Kenneth Lumia wrote:

>Ake,
>
>Before you get too involved with trying to figure out a method,
>you may want to try to reduce the number of significant digits
>in the problem.  The final method chosen would depend on the
>accuracy required on the output.  I would think you could round
>many of the factors to fewer digits and still get a reasonably
>accurate answer.  Try using a spreadsheet with columns setup
>using the "full accuracy" numbers, and other columns with the
>coefficients rounded to 3 or 4 digits.  In otherwords, simplify
>before you design.
>
>Once done, you can then decide where the decimal point
>should go and then multiply the coefficients by powers of 2
>and keep track of where the decimal point should be.
>A convenient multiple is 2**8 which will fix the decimal
>between two bytes.
>
>
>Ken
>
>
>----- Original Message -----
>From: "Ake Hedman" <akhe@EUROSOURCE.SE>
>To: <PICLIST@MITVMA.MIT.EDU>
>Sent: Sunday, August 22, 2004 10:55 AM
>Subject: Float expression to integer math
>
>
>I'm totally stuck with a math problem and though I should  ask the list
>for help as a last resort
>
>I need to calculate the dew point in C using integer maths. The dew
>point can be calculated with the following expression
>
>logEw =3D ( 0.66077 + ( ( 7.5 * T )  /  (237.3 + T ) ) + ( log10(RH) - 2=
 )
>dp =3D ( (0.66077 - logEw ) * 237.3 )  /  ( logEw - 8.16077  )
>
>which is derived from
>
>EW =3D 10^ ( 0.66077+(7.5*T/ (237.3+T)) )     //  this is the saturation
>vapor pressure over water.
>EW_RH =3D EW * RH / 100                             // multiply with
>relative humidity
>Dp =3D ((0.66077-log10(EW_RH))*237.3)  / (log10(EW_RH)-8.16077) // this =
is
>the dewpoint
>
>My usual approach to this is to try to make everything in the expression
>integers so I end up with a result that is a number of factors bigger
>than the actual and that way preserving decimals. But in this case an
>long is to small for such an approach.
>
>I can get the ( log10(RH) - 2 ) expression from a table.
>
>My input values for T and RH is t =3D 100*T and rh =3D 100 * RH that is =
both
>is preserved with two decimals.
>
>Any suggestions appreciated....
>
>Regards
>/Ake
>
>
>
>--
> ---
>Ake Hedman (YAP - Yet Another Programmer)
>eurosource, Brattbergavdgen 17, 820 50 LOS, Sweden
>Phone: (46) 657 413430 Cellular: (46) 73 84 84 102
>Company home: http://www.eurosource.se
>Kryddor/Te/Kaffe: http://www.brattberg.com
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>
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> =20
>
Ken,

thanks for setting me on the track. After some analysis in the spread=20
sheet this turned out to be trivial (as always... ). Thanks again!

Regards
/Ake



--=20
 ---
Ake Hedman (YAP - Yet Another Programmer)
eurosource, Brattbergav=E4gen 17, 820 50 LOS, Sweden
Phone: (46) 657 413430 Cellular: (46) 73 84 84 102
Company home: http://www.eurosource.se     =20
Kryddor/Te/Kaffe: http://www.brattberg.com
Personal homepage: http://www.eurosource.se/akhe
Automated home: http://www.vscp.org

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