open(IN,"<ch";
@fil=stat(IN);
$num=$fil[7]/9;
binmode(IN);
for(1 .. $num){
 read(IN,$lat,4);
 read(IN,$lon,4);
 read(IN,$pyt,1);
 if(($lat lt $lah) && ($lat gt $lab) && ($lon lt $loh) && ($lon gt $lo
+b)){
  print "OK";
 }
}

      [download]

      But it is never OK :-(

[reply]
[d/l]
[select]

        Where are the values of $lah, $lab, $loh, $lob set?

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        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.
[reply]
[d/l]

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