On 8/29/2010 3:08 AM, Nick wrote:
The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143 ?
#don't forget 2,3,5,7. this function doesn't deliver those as output.
def is_prime(b): #checks a number greater than 7 to see if it is
prime and returns if is.
if b % 2 != 0 and b % 3 != 0 and b % 5 != 0 and b % 7 != 0:
print b,
def factor(num):
x = num
b = 1
c = num
while b <= c: #starts at 1 and searches all the numbers up
to the number you put in
if x % b == 0:
is_prime(b)
b += 1
else:
b += 1
print "Don't forget to consider primes 2, 3, 5, and 7\n"
#600851475143
#shows what numbers in given range are prime
'''
def is_prime(b):
return [2,3,5,7] + [x for x in xrange(3, 10000, 2) if x % 2 != 0
and x % 3 != 0 and x % 5 != 0 and x % 7 != 0]
'''
I'm looking for some help with this problem. I realize my code is
inefficient for such a big number, and also I'm not quite sure it
works perfectly in and of itself.
Did you test the program? That is one way to tell whether it works
perfectly. What you showed above will do one visible thing - it will
print "Don't forget to consider primes 2, 3, 5, and 7\n". The rest is a
somewhat confusing collection of function definitions and comments. You
never call the functions so nothing else will happen.
As Alan said - research prime factors to see how others approach it.
My current reasoning was something of this sort: Find all the factors
of a number, then reduce them to just the prime factors
Very inefficient. IMHO the proper way is to generate a list of all the
prime numbers up to the square root of 600851475143, then test each
(starting with the largest and working down) till you discover a factor.
That then is the answer.
There are many published algorithms for generating primes.
and you can then multiply them together to get that number thus having
the prime factors. I need a lot of help haha. Thanks in advance
everyone. If anyone has a good resource to point me to other than the
open book project and dive into python it would be much appreciated.
Would it be useful for me to buy a book, and if so what are some
easily accessible ones? I feel dive into python is just too advanced
for me. I understand a lot of the teachings, but the examples seem
unwieldy and esoteric.
--
Bob Gailer
919-636-4239
Chapel Hill NC
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