On Wed, Jul 18, 2007 at 10:58:58AM -0400, Rutherdale, Will wrote: > Tim, I have a couple of feedback comments on your text. > > A) I would not characterise 32-bit signed integers as giving 10 digits > of precision as you did. They give log10( 2^31 ) ~= 9.3319 digits of > precision. Since you can't count on the full 10th digit, I would > truncate and tell people you get 9 digits of precision. Similarly a > signed 64-bit integer gives 18.9649 or really just 18 digits of > precision to be safe, but 128-bit signed integers give you a full 38 > digits. > > B) long double is not usually 96 bits, but rather 80 bits. Most > machines that people use follow the IEEE 754 standard, which says >=79 > bits but is normally implemented as 80 bits.
Okay. > Good explanation generally. Thanks for the feedback Will. I'll make some changes. > For the general list, I'm still interested in the issue of alternative > representations for financial work. Has anyone had much experience with > the ( NUMERIC , Math::BigFloat ) combination? Is that used generally by > people in the field? Might be worth meditating on that question over at http://perlmonks.org Tim. > > -Will > > > > -----Original Message----- > > From: Tim Bunce [mailto:[EMAIL PROTECTED] > > Sent: Tuesday 17 July 2007 19:02 > > To: Christopher Sarnowski > > Cc: [EMAIL PROTECTED]; dbi-users@perl.org > > Subject: Re: float bug? perl 5.8, DBI and oracle 10.2.0 > > > > . . . > > > > Funnily enough I wrote a section on this topic back in May > > 2006 for the > > 2nd edition of DBI book (which is currently shelved, by the way). > > I've appended the relevant chunk of the rough draft. Comments welcome. > > > > Tim. > > > > . . . > > > > =head3 Perl Integer Values > > > > Integers are typically stored as 32 or 64 bit (4 or 8 byte) values > > depending on how perl was configured. You can check the size > > of integers > > in your perl by running C<perl -V> and looking for C<ivsize=> > > in the output. > > The range of a 32 bit integer is -2,147,483,648 to 2,147,483,647 > > (10 digits of precision). The range of a 64 bit integer is > > -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 (19 digits > > of precision). > > . . . > > =head3 Perl Floating Point Values > > > > Floating point values are typically stored in 64 bits or sometimes 96 > > bits (that's 8, or 12 bytes) depending on how your perl was > > configured. > > You can check the size used in your perl by running C<perl -V> and > > looking for C<nvsize=> in the output. The 64 bit floats are known as > > I<doubles> and have approximately 15 digits of precision > > between 1e-307 > > to 1e+308, and the 96 bit floats are known as I<long doubles> and have > > approximately 18 digits of precision between 1e-4931 to 1e+4932. Some > > systems support 128 bit I<quad doubles> with even greater > > precision and > > scale. > > > > It's becoming more common for perl to be configured with 64 bit > > integers but still using 64 bit floating point values. But a 64 bit > > integer has 19 digits of precision whereas a 64 bit floating point > > value only has approximately 18. This is important to know because it > > means that a large integer may loose precision if it's involved in a > > calculation that causes it to be converted to a floating point value > > (which is basically anything more involved that addition or > > subtraction > > of another integer). > > . . . > > > > Some databases support very large integers. Oracle, for > > example, supports > > integers with 38 digits of precision. That's far beyond the > > 10 digits of a > > simple 32bit integer and even the 19 digits of a 64bit > > integer. Because of > > this the DBD::Oracle driver returns integers, and indeed all > > other numberic > > types, as strings. > > > > The DBI will probably gain a way to hook into the fetching of a > > value from the database so that the use of Math::BigInt, for example, > > can be made transparent and not clutter up the code. But that hasn't > > happened yet. XXX > > > > Many databases support multiple sizes of integer types from 1 to 8 > > bytes in size with INTEGER (4 bytes) and SMALLINT (2 bytes) being > > the most common.N<Oracle is a little unusual in that it only has > > one underlying numeric type which is variable width and all other > > numeric types are aliases for it.> > > > > > > - - - - - Appended by Scientific Atlanta, a Cisco company - - - - - > This e-mail and any attachments may contain information which is confidential, > proprietary, privileged or otherwise protected by law. The information is > solely > intended for the named addressee (or a person responsible for delivering it to > the addressee). If you are not the intended recipient of this message, you are > not authorized to read, print, retain, copy or disseminate this message or any > part of it. If you have received this e-mail in error, please notify the > sender > immediately by return e-mail and delete it from your computer.
