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How do I determine if an integer is a power of two?

For Beginners

Started by joshh December 24, 2001 12:34 AM

5 comments, last by joshh 22 years, 6 months ago

joshh

122

Author

December 24, 2001 12:34 AM

How do I determine if an integer is a power of two? josh

Pretender

122

December 24, 2001 12:37 AM

if (pow(2, log(2, number)) == number)
blah;

ought to work

invective

118

December 24, 2001 12:53 AM

If you want only to deal with integers, you can also just check that there is one and only one bit set in the entire number (in which case it must be a power of two).

  bool powerOfTwo (unsigned int number){if (number == 0)  return false;unsigned int numberOfbits = 0;unsigned int bitmask = 1;for (i=0; i< sizeof (unsigned int); i++){  //Check if current bit is set with logical and  if (bitmask & number)     numberOfBits++;  //If more than one bit is set, this is not a power of two  if (numberOfBits > 1)     return false;  //Get Ready to check the next bit  bitmask <<= 1;}return true;}

alexk7

138

December 24, 2001 01:17 AM

invective has a little error: you must multiply the result of sizeof() by CHAR_BIT to obtain bits instead of bytes.

bool isPowerOfTwo(unsigned int number){   bool found = false;   unsigned int mask = 1;   for (int i = 0; i < (sizeof(number) * CHAR_BIT); ++i)   {      if (number & mask)         if (found) return false;         else found = true;      mask <<= 1;   }   return found;}

alexk7

joshh

122

Author

December 24, 2001 01:38 AM

Thanks for all the replies, it is exactly what i needed to know.

joshh