Re: When is a string a number?
- From: davidnicol@xxxxxxxxx (David Nicol)
- Date: Wed, 4 Oct 2006 14:00:45 -0500
On 10/4/06, Martin J. Evans <martin.evans@xxxxxxxxxxxx> wrote:
With DBI/DBD::Oracle all values read from the database are scalars. As
everyone will know, whether something read from the database is a string
or a number in Perl purely depends on the context it is used in so:
internally, there are flags in the SV structure that hint as to what conversions
have been done on the SV already, for efficiency's sake.
I have to admit I don't know how the JSON module knows what is a number
and what is a string in Perl but I see the same issue with Data::Dumper
so I presume there must be some way to find out if a perl scalar is a
number or a string.
It seems like the pure perl Dumper tests values with a regex and optimizes
to numbers when the stringification matches a number template, while
the XS version checks the flags in the SV structure.
The problem gets a lot worse for me since I do some arithmetic on values
pulled from the database before converting them to JSON and this is
where Perl seems to change them into numbers e.g.
I don't want to have to do ($var +0) on all the number fields I pull
from the database (to turn them into numbers) and neither do I want to
do a '$var .= ""' (to turn all the fields into strings).
you might have to do exactly that. "$var" will produce a string version. Sorry.
Apparently you can tell the JSON module to make everything strings:
http://search.cpan.org/~makamaka/JSON-1.07/lib/JSON.pm#AUTOCONVERT
As an aside (and probably a perl question rather than a DBI one) does
anyone know why the type of a scalar changes when you use it on the
right side of an assignment:
perl -MData::Dumper -le '$a="1"; print Dumper($a); $b += $a; print
Dumper($a);'
$VAR1 = '1';
$VAR1 = 1;
The has-been-evaluated-as-a-number flag got set on $a when it was
evaluated as a number; then Dumper, with both available, chose the
number format.
How does JSON and Data::Dumper know whether Perl thinks something is a
number or a string?
inspecting the flags; except pure-perl Dumper apparently uses a
regular expression
to identify numbers. Those are guesses. The source is available for
your inspection.
--
The Country Of The Blind, by H.G. Wells
http://cronos.advenge.com/pc/Wells/p528.html
.
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