Question

How to round a number to the previous power of 2 in Java

Answer 1

public class PowerOfTwo {

    /**
     * Rounds an integer down to the previous power of 2.
     *
     * return the previous power of 2 less than or equal to n
     */
    public static int roundToPreviousPowerOf2(int n) {
        if (n <= 0) {
            return 0;
        }
        
        return (int) Math.pow(2, Math.floor(Math.log(n) / Math.log(2)));
    }

    public static void main(String[] args) {
        int num = 31;
        
        System.out.println("Previous power of 2: " + roundToPreviousPowerOf2(num));
    }
}



/*
run:

Previous power of 2: 16

*/

 

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Answer 2

public class PowerOfTwo {
    public static int roundToPreviousPowerOf2(int n) {
        if (n <= 0) return 0;
        
        return 1 << (31 - Integer.numberOfLeadingZeros(n));
    }

    public static void main(String[] args) {
        int num = 21;
        
        System.out.println("Previous power of 2: " + roundToPreviousPowerOf2(num));
    }
}



/*
run:

Previous power of 2: 16

*/

 

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