How to find the exponent of 2 in an integer in Java?

Question

I want to find the power of 2 that any integer contains. Like 12 = 2*2*3 so answer should come as 2, 28 = 2*2*7 so answer should come as 2 and so on.

int powerOf2InNumber = (int)Math.floor(Math.log(number) / Math.log(2));

I tried the above code but in cases like 28,26,10 etc i'm getting wrong answer.

You specifically want to find how many times 2 divides it _exactly_. The logarithm won't be involved there. — Louis Wasserman, Jun 06 '20 at 20:25
You should try it with a counter, a `while` loop and the modulo operator — QBrute, Jun 06 '20 at 20:26

score 10 · Accepted Answer · answered Jun 06 '20 at 20:37

10

There is a handy built-in function,

int powersOf2 = Integer.numberOfTrailingZeros(number);

answered Jun 06 '20 at 20:37

harold

61,398
6
86
164

It might be worth mentioning for the uninitiated that this is the number of trailing zeroes of a binary string of the number and not the decimal number. Of course there's always the JavaDoc. – WJS Jun 07 '20 at 00:54

Matt · Answer 2 · 2020-06-06T20:41:10.790

0

This should do the trick:

int check = 28;
int count = 0;
while(check % 2 == 0) {
    check /= 2;
    count++;
}

Check ends up as the other factor. I.e the 7 in 2 * 2 * 7. Count is your answer.

edited Jun 06 '20 at 20:41

answered Jun 06 '20 at 20:34

Matt

111
7

score 0 · Answer 3 · answered Jun 06 '20 at 20:44

What I think you're asking is: the number of times 2 goes into a number?

int countPowerOfTwo(int number) {
  int count = 0;
  while (abs(number) > 0) {
    if (number % 2 != 0) {
      return count;
    }
    count++;
    number = number / 2;
  }
  return count;
}

WJS · Answer 4 · 2020-06-08T19:35:58.987

The best way, imo, has already been provided using Integer.numberOfTrailingZeros. It is taken from Hacker's Delight which is an excellent book and worth the price. Another method is as follows:

int b = 32*75;
int powerOf2 = BitSet.valueOf(new long[]{b}).nextSetBit(0);
System.out.println(powerOf2);

Prints

Note: For completeness you weren't far off in your attempt and logarithms can be used along with some basic bit manipulation. So you can do the following:

int number = 32*75;
int powerOf2 = (int)(Math.log(number & -number)/Math.log(2))

How to find the exponent of 2 in an integer in Java?

4 Answers4