From: "Brian Schröder" <ruby.brian@...>
Date: 2005-04-20T07:55:17+09:00
Subject: Re: String<->float conversion headaches

On 20/04/05, Michael Campbell <michael.campbell@gmail.com> wrote:
> On 4/19/05, Brian Schr�der <ruby.brian@gmail.com> wrote:
> > On 19/04/05, pmak@aaanime.net <pmak@aaanime.net> wrote:
> > > The corresponding column in the MySQL database is of type DECIMAL(7,2),
> > > but the corresponding Ruby structure came out as a String instead of a
> > > decimal type. I haven't updated my database driver in a few years,
> > > though; perhaps I'll look into that. Thanks.
> > >
> > >
> >
> > DECIMAL(7.2) should not be converted to a float value, as floats can't
> > represent all instances of DECIMAL(7.2). For example 0.1 can not be
> > represented as a float.
> >
> > irb(main):010:0> "%.18f" % 0.1
> > => "0.100000000000000006"
> 
> In general, using non BCD type number encoding systems, *No* real
> number that isn't a summation of powers of 2 can be represented by a
> float.
> 
> By your logic then, does this mean that they just shouldn't be used
> for any "DECIMAL(x, y)" (y > 0) field in a database?
> 
> 

That depends on what you are using the numbers for. If it is for
simulation of something the float resolutions are good, but in this
special example decimal(7,2) hinted a monetary value to me, and there
it may be bad if you sum up rounding errors.

best regards,

Brian

-- 
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