A math question, I suppose

I need to have a simple operation to convert a power value to an absolute value. For example 2^23 needs to be converted to 23. Any simple math operation that would permit this? Mod? I'm out of ideas...

I don't follow.

You want to parse a string "2^23" and retrieve "23"?

Or you want to take the result of 2^23 and get a result of 23 again?

Local a = Ceil(2^23)

Local b = 1
While a>2
	a = a/2
	b = b+1
Wend
Print b

Generally, if a=b^c then c=log(a)/log(b)

fredborg, indeed!

Thanks!!! :D

Yet, some problems arise, as this is for use with bit flags:

Graphics 800, 600,32,2
For i = 0 To 31
a = Ceil(2^i)

Local b = 1
While a>2
	a = a/2
	b = b+1
Wend
Print "2^"+i+"="+b
Next
WaitKey()

Nomen luni's idea:

Graphics 800, 600,32,2
For i = 0 To 31
a=2^i
c=Log(a)/Log(2)
Print "2^"+i+"="+c
Next
WaitKey()

Nomen luni's revision 2:

Graphics 800, 600,32,2
For i = 0 To 31
   a=2^i
   c=Log(a) * 1.4427
   Print "2^"+i+"="+a+", which is bit "+c
Next
WaitKey()

Now, this way I can get the bit value, and convert it back to the bit position, very nice. But I guess I'll have to forget using bit 31.

Ah, well then this works:

For i = 0 To 31
a = Ceil(2^i)

Local b = -1
While a<>0
	a = a Shr 1
	b = b+1
Wend
Print "2^"+i+"="+b
Next

Maybe I'm wrong, but Nomen's idea seems more a performance crown. :P

Really ;(

Without running I would have put my money on SHR reather than Log. Oh well

H&K, but what is more performing:
- A solution where you have an undetermined number of Shr in a looped Blitz3D code
- A solution with a single Log() and multiplication
?

But definately, fredborg's idea works for all bit values. Nomen's idea doesn't work for bit 31, from what I tried. Maybe I should just do a bit table LOL. I'm undecided!

Here's the idea:

Function Get_ON_bit(value%)
   Local bit% = -1
   While value <> 0 And bit < 32
      value = value Shr 1
      bit = bit + 1
   Wend
   Return bit
End Function

This is, considering the value only has one bit set, or is masked with one bit.

The log method may not be as speedy as it appears as I imagine the log function is doing some internal loops to come up with an answer.

On my system, "value a = b Shr c" takes 2ns, "value = Log(b)" takes 44ns.

So, considering the odds of performing 30 Shr, vs the very solid 44ns of the Log(), I think fredborg's solution is the best for performance, and is giving 100% of the results I need. And with ferdborg's version, it's even faster than consulting a table in a loop. And if I want to convert absolute value to bit shifted value, that's as simple as "1 Shl 23"...

Cheers.

So without testing, my money was on the winner ;)