Doing Simple Math - getting the wrong answer

Probably a 'newbie' error, but it has me cornered...

I have a routine that is trying to take degrees C and convert to degrees F.. However the math is not working.

Here is how it works:

1.) I take data into an array and read that data tha deals with temperature and convert from hex to decimal

' read data for temperature Temp
SSD4= (ssmax[24]>>4)
SSD3= (ssmax[24] & $f)
SSD2= (ssmax[23]>>4)
SSD1= (ssmax[23] & $f)

' convert to Decimal:

Dec_temp=(ssd4*4096)+(ssd3*256)+(ssd2*16)+ssd1

2) I read these values out into a screen to show degrees (C), then write it out to the screen:

serout2 cpinout, 84,["Temp(C): ",#Dec_temp dig 3, #Dec_temp dig 2,#Dec_temp dig 1, ".",dec1 Dec_temp]

THIS WORKS FINE.. I get the proper 'Degrees C'...

"Temp(C): 025.5 " shows up in HyperTerminal. No problem..

BUT! Here is where it gets funky.. What if I wanted to see Degrees F?

3) I multiply:
Dec_tempf=(Dec_temp*(9/5) + 32) ' convert to farenheit

And output with this:

serout2 cpinout, 84,["Temp(F): ",#Dec_tempf dig 3, #Dec_tempf dig 2,#Dec_tempf dig 1, ".",dec1 Dec_tempf]

The display The same serout2, but now in (F)

It shows "Temp(F): 025.5" - What the heck?? That is the same as (C)???

So then I decide to look at the numbers underneath:

First - theBase # is "223" This is the value in #Dec_temp

Then look at the math:
Testmath1 - (Dec_temp*(9/5)+32) = 255 -- this is wrong

Or even look at **:
Testmath2 - (Dec_temp**(9/5)+32) 32 -- still odd..

Can anyone solve this puzzle?

Thanks

Tom

Quote:

---

First - theBase # is "223" This is the value in #Dec_temp

Then look at the math:
Testmath1 - (Dec_temp*(9/5)+32) = 255 -- this is wrong

---

PBP uses full precedence of math operators, and also uses braces to force out of order of calculation.
Secondly it also only works in integers.

As such (9/5) = 1.8 = 1 ( and is calculated first )
hence Dec_tempf = 223 * 1 +32 = 255

try ( Dec_temp*( 90 / 5 ) + 320 ) / 10
ie ( Dec_temp*18 + 320 ) / 10

Also, if you just leave it at ( Dec_temp*18 + 320 ) you can use simple math to get the basic temp and the first decimal.

Andrew

Hi Tom,

PBP can't handle fractions very well, the "**" and "*/" operators however makes it possible. 9/5 is 1.8 which PBP will trunctate to 1, not very useful. The "**" operator can handle the fractional part, this operaton makes an "invisible" division by 65536. 0.8*65536=52428.8 which we round up to 52429. Since your Dec_temp variable contains the temerature in tenths of a degree your formula needs to be ......

Dec_tempf=Dec_temp*(9/5) + 320 'Farenheit * 10

We rewrite this to ...
Dec_tempf=Dec_temp*1.8 + 320 'Farenheit * 10

Which is the same as .....
Dec_tempf=Dec_temp*1+Dec_temp*0.8 + 320 'Farenheit * 10

Using "**", we end up with ......
Dec_tempf=Dec_temp + Dec_temp**52429 + 320

Another thing, you can replace ......

' read data for temperature Temp
SSD4= (ssmax[24]>>4)
SSD3= (ssmax[24] & $f)
SSD2= (ssmax[23]>>4)
SSD1= (ssmax[23] & $f)

' convert to Decimal:

Dec_temp=(ssd4*4096)+(ssd3*256)+(ssd2*16)+ssd1

...... with ......

Dec_temp.highbyte = ssmax[24]
Dec_temp.lowbyte = ssmax[23]

/Ingvar

Fahrenheit = Celsius * (212 - 32)/100 + 32


Example 30 Celsius to Fahrenheit:


30 * 180 / 100 + 32 = 86 Fahrenheit


Best regards,

Luciano