Check Evil Number in Python

What You’ll Learn

An evil number has an even count of 1 bits in binary; an odd count means odious. This tutorial covers Hamming weight, bit_count(), manual loops, a live preview, worked Python examples, edge cases, and complexity.

📝 Problem & Approach

Given a nonnegative integer n, decide whether its binary popcount is even.

# 15 → 1111  → 4 ones → evil
# 7  → 111   → 3 ones → odious
# 0  → 0     → 0 ones → evil

Inputs & Outputs

Minimal workflow

function is_evil(n):  // n >= 0
    ones = count_ones_in_binary(n)
    return (ones mod 2) == 0

Method comparison

🔮 Live Preview

Nonnegative integers only for this definition (within JavaScript safe range).

Press "Classify" to see result.

Examples Gallery

Three complete Python programs — bit_count(), range scan with division, and Kernighan popcount. Click View Output to reveal sample console results.

def is_evil(n: int) -> bool:
    if n < 0:
        return False
    return n.bit_count() % 2 == 0


number = 15
if is_evil(number):
    print(f"{number} is an Evil Number.")
else:
    print(f"{number} is not an Evil Number.")

15 is an Evil Number.

def is_evil_nonneg(num: int) -> bool:
    ones = 0
    while num > 0:
        if num % 2 == 1:
            ones += 1
        num //= 2
    return ones % 2 == 0


print("Evil numbers in the range 1 to 10:")
for i in range(1, 11):
    if is_evil_nonneg(i):
        print(i, end=" ")

Evil numbers in the range 1 to 10:
3 5 6 9 10

def popcount_kernighan(n: int) -> int:
    ones = 0
    while n:
        n &= n - 1
        ones += 1
    return ones


def is_evil_kernighan(n: int) -> bool:
    if n < 0:
        return False
    return popcount_kernighan(n) % 2 == 0


for n in (15, 7, 0, 3):
    label = "evil" if is_evil_kernighan(n) else "odious"
    print(f"{n}: {label} (ones={popcount_kernighan(n)})")

15: evil (ones=4)
7: odious (ones=3)
0: evil (ones=0)
3: evil (ones=2)

🔎 Worked Walkthrough — n = 15

Trace popcount for the classic evil example.

For contrast, 7 = 111 has 3 ones → odious.

⚖️ Facts Worth Knowing

Handy follow-ups interviewers sometimes ask.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices