Armstrong number program in Python

An Armstrong number is equal to the sum of its own digits, each raised to the power of the number of digits. 153 has 3 digits, and 1³ + 5³ + 3³ = 1 + 125 + 27 = 153, so 153 is an Armstrong number.

The program

def is_armstrong(n):
    digits = str(n)
    power = len(digits)
    total = sum(int(d) ** power for d in digits)
    return total == n


n = int(input("Enter a number: "))

if is_armstrong(n):
    print(f"{n} is an Armstrong number")
else:
    print(f"{n} is not an Armstrong number")

Typed in this example: 153. Press Run to type your own.

Enter a number: 153
153 is an Armstrong number

How it works

• str(n) gives the digits as text, and len(digits) counts them. For 153, power is 3.
• int(d) ** power turns each digit back into a number and raises it to that power.
• sum(...) adds the results. If the total equals the original number, the function returns True.
• The function is meant for positive numbers. A minus sign is not a digit, so int("-") would raise a ValueError for negative input.

Finding all of them below 1000

def is_armstrong(n):
    digits = str(n)
    power = len(digits)
    return sum(int(d) ** power for d in digits) == n


found = [n for n in range(1, 1000) if is_armstrong(n)]
print(*found)

1 2 3 4 5 6 7 8 9 153 370 371 407

Every one-digit number is an Armstrong number, because a single digit to the power 1 is itself. There are no two-digit ones, and only four with three digits. Change the range to range(1, 10000) to find the three four-digit ones.

Related programs: Strong number.