JavaScript - Binary logarithm using bitwise operators

JavaScript Bit Manipulation: Exercise-8 with Solution

Binary Logarithm (log2n)

Write a JavaScript program to calculate the binary logarithm (log₂n) using bitwise operators.

From handwiki.org -

Binary logarithm:

In mathematics, the binary logarithm (log₂n) is the power to which the number 2 must be raised to obtain the value n. That is, for any real number x,

For example, the binary logarithm of 1 is 0, the binary logarithm of 2 is 1, the binary logarithm of 4 is 2, and the binary logarithm of 32 is 5.

The binary logarithm is the logarithm to the base 2 and is the inverse function of the power of two function. As well as log₂, an alternative notation for the binary logarithm is lb (the notation preferred by ISO 31-11 and ISO 80000-2).

Test Data:
(1) -> 0
(2) -> 1
(4) -> 2
(32) -> 5

Sample Solution:

JavaScript Code:

// Define a function to calculate the logarithm base 2 of a number
const log_Two = (n) => {
  // Check if the input is not a number
  if (typeof n!= "number") {
     return 'It must be number!' // Return an error message
  }
  let result = 0; // Initialize the result variable to 0
  // Loop to find the logarithm base 2 of the number
  while (n >> 1) {
    n >>= 1; // Right shift the number by 1 bit
    result++; // Increment the result counter
  }
  return result; // Return the logarithm base 2 of the number
}
console.log(log_Two(1)) // Test the function with the number 1
console.log(log_Two(2)) // Test the function with the number 2
console.log(log_Two(4)) // Test the function with the number 4
console.log(log_Two(32)) // Test the function with the number 32

Output:

0
1
2
5

Flowchart:

Live Demo:

See the Pen javascript-bit-manipulation-exercise-8 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 calculates the binary logarithm of a number using bitwise operations to count the number of shifts.
• Write a JavaScript function that uses recursion to compute log base 2 of a number without Math.log2.
• Write a JavaScript function that validates the input is a power of two and returns the exponent if true, or an approximation otherwise.
• Write a JavaScript function that compares the result of your bitwise log2 implementation with Math.log2 for accuracy.

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