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Update the FAQ to include an entry about binary versus decimal numbers. (CVS 2648)

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drh
2005-08-31 02:46:21 +00:00
parent f6ac657f27
commit b64d5ee42c
3 changed files with 27 additions and 8 deletions
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21
www/faq.tcl
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@ -1,7 +1,7 @@
#
# Run this script to generated a faq.html output file
#
set rcsid {$Id: faq.tcl,v 1.29 2005/06/16 19:48:40 drh Exp $}
set rcsid {$Id: faq.tcl,v 1.30 2005/08/31 02:46:21 drh Exp $}
source common.tcl
header {SQLite Frequently Asked Questions</title>}
@ -484,6 +484,25 @@ faq {What is an SQLITE_SCHEMA error, and why am I getting one?} {
</pre></blockquote>
}
faq {Why does ROUND(9.95,1) return 9.9 instead of 10.0?
Shouldn't 9.95 round up?} {
<p>SQLite uses binary arithmetic and in binary, there is no
way to write 9.95 in a finite number of bits. The closest to
you can get to 9.95 in a 64-bit IEEE float (which is what
SQLite uses) is 9.949999999999999289457264239899814128875732421875.
So when you type "9.95", SQLite really understands the number to be
the much longer value shown above. And that value rounds down.</p>
<p>This kind of problem comes up all the time when dealing with
floating point binary numbers. The general rule to remember is
that most fractional numbers that have a finite representation in decimal
do not have a finite representation in binary. And so they are
approximated using the closest binary number available. That
approximation is usually very close, but it will be slightly off
and in some cases can cause your results to be a little different
from what you might expect.</p>
}
# End of questions and answers.
#############