Power of 2
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Actually, I've been using this for years doing embedded programming. Every clock cycle is precious ;)
Ryan
"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"
Hi Ryan, Thanks for the help. The code is great and the application is for an embedded -> PC interface. cheers Rich "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far the Universe is winning." -- Rich Cook
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Crikey. I thought this was well known. Just do this:
if (x & (x-1))
{
// x is <edit>not</edit>a power of two
}
else
{
// x is <edit>not</edit> a power of two
}sorry, got the two cases round the wrong way... :-O
Ryan
"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"
-- modified at 6:04 Monday 6th March, 2006
Hi Ryan, Is there is quick way for now making x a power of 2 after you have decided that it is not? regards, Rich "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far the Universe is winning." -- Rich Cook
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Modified version is still wrong - It will treat any integer as a power of 2. Steve
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Modified version is still wrong - It will treat any integer as a power of 2. Steve
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Hi All, Is there a fast way of checking to see if an
intis a power of 2 (i.e. 2, 4, 8, 16, 1024)? I know the long way using a loop, but I was hoping for a simple way of doing this. regards, Rich "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far the Universe is winning." -- Rich Cook#define ISPOWER2(x) (!((x)&((x)-1)))
"Let us be thankful for the fools. But for them the rest of us could not succeed." - Mark Twain
"There is no death, only a change of worlds." - Native American Proverb
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Read this function, it could be another start point! This function that I wrote last year find the bigger long, greater than the input value, that is a power of 2. And it is quite fast!:)
unsigned long Next2Power(unsigned long x){ unsigned long y=1, x1=x; if(x==0) return 0; while(x1!=0){ x1>>=1; y<<=1; } y>>=1; if(y!=x) y<<=1; return y; }Have a nice code day ;)
_Russell_ wrote:
This function that I wrote last year find the bigger long, greater than the input value, that is a power of 2.
Why not just use
log2()?
"Let us be thankful for the fools. But for them the rest of us could not succeed." - Mark Twain
"There is no death, only a change of worlds." - Native American Proverb
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Hi Ryan, Is there is quick way for now making x a power of 2 after you have decided that it is not? regards, Rich "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far the Universe is winning." -- Rich Cook
RichardS wrote:
Is there is quick way for now making x a power of 2 after you have decided that it is not?
Not that I know of. I've never had to do this. Would you round up or down?
Ryan
"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"
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RichardS wrote:
Is there is quick way for now making x a power of 2 after you have decided that it is not?
Not that I know of. I've never had to do this. Would you round up or down?
Ryan
"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"
Round up. So far I have a loop (for 32-bits) shifting 0x80000000 down until the & with x is successful, then shifting it back up by one. regards, Rich "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far the Universe is winning." -- Rich Cook
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_Russell_ wrote:
This function that I wrote last year find the bigger long, greater than the input value, that is a power of 2.
Why not just use
log2()?
"Let us be thankful for the fools. But for them the rest of us could not succeed." - Mark Twain
"There is no death, only a change of worlds." - Native American Proverb
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Because my function is faster (I think) than log2(). But if I'm wrong let me know how have a faster function. And why. thanks
Have a nice code day ;)
_Russell_ wrote:
...let me know how have a faster function.
This one benchmarks faster than the one you've shown:
unsigned long Next2Power(unsigned long x)
{
x--;
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
x++;return x;}
"Let us be thankful for the fools. But for them the rest of us could not succeed." - Mark Twain
"There is no death, only a change of worlds." - Native American Proverb
-
Hi Ryan, Is there is quick way for now making x a power of 2 after you have decided that it is not? regards, Rich "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far the Universe is winning." -- Rich Cook
#define CK_ISPOW2(n) ( ((n)&((n)-1)) == 0 )
ullong FkGrainDn2( ullong S, ullong G )
{
// if( !CK_ISPOW2(G) ) return( FkGrainDnN(S, G) );
S = S & ~(G-1);
return(S);
}ullong FkGrainUp2( ullong S, ullong G )
{
// if( !CK_ISPOW2(G) ) return( FkGrainUpN(S, G) );
G -= 1;
S = (S+G) & ~G;
return(S);
}where: S is value to round G is a power of 2 ...cmk Save the whales - collect the whole set