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 numbernand unsets all even positioned bits in that number. - Line 8: We initialize variable
nwith15as the value. - Line 9: We call
unSetEvenPositionedBits()for the value15and print result to the console. - Lines 10 to 13: We overwrite values in the variable
nand print results to the console.
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