Check if a Number is a Power of 2

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How to check if a number is a power of 2.  To understand this question, let’s take some example.

Example –

Input – 16   – 16 is a power of 2 (2^4).

Input – 15 – 15 is not a power of 2.

Input – 32- 32 is a power of 2 (2^5).

We can use multiple approaches to check whether a number is a power of 2 or not.

How to Check if a Number is a Power of 2

Method 1 : Check whether a number is perfectly divisible by 2.

In this approach, We’ll check whether a number is perfectly divisible by 2. If a number is perfectly divisible by 2 then it’s remainder is 0.

We’ll repeat this step until n becomes 1. In each step, we check if n%2 becomes non-zero and n is not 1 then n is not a power of 2. If n becomes 1 then it is a power of 2.

Method 2 : Bit Manipulation

Let’s consider the binary representation of any number that is a power of 2. We’ll find only one bit is set in their binary representation.

2    =  00000010 
4    =  00000100 
8    =  00001000 
16   =  00010000 

And number which is one less has following binary representation

1 = 00000001 
3 = 00000011 
7 = 00000111
   0 0 0 0 0 0 1 0  (Binary representation of 2)
&  0 0 0 0 0 0 0 1  (Binary representation of 1)
-------------------
   0 0 0 0 0 0 0 0

We have discussed two approaches to check whether a number is a power of 2 or not. Let’s implement both the approaches in multiple programming languages.

C Code to Check whether a Number is a Power of 2

METHOD 1: Check if a number is perfectly divisible by 2

#include <stdio.h>
int main(void) {
 
 int num, flag = 1;
        printf("Enter a number \n");
        scanf("%d", &num);
 
 while(num > 1) {
 
 if(num % 2 != 0){
      flag = 0;
      break;
 }
 
 num = num / 2;
 }
 
 if(flag) {
 printf("Input number is a power of 2");
 } else {
 printf("Input number is not a power of 2");
 }
 
 return 0;
}

METHOD 2 – Bit Manipulation

#include <stdio.h>
 
int isPowerOfTwo(int num) {
 return num && !(num & (num-1)); 
}
int main(void) { 
 int num;
 
 printf("Enter a number\n");
 scanf("%d", &num);
 
 if(isPowerOfTwo(num)) {
     printf("Input number is a power of 2");
 } else {
     printf("Input number is not a power of 2");
 }
 
 return 0;
}

Java Code to Check if a Number is a Power of 2

METHOD 1: Check if a number is perfectly divisible by 2

import java.util.*;
 
public class PowerofTwo {
 
    public static void main(String[] args) {
        
        int num, flag = 1;
        
        System.out.println("Enter a number");
        Scanner in = new Scanner(System.in);
       
        num = in.nextInt();
 
        while(num > 1) {
            
            if( num % 2 != 0) {
                flag = 0;
                break;
            }
            
            num = num/2;
        }
        
        if(flag == 1)
            System.out.println("Input number is a power of 2");
        else
            System.out.println("Input number is not a power of 2");
    }
    
}

METHOD 2 – Bit Manipulation

import java.util.*;
 
public class PowerofTwo {
 
    
    public static boolean isPowerOfTwo(int num) {
        
        return (num > 0) && ((num & (num - 1)) == 0);
    }
    
    public static void main(String[] args) {
        
        int num;
        
        System.out.println("Enter a number");
        Scanner in = new Scanner(System.in);
        
        num = in.nextInt();
        
        if(isPowerOfTwo(num))
            System.out.println("Input number is a power of 2");
        else
            System.out.println("Input number is not a power of 2");
    }
    
}

Check if a Number is a Power of 2

Check if a Number is a Power of 2 Posted on 12-10-2016  How to check if a number is a power of 2. To understand this question, let’s take some example. 3/10 399

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