2^x = ?

BlitzMax Forums/BlitzMax Programming/2^x = ?

DavidDC

(Posted 2007) [#1]

I clearly don't understand how Power works as this code

Local check_flag:Int = 0

For Local conflict_id:Int = 0 Until 8
			
	check_flag = 2^conflict_id

	DebugLog "2^" +conflict_id + " = " +check_flag

Next

Produces this output:

DebugLog:2^0 = 1
DebugLog:2^1 = 2
DebugLog:2^2 = 4
DebugLog:2^3 = 7
DebugLog:2^4 = 15
DebugLog:2^5 = 32
DebugLog:2^6 = 63
DebugLog:2^7 = 127

Why aren't the numbers (bar 1) all even?

Floyd

(Posted 2007) [#2]

This happens because of very small inaccuracies in the exponentiation function ( corresponding to the MinGW pow( ) function ) and the fact that floating point values are coverted to integer by chopping off anything after the decimal point. Thus a number like 7.9999999... becomes 7 rather than 8.

The problem will be fixed with the next BlitzMax update, which uses a recent version of MinGW and an improved pow().

DavidDC

(Posted 2007) [#3]

Thanks Floyd. That makes sense - nice to know it's being fixed!

TomToad

(Posted 2007) [#4]

BlitzMAX doesn't use MinGW's pow() function at all. Mark opted to write a faster, but less accurate version which is called bbFloatPow().
The newer version of MinGW does handle floating point math more accurately, which in turn will allow bbFloatPow() to return more accurate results. But if you are not wanting to wait for the next BMAX version with newer MinGW, or not wanting to fudge about trying to get the newer MinGW to work with the current BMAX, then you can call MinGW's pow() function directly.

Extern
	Function pow:Double(x:Double,y:Double)
End Extern

Local check_flag:Int = 0

For Local conflict_id:Int = 0 Until 8
			
	check_flag = pow(2,conflict_id)

	DebugLog "2^" +conflict_id + " = " +check_flag

Next

H&K	(Posted 2007) [#5]

Local check_flag:Int = 0

For Local conflict_id:Int = 0 Until 8
			
	check_flag = Ceil (2^conflict_id)

	DebugLog "2^" +conflict_id + " = " +check_flag

Next

DavidDC

(Posted 2007) [#6]

Thanks for those great (and instructive!) alternatives, but I think I'll just play it safe and check against this array:

Global CheckFlag:Int[] = [..
%0000000000000001,..
%0000000000000010,..
%0000000000000100,..
%0000000000001000,..
%0000000000010000,..
%0000000000100000,..
%0000000001000000,..
%0000000010000000,..
%0000000100000000,..
%0000001000000000,..
%0000010000000000,..
%0000100000000000,..
%0001000000000000,..
%0010000000000000,..
%0100000000000000,..
%1000000000000000..			
]

Actually... scratch that. I'll do this instead :)

Local check_flag:Int = 1

For Local conflict_id:Int = 0 Until 8
	
	DebugLog "2^" +conflict_id + " = " +check_flag
	check_flag :Shl 1

Next

David

Dreamora

(Posted 2007) [#7]

congrats.

First one that realized that 2^x is crap anyway and that shl and shr aren't there just for fun :-)

xlsior

(Posted 2007) [#8]

With the latest MinGW, Blitzmax 1.24 returns the following results:

2^0 = 1
2^1 = 2
2^2 = 4
2^3 = 8
2^4 = 16
2^5 = 32
2^6 = 64
2^7 = 128