determine if int is power of 2

[Moe Sisko]

Using dotnet 2.0
I wrote some code to figure out if a number is a power of 2 :

==
private bool IsPowerOf2(int num)
{
int j = 0x1;
// test bits 0 .. 30. Don't use sign bit (bit 31).
for (int i = 0; i < 31; i++)
{
if (num == j)
return true;

j = j << 1;
}
return false;
}
==

It seems to work, but I got to wondering if there is a more efficient way to
do this. Any ideas ?

TIA

 

[Jeroen Mostert]

Using dotnet 2.0
I wrote some code to figure out if a number is a power of 2 :

==
private bool IsPowerOf2(int num)
{
int j = 0x1;
// test bits 0 .. 30. Don't use sign bit (bit 31).
for (int i = 0; i < 31; i++)
{
if (num == j)
return true;

j = j << 1;
}
return false;
}
==

It seems to work, but I got to wondering if there is a more efficient way to
do this. Any ideas ?

http://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2

private bool IsPowerOf2(int num) {
return num > 0 && (num & (num - 1)) == 0;
}

 

[Moe Sisko]

You could return false as soon as j > num. Is this a homework exercise?

Thanks. No, it's not a homework exercise (although granted, it probably

Why "int j = 0x1" instead of "int j = 1"?

No real reason - it just helps to remind myself that I'm doing bitwise
manipulation stuff.

 

[Moe Sisko]

http://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2

private bool IsPowerOf2(int num) {
return num > 0 && (num & (num - 1)) == 0;
}

Thanks, that's great !