Why won't my python code display any output for my algorithm?

Question

I am making an algorithm with python to find perfect numbers up to 100000000000000. I have created some code for this that should do it, and it does not throw up any errors, but the code just outputted nothing and is running continuously. I have checked, and the first perfect number is six, so why is my program taking so long to get there?

Here is my code:

    number = 1
    divisor = 2
    factors = 1

    if number < 100000000000000:

        while True:
            number2 = number/divisor
            if isinstance(number2, int):
                factors = factors + divisor + number2
                divisor = divisor + 1

            if divisor == number:
                if factors  == number:
                    print(number)
                    number = number + 1

                break

`while True:` _never_ breaks out. You presumably wanted your `if` to be inside the `while` loop, but I'm still not sure the code logic holds for what you're trying to accomplish. — roganjosh, Jun 06 '19 at 20:30
`while True` is always very dangerous. The likely culprit here is that you're not reaching the `break` statement - I would advise instead doing `for number in range(100000000000000)`, which will just iterate through the numbers one by one — Green Cloak Guy, Jun 06 '19 at 20:31
@GreenCloakGuy they cannot ever reach the `break` because its indentation is outside of the `while` loop. I'm not sure I understand why a `while True` loop is always "very dangerous" - it is no different than `while some_condition` and relying on that condition to switch — roganjosh, Jun 06 '19 at 20:35
@roganjosh I'm being generous and assuming the indentation is posted wrong, as it so often is in amateur StackOverflow posts. That would also be the issue, though — Green Cloak Guy, Jun 06 '19 at 20:36
Even if indentation is not at fault, the if number < 10000000, is before the while loop, so it is tested once. With number = 1, all the if conditions are false except number < 100000000 — , Jun 06 '19 at 21:20
See this lovely [debug](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) blog for help. If nothing else, a simple `print` statement to trace values would immediately show the problem; follow up with more to find the rationale. Just staring at your code doesn't reveal the problem as often as we expect. :-) — Prune, Jun 06 '19 at 22:25

score 0 · Accepted Answer · answered Jun 06 '19 at 23:02

In both of these examples, I used the variable "j" for the target number.

This was my first thought, but please do not use it on anything bigger than 10000:

print([i for i in range(1,j+1) if i == sum(k for k in range(1,i//2+1) if i%k == 0)])

Since perfect numbers cannot be prime, I decided to alter a sieve to reduce the amount of numbers to calculate. The sieve can be found here Sieve of Eratosthenes. There is a good explanation there and on the wiki about the sieve.

def SieveOfEratosthenes(n):
    prime = [True for i in range(n+1)] 
    p = 2
    while (p * p <= n):
        if (prime[p] == True): 
            for i in range(p * 2, n+1, p): 
                prime[i] = False
        p += 1
    for p in range(2, n):
        if not prime[p]:
            #this is my change
            if p == sum(k for k in range(1,p//2+1) if p%k == 0):
                yield p
a = SieveOfEratosthenes(j)
b = next(a)
print(b)
try:
    while b < j:
        b = next(a)
        print(b)
except StopIteration:
    print("Done")

These work in theory, but I can't get it to work in a "reasonable" amount of time.

Hopefully these will help until someone can post a more efficient solution.

score 0 · Answer 2 · answered Jun 07 '19 at 20:45

Even if you fix your code, it's the wrong approach for looking for perfect numbers - you're not likely to find more than the first four, and certainly not perfect numbers up to 100,000,000,000,000.

A better approach would be to search for Mersenne primes using a Lucas-Lehmer primality test and when you find one, compute its companion perfect number. This doesn't take much code and will easily eclipse your current approach.

Why won't my python code display any output for my algorithm?

2 Answers2