Unable to get floating point number (decimal value) from integer

Comments

• Electrodude Posts: 1,683

  2016-07-30 22:23 edited 2016-07-30 22:24

  pst.dec prints integers, not floats. Floatmath.spin has methods to convert floats to strings, which you can then display with pst.str.
  
  Also, you can't use arithmetic operators (like +, -, *, and /) on floats - they only work on integers. You must use f32.fadd, f32.fmul, etc. instead.
  
  For example, the last line of the second piece of code you wrote should be:

  pst.str(fstr.floattostring(f32.fdiv(f32.fmul(f32.ffloat(temp1), 3.3),1023.0)))
  

  Note the 1023.0 to tell the compiler it should be a float and not an integer.
  
  That could be further simplified to the following, which saves a runtime divide:

  pst.str(fstr.floattostring(f32.fmul(f32.ffloat(temp1), constant(3.3 / 1023.0))))
  

  
  I'm not sure if you really need the constant() in there, or if the 1023 inside the constant() needs the decimal point - I never really understood the rules for floating point constants. Floating point math with arithmetic operators works at compile time, but at runtime you need to use f32 methods.
• Phil Pilgrim (PhiPi) Posts: 23,514

  2016-07-31 01:56

  I'm not sure constant(3.3 / 1023.0) works like that on floats. Better just to do the math on a calculator and use .003225806452?
  
  -Phil
• Electrodude Posts: 1,683

  2016-07-31 04:59

  I'm 99% sure you actually can do that. I know, it's wierd, but you can do that. I have working code that does round(1.14*float(x)) at compile time and works just fine, but I've never tried actually doing constant(float_expr) at compile time.
  
  If you can't do that inside constant(), there's some place inside of CON blocks where you can.
  
  Just try it and see what works.
• kwinn Posts: 8,697

  2016-07-31 13:30

  Another option is to avoid floating point altogether by doing the calculation with 32 bit integers. Faster and simpler.
  
  Multiply the temperature reading from the MCP by 3226 (( 3.3 / 1023 ) x 1,000,000 )
  Convert the result to decimal and place the decimal point where needed.
• Duane Degn Posts: 10,588

  2016-07-31 14:25

  If you're just doing simple arithmetic, you're better off using integers.
  
  To place a decimal point within the number in the correct location (as suggested by kwinn) I use a method I call "DecPoint". Here's a link to this method.
• Duane Degn Posts: 10,588

  2016-07-31 14:34 edited 2016-07-31 14:36

  Here are a couple more methods I use a lot when doing integer math on the Prop. These are methods Tracy Allen suggested when I was concerned about doing math with large numbers. These methods should prevent 32-bit overflow issues in many cases.
  

  PUB TtaMethodSigned(N, X, localD) | sign  ' return X*N/D
  
    sign := 1
    
    if N < 0
      -sign
      -N
    if X < 0
      -sign
      -X
    if localD < 0
      -sign
      -localD
      
    result := TtaMethod(N, X, localD) * sign
  
  PUB TtaMethod(N, X, localD)   ' return X*N/D where all numbers and result are positive =<2^31
    return (N / localD * X) + (binNormal(N//localD, localD, 31) ** (X*2))
  
  PUB BinNormal (y, x, b) : f                  ' calculate f = y/x * 2^b
  ' b is number of bits
  ' enter with y,x: {x > y, x < 2^31, y <= 2^31}
  ' exit with f: f/(2^b) =<  y/x =< (f+1) / (2^b)
  ' that is, f / 2^b is the closest appoximation to the original fraction for that b.
    repeat b
      y <<= 1
      f <<= 1
      if y => x    '
        y -= x
        f++
    if y << 1 => x    ' Round off. In some cases better without.
        f++
  

  
  I added the simple "TtaMethodSigned" method to make Tracy Allen's "TtaMethod" a bit more useful. If you know the three numbers you're using are all positive, you can skip the "TtaMethodSigned" method and simply call the "TtaMethod."