Just estimate...

[Michael Vogel]

...how long this program will take and what value will be displayed?

Code: Select all

a.q
cube.q
a=1
Repeat
	a+1
	cube=a*a*a
Until cube<>Pow(a,3)
Debug a

[Trond]

Strange.

[dell_jockey]

hmmm, on 4.01 (XP) I got '257', whose third power is only 16.974.593 - way, way below the maximum limit of a quad

Although you seem to have stumbled over something, it's perfectly sensible: POW expects float arguments (so 'a' will be typecast to a float by POW) and returns a float. 16.974.593 is still way below the maximum limits of floats too, but floating point rounding errors WILL creep in, so it's actually astonishing the loop gets executed that often, really.

It goes to show that one shouldn't compare floats with ints/quads...

[Dr. Dri]

it's weird indeed

Code: Select all

a.q
cube.q
a=1
Repeat
   a+1
   cube=a*a*a
Until cube <> Pow(a,3)
Debug a
Debug cube
Debug Pow(a, 3)

[EDIT]
i got it !
pow is interpreted as a float in the test and as a double in the debug

Code: Select all

a.q = 257

cube_q.q = a * a * a
cube_f.f = a * a * a
cube_d.d = a * a * a

Debug cube_q
Debug cube_f
Debug cube_d
Debug Pow(a, 3)

Dri

[Trond]

The problem is not in the quad/int/float comparision, it's inside Pow():

Code: Select all

cube.f = Pow(257., 3.)
Debug cube

PB actually has two Pow()s. The double precision one returns the correct answer, the single precision one does not. In my opinion it should, since it's within the range of a float.

[Kaeru Gaman]

I think it's the coparison.

change the one line into:

Code: Select all

Until cube <> Int(Pow(a,3))

and the code will run until the limit of a LONG.

[Trond]

It's not the comparision. When you use Int() the double precision version of Pow() is used.

[Kaeru Gaman]

> When you use Int() the double precision version of Pow() is used.
where can I get more information about the two versions of Pow()?
how can I force the use of one of them?


> the single precision one does not. In my opinion it should, since it's within the range of a float.
not only the range, also the precision counts.

a 4byte float uses 23bit for the mantissa.
that means, if the value exceeds 2^24 = 16.777.216, the precision gets worse than 1.
(meaning, a variance grater 1 can occur)

as you see, the code runs out when Pow(a,3) = 16.974.593,
wich is the first cube that is bigger than 2^24

the last attempt that works is 256^3, wich is 2^24....

[Trond]

> When you use Int() the double precision version of Pow() is used.
where can I get more information about the two versions of Pow()?
how can I force the use of one of them?

Look at the generated asm code.