Incorrect calculations... please help.

[bearpawz]

Ok, Im writing a program, which uses numbers that i require to be 100ths accurate (ie: 000.00)

Obviously trying to do any type of floating point math is a PITA so I just decided I'll multiply everything x 100 and let the display put a decimal point where needed.

Here is my code:


FuelLevel VAR WORD ' Store Current Fuel Level
GalLeft Var WORD ' Gallons Left to consume
GalUse Var WORD ' Gallons Used
MPG VAR WORD ' Holds Miles / Gallon
Range Var WORD ' Holds Range Value
Miles Var Word ' Miles Traveled

Gallons Con 1600 ' Aprox 16.00 Gallons per tank
Miles = 3280 ' Simulated Milage of 32.8 Miles
FuelLevel = 1475 ' Debug Fuel Level = 14.75 Gallons

' __________________________________________________ ________________________________________

Start:
Gosub CalcGalLeft ' Calculate gallons left
Gosub CalcGalUse ' Calculate the gallons used
Gosub CalcMPG ' Calculate MPG
Gosub UpdateDisplay ' Update the display
Goto Start
END

' ------------------------------------------ Sub Routines -----------------------------------

CalcGalLeft:
GalLeft = Gallons - (Gallons - FuelLevel) ' 16.00 - (16.00 - 14.75) = 14.75
' Note: this is same as FuelLevel to begin with so why do I have this here???
Return

CalcGalUse:
GalUse = Gallons - FuelLEvel ' 16.00 - 14.75 = 01.25
Return

CalcMPG:
MPG = Miles / GalUse ' MPG = 32.80 / 01.25 = 26.24
Return

UpdateDisplay:
debug "Level: ", DEC2 GalLeft/100,".",DEC2 GalLeft, 13,10
DEBUG "MPG: ", DEC2 MPG/100,".",DEC2 MPG, 13,10
DEBUG "Range: ", DEC3 Range,".",Dec2 Range, 13,10
DEBUG "Miles: ", DEC4 Miles/100,".",Dec2 Miles, 13,10,13,10
Return



This is the output:

Level: 14.75
MPG: 00.26 <----------- This *SHOULD* be 26.24
Range: 046.46
Miles: 0032.80


MPG should be displayed as 26.24.... but its only showing .26. I cant find whats wrong... Sometimes all you need is another set of eyes :)

[Mith]

Let's do the math....
GalUse = 1600 - 1475 'which equals 125, this is fine

MPG = 3280/125 'which equals 26.24 and the .24 is lost because PBP doesn't handle floating point. Therefore in your debug statement MPG/100 is equal to 26/100 which is less than 0 and is displayed as 0. DEC2 MPG displays 26 which is correct.

Peter

[bearpawz]

AH!, OK... Now I see it. Thanks....

If anyone ever follows this thread or stumbles accross it, here was my solution:

CalcMPG:
MPG = Miles * 100 ' Add 2 decimal places to the miles Variable
MPG = DIV32 GalUse ' perfome a divide on the 32bit result
Return

My output now looks like

Level: 014.75
MPG: 026.24
Range: 387.04
Miles: 032.80

which is 100% what it should be. So now here comes the fun part... scaling this all into a 24 segment bargraph for LEVEL, MPG and RANGE...

Thanks again Mith, Like I said some times it just takes a different set of eyeballs to see what someone else cant... :)

[Archangel]

Hi Bearpawz,
What are you measuring the fuel consumption with? And are you measuring the miles traveled with a VSS signal?
Just curious.
JS