AppleScript Math Issue

Question

Can anyone explain why this happens? Just seems like an odd requirement to ensure accurate math.
set a to 12.0
set b to 9.6
set c to b / a
--c is 0.7999999999999999 instead of 0.8 (unless casted to string in next line)
--set c to (c as string) as real
if (c is 0.8) then
  display dialog "c is 0.8"
else
  display dialog c
end if

Answer 1

Hello
My pleasure. Glad to know it helped.
Well, there's one line I'd like to retract in my example, for it is misleading, that is -
set a to 12.0
set b to 9.6
set c to b / a
return roundn(c, -10) = 0.8 -- => true
Indeed it returns true but it is only happy coincidence.
We must carefully avoid "raw" real numbers in any comparison and round the both operands except for literal integers. Such as -
return roundn(c, -15) = roundn(0.8, -15)
or preferrably for performance's sake -
return roundn(c - 0.8, -15) = 0.0
Here's some unhappy coincidence -
set x to 0.3 + 0.4
set y to 0.3 + 0.4
--return x = y -- => true
return roundn(x, -1) = y -- => false
--return roundn(x, -2) = y -- => false
--return roundn(x, -3) = y -- => false
--return roundn(x, -4) = y -- => false
--return roundn(x, -5) = y -- => false
--return roundn(x, -6) = y -- => true
--return roundn(x, -7) = y -- => true
--return roundn(x, -8) = y -- => false
--return roundn(x, -9) = y -- => false
--return roundn(x, -10) = y -- => false
--return roundn(x, -11) = y -- => true
--return roundn(x, -12) = y -- => true
--return roundn(x, -13) = y -- => false
--return roundn(x, -14) = y -- => true
--return roundn(x, -15) = y -- => false
--return roundn(x, -16) = y -- => true
--return roundn(x, -17) = y -- => true
--return roundn(x, -18) = y -- => true
--return roundn(x, -15) = roundn(y, -15) -- => true (trivial if x = y)
--return roundn(x - y, -15) = 0.0 -- => true (ditto)
on roundn(n, d)
        number n : source number
        integer d : target rounding place so that n is rounded to 10 ^ d's place
    set h to 10 ^ d
    (round n / h rounding to nearest) * h
end roundn
Let us tame the floating devils by proper rounding!
All the best,
H