Comments on: Hacker’s Delight

By: Rickyah

Rickyah — Thu, 06 Nov 2008 11:21:39 +0000

Sorry Liam, i totally misunderstood you

By: Liam

Liam — Wed, 05 Nov 2008 17:21:09 +0000

Just typing that out quickly I have missed a set of parenthesis
return x ? (x & (x – 1)) == 0 : false;

By: ent

ent — Wed, 05 Nov 2008 17:20:31 +0000

Liam is right,

That function is not exactly correct because is returning true for zero (zero is not a power of two).

So, rename the function to IsPower2OrZero() or add to that function the check against zero.

Sorry for the bug.

By: Liam

Liam — Wed, 05 Nov 2008 17:17:59 +0000

RickyAH thanks for your concern about me being unable to read code, but I fully understand it and understand the floor in the solution.

bool IsPow2(int x)
{
return x ? x & (x – 1) == 0 : false;
}
bool IsPow2_org(int x)
{
return (((x) & ((x) – 1)) == 0);
}
#include
int main()
{
int i = 0;
std::cout < }

By: RickyAH

RickyAH — Wed, 05 Nov 2008 16:51:43 +0000

Liam I think you misunderstood the code abobe.

A power of two number express in binary ALWAYS has exactly ONE bit equal to 1:
2 = 0010
4 = 0100
8 = 1000
…

take that number and the previous:
1 = 0001
3 = 0011
7 = 0111

Perform an ‘and’ bit operation:
0010 & 0001 == 0000
0100 & 0011 == 0000
1000 & 0111 == 0000

For any number not power of two, e.g. 7 the result is not zero:
0111 & 0110 = 0110 == 6 != 0

By: Liam

Liam — Wed, 05 Nov 2008 15:44:31 +0000

Is zero really a power of two?

By: Rickyah

Rickyah — Wed, 05 Nov 2008 08:25:53 +0000

I’ve seen that code before, but as a macro. It was in a classroom

By: ent

ent — Tue, 04 Nov 2008 14:59:31 +0000

bool IsPow2(int x)
{
return (((x) & ((x) – 1)) == 0);
}

By: Zalo

Zalo — Tue, 04 Nov 2008 13:42:52 +0000

Does this book tell how to enter into the FBI or the NASA computers?

By: Liam

Liam — Tue, 04 Nov 2008 11:57:35 +0000

O dear that is multiple not power of lol.
I suspect it would use the same type of logic thought, just testing the upper bit with the rest zeroed out.