or instead, after selecting, I can use sprintf:

$float_column = sprintf("%1.16e", $float_column);

The first solution is okay if I know in advance which columns are

floats,

while the second solution seems better if I need to dynamically deal

with unknown column types (e.g., "select *").

Does anyone have any better (or just other) ideas?

On Tue, 2008-10-07 at 09:02 -0700, Douglas Wilson wrote:

sprintf is the preferred way of rounding off floats in Perl. There is a

whole branch of mathematics, numerical analysis, which deals with

problems like this, so don't expect things to be easy.

BTW, don't use "SELECT *". If your DBA rebuilds the tables with a

different column order, your code will fail. Specifically name the

columns (or if you really have to, use the _hashref methods to fetch;

that way you can select the correct column via the hash).

On Tue, 2008-10-07 at 09:02 -0700, Douglas Wilson wrote:

BTW, don't use "SELECT *". If your DBA rebuilds the tables with a

different column order, your code will fail. Specifically name the

columns (or if you really have to, use the _hashref methods to fetch;

that way you can select the correct column via the hash).

All of the common languages suffer from rounding problems inherent in

IEEE rounding (round-to-even and other issues). Math::Round resolves

these issues.

$a=4712.5;

$b=round $a;

printf "%s %5.0f %5.0f\n", $a, $a, $b;

I hit this while comparing SAS output (which does do proper (within

machine limits) "mathematical" rounding that we all learned about in

school. The C libraries from which Perl and other C-lib derived

languages take their rounding functions are the cause of some

potential grief here and comparison with 4 other languages vs. SAS

showed SAS getting it right each time. Using Math::Round - so did

Perl. We're talking way down to DP as well.

This caused the group I work with some issues as I was rewriting some

SAS code in Perl and our regression tests were not matching - and we

were talking adding up millions of rows of data with N DP to be

concerned about. Math::Round fixed that.