JavaScript - Check a number is a power of 4 or not

JavaScript Bit Manipulation: Exercise-6 with Solution

Power of Four Check

Write a JavaScript program to check if a given positive number is a power of four or not using bit manipulation.

The expression n & (n-1) will unset the rightmost set bit of a number. If the number is a power of 2, it has only a 1-bit set, and n & (n-1) will unset the only set bit. So, we can say that n & (n-1) returns 0 if n is a power of 2; otherwise, it's not a power of 2.
The mask 0xAAAAAAAA will allow us to determine where the set bit is located. In all of its odd positions, the mask 0xAAAAAAAA has a value of 1. The set bit in n will be even if the expression (n & 0xAAAAAAAA) is true.
_(0xAAAAAAAA)16 = _{(10101010101010101010101010101010)}2.

Test Data:
(256) -> true
(4) -> true
(2) -> false
("16") -> "It must be number!"

Sample Solution:

JavaScript Code:

// Define a function to check if a number is a power of four
const Power_of_four = (n) => {
      // Check if the input is not a number
      if (typeof n!= "number") {
          return 'It must be number!' // Return an error message
      }
      // Check if the number is not zero, is a power of two, and has only one bit set in even position
      return n != 0 && (n & (n - 1)) == 0 && (n & 0xAAAAAAAA) == 0;
}
console.log(Power_of_four(256)) // Test the function with the number 256
console.log(Power_of_four(4)) // Test the function with the number 4
console.log(Power_of_four(2)) // Test the function with the number 2
console.log(Power_of_four("16")) // Test the function with the string "16"

Output:

true
true
false
It must be number!

Flowchart:

Live Demo:

See the Pen javascript-bit-manipulation-exercise-6 by w3resource (@w3resource) on CodePen.

* To run the code mouse over on Result panel and click on 'RERUN' button.*

For more Practice: Solve these Related Problems:

• Write a JavaScript function that checks if a number is a power of two and then verifies if the set bit is in an even position using a bitmask.
• Write a JavaScript function that uses logarithms to determine if a number is a power of four by checking if log2(n) is even.
• Write a JavaScript function that recursively divides a number by 4 to see if it reduces to 1.
• Write a JavaScript function that validates input to ensure it is a positive integer before checking for power of four.

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