Python Program to Check a Neon Number

Beginner · Numbers

A neon number is a number where the digits of its square add up to the number itself. 9 is a neon number: 9 x 9 = 81, and 8 + 1 = 9.

Program

neon_number.py

num = 9 # or: num = int(input("Enter a number: "))

square = num * num

digit_sum = 0

temp = square

while temp > 0:

digit_sum += temp % 10 # add the last digit

temp //= 10 # then drop it

print(f"Square of {num}: {square}")

print(f"Sum of the digits of {square}: {digit_sum}")

if digit_sum == num:

print(num, "is a neon number")

else:

print(num, "is not a neon number")

Output

Square of 9: 81
Sum of the digits of 81: 9
9 is a neon number

The program first squares the number: 9 x 9 = 81.
The while loop adds up the digits of the square, taking the last digit with % 10 and dropping it with //= 10.
If that digit sum equals the original number, the number is neon.
The loop works on temp, a copy, so square is still there to print.

def is_neon(n):

return sum(int(d) for d in str(n * n)) == n

found = [n for n in range(10001) if is_neon(n)]

print("Neon numbers from 0 to 10000:", found)

Output

Neon numbers from 0 to 10000: [0, 1, 9]

Only 0, 1 and 9 are neon numbers: for anything bigger, the digits of the square can never add up to as much as the number.

Set num = 12 and work out by hand why it is not a neon number.

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