C / C++ / MFC

Have a nice code day

unsigned long Next2Power(unsigned long x){<br />
	unsigned long y=1, x1=x;<br />
	if(x==0) return 0;<br />
	while(x1!=0){<br />
		x1>>=1;<br />
		y<<=1;<br />
	}<br />
	y>>=1;<br />
	if(y!=x) y<<=1;<br />
	return y;<br />
}

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.

"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

Have a nice code day

_Russell_ wrote:

...let me know how have a faster function.

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

double val = 0.0;
val = log10(y) / log10(2.0);

//Check that 'val' is pure int (Here you use ur own code is u optimize)
int val2 = int(val);

if (val == val2) 
     // y is power of 2
else
     // y is not power of 2

Divyang Mithaiwala wrote:

my suggesion is not wrong for it.

2^x = y
log 2^x = log y
x log 2 = log y

x = log y /log 2 

int y = 1024;
if(y == (int)(::pow(2.0, ::floor(::log((double)y)/::log(2.0)))))
{
  //  true
}
else
{
  // false
}

Taka Muraoka wrote:

BTW, all these suggestions to use sqrt() and log() are nuts - these functions are insanely slow!

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...