Question: How do you find out if an unsigned integer is a power of 2?
Answer: The trick here is to know that an unsigned integer which is a power of 2 has only one of its bits as 1. So a simple solution would be to loop through the bits and count the number of 1s.
There is another solution which uses bit manipulation.
isPower = (x !=0 && !(x & (x-1)))
x & (x-1) will always give you a 0 if x is a power of 2. !(x & (x-1)) should give us what we want but there is one corner case. If x is 0, then the second term alone would return true when the answer should be false. We need the check to make sure x is not 0.
Most interviewers would be happy with the first solution. The second solution might be quite hard to come up with on the spot unless of course you are a genius or you have read the solution before.
If you have any questions, please feel free to send me an email at support@mytechinterviews.com. If you have any interview questions which you feel would benefit others, I would love to hear about it.
If you're looking for some serious preparation for your interviews, I'd recommend this book written by a lead Google interviewer. It has 189 programming questions and solutions:
int pow_of_two(unsigned num){
/*count the numbers of ones */
int i=0,count=0;
while(i!=31){
if(num&(1<<i)){
count++;
}
if(!(count%2 == 0)){
printf("Pow of 2");
}
else {
printf("Nah not power of 2");
}
}
/*not optimized but easy to understand*/
Easy way to find whether a number is power of 2.
/*
* A program to find whether a given number is a power of 2.
* jagannath_pattar@yahoo.co.in
*/
#include
void power_of_2(int num)
{
num = num & num-1;
if(num)
printf(“\nNumber is not power of 2\n”);
else
printf(“\nNumber is power of 2\n”);
}
main()
{
unsigned int num;
printf(“Enter the number: “);
scanf(“%d”, &num);
power_of_2(num);
}