How to unset all even positioned bits

Problem statement

Given a number n, unset all bits at even positions. The indexing is zero-based indexing where the rightmost bit/least significant bit is 0-th index.

Consider that the maximum number can be 32-bit.

Example 1

  • Input: 18
  • Output: 2

The binary representation of 18 is 10010. So, the even-positioned bits are 10010. When we turn off all the even positioned bits, we get 00010, that is, 2.

Example 2

  • Input: 15
  • Output: 10

The binary representation of 15 is 1111. So, the even-positioned bits are 1111. When we turn off all the even positioned bits, we get 1010, that is, 10.

Solution

The solution is to generate a mask where the even positioned bits are zero and the odd positioned bits are ones. Since the maximum number of bits is 32, the bits mask is as follows:

10101010101010101010101010101010

In the mask above, all the even positioned (zero-based indexing is followed) bits are unset and all the odd positioned bits are set.

The above bits representation in hexadecimal is 0xaaaaaaaa.

Now, if we take a bitwise AND between the input number, the mask unsets the bits at even positions.

Code example

Let’s look at the code below:

class Main{
    static int unSetEvenPositionedBits(int n) {
        return (n & 0xaaaaaaaa);
    }
    public static void main(String[] args) {
        int n = 15;
        System.out.println("Unsetting even positioned bits of " + n + " we get " + unSetEvenPositionedBits(n));
        n = 18;
        System.out.println("Unsetting even positioned bits of " + n + " we get " + unSetEvenPositionedBits(n));
        n = 100; 
        System.out.println("Unsetting even positioned bits of " + n + " we get " + unSetEvenPositionedBits(n));
    }
}

Code explanation

  • Lines 3 to 5: We define the unSetEvenPositionedBits() method that takes an input number n and unsets all even positioned bits in that number.
  • Line 8: We initialize variable n with 15 as the value.
  • Line 9: We call unSetEvenPositionedBits() for the value 15 and print result to the console.
  • Lines 10 to 13: We overwrite values in the variable n and print results to the console.

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