binary ids

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

Hi monks, i have a question concerning binary identifiers coming from sql server. I'm using DBI to retrieve rows that contain a binary data type, and then i use unpack.

# ... DBI stuff ... grab row with binary datatype
$parent_hex = '0x0' . unpack("H*",$row->{parentid});
[download]

I want to determine whether $parent_hex is zero but for some reason the below comparison doesn't work

if ( !$parent_hex ) {
     #do something
}
else {
     #do something different
}
[download]

how can I successfully compare $parent_hex to 0x000000000? thanks, Michael

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Re: binary ids by Paladin (Vicar) on Feb 10, 2003 at 17:26 UTC
You could do something like the following: `if ( hex($parent_hex) != 0) { # $parent_hex isn't 0 }` [download] `hex` will convert a hex string into it's numerical value. Then compare that with 0.	[reply] [d/l] [select]
Re: Re: binary ids by tachyon (Chancellor) on Feb 10, 2003 at 18:07 UTC
In fact all you need is `if( $parent_hex == 0 ) { blah }` as the numeric comparison forces perl to convert the sv to a number and 0x00000 is correctly interpretted as 0 automagically. The problem with the `if (! $parent_hex) {}` example given is that without forcing numeric context perl sees the sting '0x0' which is not a null string '' and thus not false in this context. cheers tachyon s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print	[reply] [d/l] [select]
Re^3: binary ids by diotalevi (Canon) on Feb 10, 2003 at 21:08 UTC
This is misleadingly incorrect. You assume that just because the perl compiler will read a plain `0x0005` as the numeric value 5 that the same thing in a scalar behaves the same way. In this case strings with leading numbers are evaluated in numeric context as the value of the leading number. In this case this means that `$var = "0x0005"; $var == 5` is false - in fact, `$var == 0` because `$var` begins with the string "0". The correct answer is to use the hex() function. You'd have to resort to using eval to get your idea to work (which re-invokes the compiler which has its own rules...). Seeking Green geeks in Minnesota	[reply] [d/l] [select]
Re: Re^3: binary ids by tachyon (Chancellor) on Feb 10, 2003 at 21:22 UTC
Re: binary ids by BrowserUk (Patriarch) on Feb 10, 2003 at 18:20 UTC
As the only value of `$row->{parentid}` that could produce a hex value of 0x00000000 is a string of 4 ascii 0 (zero) bytes, then you could do `if ( $row->{parentid} =~ /^\0{4}$/ ) { # do something; } else { $parent_hex = '0x0' . unpack 'H', $row->{parentid}; # do something else }` [download] Examine what is said, not who speaks. The 7th Rule of perl club is -- pearl clubs are easily damaged. Use a diamond club instead.*	[reply] [d/l] [select]