Finding prime factors logic error python

Question

Not sure why trying to find prime factors like this doesn't work:

primes= [2,3,5]
maximum = int(input("Highest number checked: "))
x = 2
prime = False
while x < maximum:
    y = len(primes)
    for n in range(2,y):
        if x % int(primes[n]) == 0:
            prime = True
        else:
            prime = False
            break
    if prime == True:
        primes.append(x)
    x = x+1

print(primes)

Inputting 30 prints 2,3,5,5,10,20

Would you like to share the error message such that we can look into the problem or are you just using this site as your personal journal? — timgeb, Dec 10 '17 at 16:11
I just assumed someone would be able to spot my error, sorry? — Lily, Dec 10 '17 at 16:13
Well, someone (probably) already did, but it would be way easier if you gave us the complete error message. 'bool error' says nothing. — timgeb, Dec 10 '17 at 16:14
When I read your code I find I'm unsure of what your program is *supposed* to do. You seem to have predefined a tiny list of primes. Are those the only ones you are going to check for? That means you can only factor very small numbers. — President James K. Polk, Dec 10 '17 at 19:04

score 0 · Answer 1 · answered Dec 10 '17 at 16:11

0

In this line if x % int(prime[n]) == 0: change prime to primes.

answered Dec 10 '17 at 16:11

Ahmad

906
11
27

even with this change the program doesn't work, I inputted 30 and got back 2,3,5,5,10,20 – Lily Dec 10 '17 at 16:16
prime i bool variable and not list. but you write prime[n]. it is not correct – Ahmad Dec 10 '17 at 16:24
what is your problem? please explain complete – Ahmad Dec 10 '17 at 16:25
When I execute the program, if I input the maximum as 30, it should give me back a list of primes but it gives me back 2,3,5,5,10,20 which is of course wrong so I must have logic error – Lily Dec 10 '17 at 16:28
it's meant to give prime factors so of 30 would be 2,3,5 – Lily Dec 10 '17 at 16:31
please check this page for find your answer: https://stackoverflow.com/questions/16996217/prime-factorization-list – Ahmad Dec 10 '17 at 16:34

score 0 · Answer 2 · answered Dec 11 '17 at 17:40

Not sure why trying to find prime factors like this doesn't work

It doesn't work because you didn't write a (broken) prime factor finding program, you wrote a (broken) prime finding program. If we fix it and clean it up, this should be a bit more obvious:

primes = [2, 3, 5]

maximum = int(input("Highest number checked: "))

x = 2

while x <= maximum:
    is_prime = True

    for prime in primes:
        if prime * prime > x:
            break

        if x % prime == 0:
            is_prime = False
            break

    if is_prime:
        primes.append(x)

    x = x + 1

print(primes)

USAGE

% python3 test.py
Highest number checked: 100
[2, 3, 5, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]
%

This can be used as part of a prime factor finding program, but isn't sufficient on its own. You need to work through some subset of the list of primes up to N seeing which primes are factors of N. Not substantially more code than you have now.

Finding prime factors logic error python

2 Answers2