Find the biggest prime of a number [Python]

Question

def f(num):
    return num

number = f(13195)
list = [ x for x in range(number) if ((x!= 0 and x!=1) and number%x ==0)] #x (the multiplication factor) should not be 0 or 1
max = max(list)
for num in range(1,max) :
    if all(num%i!=0 for i in range(2,num)):
       if num in list:
           print num

Hello, this code is used to find the biggest prime of a number. i have tried using f(13195) and i found the correct numbers, but I got an error of too many items in range, is there any other way of solving this?

But when I typed 600851475143,it says the range has too many items, however, does it mean I should not use range? Thanks in advance !

@lejlot I tried, however, the number is still too big, it shows another error. — el psy Congroo, Oct 03 '14 at 00:21
What is the purpose of the for loop? Why not just do `print(list)`? — Code-Apprentice, Oct 03 '14 at 00:42
Also what is the purpose of the function `f`? You can change `number = f(13195)` to `number = 13195`. — Code-Apprentice, Oct 03 '14 at 00:51
@Code-Apprentice There is no purpose for that line . sorry my bad , haven't been coding for ages. — el psy Congroo, Oct 04 '14 at 00:55

score 2 · Answer 1 · answered Oct 02 '14 at 23:50

2

You can use xrange(number) instead.

However the loop will take a very long time for large numbers

Alternatively use Python3 where range behaves like xrange

answered Oct 02 '14 at 23:50

John La Rooy

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it now says Python int too large to convert to C long, let me look into it. – el psy Congroo Oct 03 '14 at 00:00
It doesn't work, don't know how to fix it, can you help me ? Thanks! – el psy Congroo Oct 03 '14 at 00:19

score 0 · Accepted Answer · edited May 23 '17 at 12:21

import math
def f(num):
    return num

number = f(600851475143)
list = [ x for x in xrange(int(math.sqrt(number))) if ((x!= 0 and x!=1) and number%x ==0)] 
max = max(list)
for num in xrange(1,max) :
    if all(num%i!=0 for i in xrange(2,num)):
       if num in list: # if sth is true it runs...
           print num

I figured it out, we need to change the range up to square-root of the number, I changed the algorithm rather than trying to find some range bigger than xrange in python

Please find detail explanation here: If you are interested:) "second reply" Why do we check up to the square root of a prime number to determine if it is prime?

Find the biggest prime of a number [Python]

2 Answers2