fract() of negative - unexpected

spikey

https://fuzearena.com/help/view/fract
It seems that

fract(-1.1) = 0.9

I expected:

fract(-1.1) = 0.1

Is that a bug?

pianofire

@spikey I would have thought so too but apparently the fractional part of a negative number is not straight forward.

According to wikipedia the fractional part of -1.3 can be 0.7, 0.3 or -0.3 !

See https://en.wikipedia.org/wiki/Fractional_part

spikey

Ah. I See, I just found out that my program was failing because of fract(-0.0) = 1

test program output:

z=0.400000 fract(z)= 0.4000000
z=0.300000 fract(z)= 0.3000000
z=0.200000 fract(z)= 0.2000000
z=0.100000 fract(z)= 0.1000000
z=-0.000000 fract(z)=1.000000
z=-0.100000 fract(z)=0.900000
z=-0.200000 fract(z)=0.800000
z=-0.300000 fract(z)=0.700000
z=-0.400000 fract(z)=0.600000
z=-0.500000 fract(z)=0.500000
fract(-0.000000)=0.000000

example code:

z = 0.5
for i = 0 to 10 loop
   z = z - 0.1
   fz = fract(z)
   print("z=", z, " fract(z)= ", fz, "\n")
   update()
repeat
test = fract(-0.000000)
print("fract(-0.000000)=",test)
update()
loop
repeat

But something is wrong here, because if I run print(fract(-0.000000)) I get 0.000000 and during the for loop I get 1.000000.
Is it to early in the morning? I don't see the why.

pianofire

@spikey that is strange I will have a look

pianofire

@spikey There are a couple of strange things here. Why does it think that 0 is negative (I am pretty sure that it shouldn't be) and why does it behave differently inside and outside of the loop?

I will raise an issue. In the meantime fract(abs(x)) seems to behave as you want

spikey

Oh, thats perfect. Thanx.

Mincus

I'm guessing the -0.0 and +0.0 are an artefact of the floating point format (it has both a + and - 0.0 due to how it works) and Fuze is taking your "-0.00" literal and converting it to the +0.0 instead whilst the 0.1 - 0.1 is giving -0.0 because of precision limitations in the floating point format.
fract() is then handling -0.0 and +0.0 differently, which may be a bug?