Re: comparison of packed signed integers

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.

Comment on Re: comparison of packed signed integers Select or Download Code

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Re^2: comparison of packed signed integers by johngg (Canon) on Oct 30, 2011 at 11:36 UTC
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]
Re^3: comparison of packed signed integers by BrowserUk (Patriarch) on Oct 30, 2011 at 12:00 UTC
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]
Re^4: comparison of packed signed integers by johngg (Canon) on Oct 30, 2011 at 12:28 UTC
Yes, but the same holds true using 'l>'. `knoppix@Microknoppix:~$ perl -E ' > $pos = pack q{l>}, 10000; > say unpack q{B}, $pos; > $neg = pack q{l>}, -10000; > say unpack q{B}, $neg; > say $pos gt $neg ? q{Good} : q{Bad};' 00000000000000000010011100010000 11111111111111111101100011110000 Bad knoppix@Microknoppix:~$` [download] I think that packing with 'N' or 'l>' will always produce the same results, it is unpacking where it makes a difference. knoppix@Microknoppix:~$ perl -E ' > for ( -2147483648, -1, 0, 1, 2147483647, 4294967295 ) > { > say qq{$_:}; > say unpack q{B}, pack q{N}, $_; > say unpack q{B}, pack q{l>}, $_; > say q{-} x 32; > }; > ' -2147483648: 10000000000000000000000000000000 10000000000000000000000000000000 -------------------------------- -1: 11111111111111111111111111111111 11111111111111111111111111111111 -------------------------------- 0: 00000000000000000000000000000000 00000000000000000000000000000000 -------------------------------- 1: 00000000000000000000000000000001 00000000000000000000000000000001 -------------------------------- 2147483647: 01111111111111111111111111111111 01111111111111111111111111111111 -------------------------------- 4294967295: 11111111111111111111111111111111 11111111111111111111111111111111 -------------------------------- knoppix@Microknoppix:~$ [download] However, I'm happy to be corrected if I'm wrong. Cheers, JohnGG	[reply] [d/l] [select]
Re^5: comparison of packed signed integers by BrowserUk (Patriarch) on Oct 30, 2011 at 12:33 UTC
Re^2: comparison of packed signed integers by gerleu (Novice) on Oct 30, 2011 at 12:06 UTC
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 $lo +b)){ print "OK"; } }` [download] But it is never OK :-(	[reply] [d/l] [select]
Re^3: comparison of packed signed integers by BrowserUk (Patriarch) on Oct 30, 2011 at 12:16 UTC
Where are the values of `$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]
Re^4: comparison of packed signed integers by gerleu (Novice) on Oct 30, 2011 at 12:36 UTC
They are set just above the given code and they compare correctly if I don't pack them and if I unpack the values in the file too !	[reply]
Re^5: comparison of packed signed integers by gerleu (Novice) on Oct 30, 2011 at 12:51 UTC
Re^5: comparison of packed signed integers by BrowserUk (Patriarch) on Oct 30, 2011 at 12:39 UTC


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