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From: Deane_Yang_at_[hidden]
Date: 20010629 22:54:11
 In boost_at_y..., "Corwin Joy" <cjoy_at_h...> wrote:
> I think that having a nice templated units library
> would be cool. One thing that seems to be missing
> from this discussion, however, is that the templates
> should have virtual base classes which expose operator
> + and * such that developers can
> use the units classes for either compile time checks or runtime
> checks. In my work, I do a lot of units conversions
> but these have to be checked and cast at runtime
> for three reasons:
> 1. I'm reading the values and units from a database and I
> don't always know what possible units are allowed in advance.
> 2. I often have to cast and format out reports in different
> units of measure depending on how the end users
> prefer to view their results.
> 3. The unit conversion ratios for me will often change at runtime,
> potentially minutebyminute
> e.g. how much is a Mexican Peso worth in US $ today, (exchange
rate)
> . how much is a Mexican Peso to be delivered in 1 year worth in US
$ today
> (exchange rate) * (Mex discount factor at 1 year).
> These kind conversions all benefit from a strict units regimen, but
it needs
> to happen at runtime.
>
I've thought about this, too. But allowing dynamic types to be defined
at runtime makes things way too complicated for me. I'll leave that
to the experts. I concede that foreign currencies are a bit
of a headache to implement using template unit classes.
>(stuff deleted)
> Sort of, there are different time units, tho, depending on the
interest
> rate convention. Example daycounts that are applied to
> interest rates include
>
> Actual/365, 30/360, Actual/Actual, business/260 etc.
> >
I don't agree with this. I've spent more time thinking about daycount
conventions than I care to admit. I finally decided that different
daycount conventions do NOT correspond to different units of time.
Rather a daycount convention is simply a function that takes two
dates and returns a value in years. I implement daycounts in a
separate class that uses but is not derived from
any of my unit classes.
> > 3) Transcendental functions
> I'm not sure I agree with sin & cos, but I definitely *disagree*
for logs.
> It seems to me that the log of a unit has well defined units.
> e.g.
> distance d1 = 5*meters;
> distance d2 = 10*meters;
> distance d3 = exp(log(d1 + d2)); // 15 meters
> area a1 = exp(log(d1) + log(d2)); // 50 meters^2
>
> i.e. in general log(measurement) has a unit that is the log()
operator
> applied
> to the individual units. Naturally supporting these is more
complicated
> since taking the log of a measurement then swaps the role that
> multiplication
> and addition play. I can certainly see why one might draw the line
> at function checking.
I've never seen exp() and log() used to compute areas like this.
And I've never seen log(meters) used anywhere in physics or anywhere
else. You can kind of see how awkward log(meters) are, if you think
about how to change from log(meters) to log(feet). It's not the usual
"multiply by a constant factor" rule. Instead, it's "add by a
constant term". So clearly these are different animals.
Another interesting case study is finance. There are lots of
logarithms and exponentials used in discounting with interest rates
and in option pricing models.
If you look at it all carefully, you'll find that the arguments to exp
() and log() are always unitless and so are the exp()'s and log()'s
themselves. In fact, this is the setting where my unit
classes have been most useful.
A good rule of thumb is to ask yourself what happens to a unit
that is defined in terms of simpler units when you change the simpler
units (like from meters to feet or hours to minutes). If the
unit doesn't simply change by a constant factor, then you should
definitely beware!
By the way, the explanations of pH and decibels that others have
posted also corroborate my claims.
Deane
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