[Solved] Consider the following Python function to check if n is a power of four:def checkPowerOfFour(n): return n > 0 and (n & (n - 1)) == 0 and (n & 0x55555555) != 0What is the subtle bug in this code? | Bit Manipulation

Bit Manipulation - Power of Four

Consider the following Python function to check if n is a power of four:

def checkPowerOfFour(n):
    return n > 0 and (n & (n - 1)) == 0 and (n & 0x55555555) != 0

What is the subtle bug in this code?

AThe mask 0x55555555 is incorrect for 64-bit integers

BThe condition (n & (n - 1)) == 0 incorrectly allows zero

CThe function does not handle negative inputs correctly

DThe bitwise AND with 0x55555555 should be equality to zero

Step-by-Step Solution

Solution:

Step 1: Understand the mask usage
The mask 0x55555555 selects bits in odd positions for 32-bit integers.
Step 2: Consider integer size
On 64-bit systems, this mask only covers lower 32 bits, missing higher bits.
Step 3: Identify the bug
For 64-bit integers, the mask should be 0x5555555555555555 to cover all bits.
Final Answer:
Option A -> Option A
Quick Check:
Mask length must match integer size [OK]

Quick Trick: Mask must cover all bits for correct power-of-four check [OK]

Common Mistakes:

MISTAKES

Trap Explanation:

PITFALL

Option B is wrong because zero is excluded by n > 0; Option D reverses mask logic.

Interviewer Note:

CONTEXT

Master "Power of Four" in Bit Manipulation

3 interactive learning modes - each teaches the same concept differently

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