bit-wise boolean ops behave differently when observed

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

My application involves data storted in a database, one field of which is a bitmapped value.

The user-defined routines provided by one of our team members uses these values correctly, to process the records in various ways.

But some queries need to look up a bit-field, alter it, and use the result in a new query.

SQL doesn't do bit-fields, so I have to do it in Perl.

I do something like this ..

$bitfield = ....
my @fields = ( $modifier =~ m{ ([+-])  (\d+) }gxms ); # extract mods
while ( @fields >= 2 ) {                # for each modification
    my $op    = shift @fields;
    my $value = shift @fields;

    if ( $op eq '+') {
        $bitfield |= $value;            # set a bit w/ OR
    }
    elsif ($op eq '-' ) {
        my $dummy;                      # no-op to force full-width
        $dummy = sprintf "%064b\n", $value;
        my $negated = ~$value;
        $trade_condition &= $negated    # clear a bit w/ negated AND
    }
}
[download]

I put in the sprintf because it cleared up a problem that negating one produced zero.

But I still have a problem that starting off with a $bitfield value of one and modifying it with $modifier = '+2+16+1024' produces 3562 when I'm not looking, or 1043 when I single-step through it. Actually, single-stepping isn't enough, I need to be observing. Using

> printf "064b\n", $bitfield

makes it come out right.

How can I make bit boolean ops reliable ... or is it a matter of raising my intelligence to stop making foolish mistakes? PS ... This is perl, v5.8.8 built for i686-linux-thread-multi

--
TTTATCGGTCGTTATATAGATGTTTGCA

Comment on bit-wise boolean ops behave differently when observed Select or Download Code

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Re: bit-wise boolean ops behave differently when observed by kyle (Abbot) on Jan 18, 2008 at 21:08 UTC
Try this: `my $bitfield = 1; my $modifier = '+2+16+1024'; my @fields = ( $modifier =~ m{ ([+-]) (\d+) }gxms ); # extract mods while ( @fields >= 2 ) { # for each modification my $op = shift @fields; my $value = shift @fields; if ( $op eq '+') { $bitfield \|= $value; # set a bit w/ OR } } print "bf: $bitfield\n"; __END__ bf: 1043` [download] That's the desired result, right? If I change the first line: `my $bitfield = '1';` [download] ...then I get 3624. This isn't the result you got (3562), but I still think the problem is that you're starting out with a string somewhere, or you're using strings where you think you have numbers. This way... `my $bitfield = '1'; $bitfield = 0+$bitfield;` [download] ...the result goes back to 1043.	[reply] [d/l] [select]
Re: bit-wise boolean ops behave differently when observed by TomDLux (Vicar) on Jan 18, 2008 at 21:40 UTC
That did it! Can I kiss you, Kyle? I replaced the $dummy stuff with `my $value = ... $value += 0; # number, not a string` [download] That cleard up the Argument "M-N" isn't numeric in bitwise and (&) error message. -- `TTTATCGGTCGTTATATAGATGTTTGCA`	[reply] [d/l]