CODEWITHSUNDEEP
javamathexponential
UnscrewedChaff
UnscrewedChaff

Reputation: 57

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

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.

Upvotes: 4

Views: 1395

Answers (4)

WJS
WJS

Reputation: 40057

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

5

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))

Upvotes: 0

Alec
Alec

Reputation: 509

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;
}

Upvotes: 0

Matt
Matt

Reputation: 111

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.

Upvotes: 0

user555045
user555045

Reputation: 64913

There is a handy built-in function,

int powersOf2 = Integer.numberOfTrailingZeros(number);

Upvotes: 12

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