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comp.lang.ruby

Numerical calculation returns string with DBI/Mysql

Ken Bloom

2/22/2007 5:08:00 PM

When I run the following query in DBI/Mysql, I'm finding that DBI is
returning the rho column as a string (rather than a float), thus forcing me
to convert it to a float with to_f. (I lose precision because the string
only returns the result to three decimal places). Any idea why DBI/Mysql is
returning the result of a numerical calculation as a string?

I'm running
mysql-5.0.34_alpha20070101-r61
mysql-ruby-2.7.2
ruby-dbi-0.0.21-r2
ruby-1.8.5_p2
on Gentoo.

dbh.execute("select childcounts.name name, childcounts.count/
parentcounts.count rho from parentcounts, childcounts where
parentcounts.left_key=childcounts.parent_left")

--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu...
2 Answers

Robert Dober

2/22/2007 6:18:00 PM

0

On 2/22/07, Ken Bloom <kbloom@gmail.com> wrote:
> When I run the following query in DBI/Mysql, I'm finding that DBI is
> returning the rho column as a string (rather than a float), thus forcing me
> to convert it to a float with to_f. (I lose precision because the string
> only returns the result to three decimal places). Any idea why DBI/Mysql is
> returning the result of a numerical calculation as a string?
>
> I'm running
> mysql-5.0.34_alpha20070101-r61
> mysql-ruby-2.7.2
> ruby-dbi-0.0.21-r2
> ruby-1.8.5_p2
> on Gentoo.
>
> dbh.execute("select childcounts.name name, childcounts.count/
> parentcounts.count rho from parentcounts, childcounts where
> parentcounts.left_key=childcounts.parent_left")
>
Hi Ken
if this is an issue it is a MySQL issue I guess, concerning ruby, do
you have a compelling reason not to write

dbh.execute("select childcounts.name name, childcounts.count whatever,
parentcounts.count rho from parentcounts, childcounts where
parentcounts.left_key=childcounts.parent_left")

and let the gem do the arithmetic? (performance maybe)

Cheers
Robert
> --
> Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
> Department of Computer Science. Illinois Institute of Technology.
> http://www.iit.edu...
>
>


--
We have not succeeded in answering all of our questions.
In fact, in some ways, we are more confused than ever.
But we feel we are confused on a higher level and about more important things.
-Anonymous

Ken Bloom

2/22/2007 10:08:00 PM

0

On Fri, 23 Feb 2007 03:17:32 +0900, Robert Dober wrote:

> On 2/22/07, Ken Bloom <kbloom@gmail.com> wrote:
>> When I run the following query in DBI/Mysql, I'm finding that DBI is
>> returning the rho column as a string (rather than a float), thus forcing
me
>> to convert it to a float with to_f. (I lose precision because the string
>> only returns the result to three decimal places). Any idea why DBI/Mysql
is
>> returning the result of a numerical calculation as a string?
>>
>> I'm running
>> mysql-5.0.34_alpha20070101-r61
>> mysql-ruby-2.7.2
>> ruby-dbi-0.0.21-r2
>> ruby-1.8.5_p2
>> on Gentoo.
>>
>> dbh.execute("select childcounts.name name, childcounts.count/
>> parentcounts.count rho from parentcounts, childcounts where
>> parentcounts.left_key=childcounts.parent_left")
>>
> Hi Ken
> if this is an issue it is a MySQL issue I guess, concerning ruby, do
> you have a compelling reason not to write
>
> dbh.execute("select childcounts.name name, childcounts.count whatever,
> parentcounts.count rho from parentcounts, childcounts where
> parentcounts.left_key=childcounts.parent_left")
>
> and let the gem do the arithmetic? (performance maybe)

Apparently extracting all data as strings is an important part of the MySQL
C API (see http://dev.mysql.com/doc/refman/5.0/en/mysql-fetc...), so
I just have to live with that way of doing things.

Someone should look through the DBI coercion code to make sure that they're
coercing data back to a float correctly, because this is a case that's
getting missed. Either DBI or DBD::MySQL is responsible for the coercion --
I'm not entirely sure which though.

I can get better precision by adding 0.000000000 to the quotient in the SQL
statement (so childcounts.count/parentcounts.count becomes childcounts.count/
parentcounts.count+0.000000000) because that forces the returned string to
be longer. It would be nice if I could get the data in the native
representation though, have high precision automatically, and not need
kludges like this.

Who can I email directly about this bug?

--Ken

--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu...