Question

I’m currently trying to find the largest prime number contained within another large number.

maxlen = 1024
for i in range(1023, -1, -1):
    maxlen -= 1
    number = ""
    for k in range(maxlen, -1, -1):
            number = pi[k] + number
            if isprime(number) == True:
                    print number

isprime() is a function that checks if the number is a prime (pretty standard).
this works pretty well up to a certain point where I get a MemoryError.

This is not because the number checked by the function is too large since it happens around the 6th run of the first for loop.

I’ve already tried gc.enable() and gc.collect() without any positive result.

Does anyone have an idea how to fix this?

Edit: definition of pi and isprime() as per request:

f = open("/root/number", "r")
pi = f.read()
f.close()

where the file “number” contains the original number in which I’d like to find the prime.

def isprime(n):
    n = abs(int(n))
    if n < 2:
            return False
    if n == 2:
            return True
    if not n & 1:
            return False
    for x in range(3, int(n**0.5)+1, 2):
            if n % x == 0:
                    return False
    return True

Traceback:

Traceback (most recent call last):
  File "./primal.py", line 36, in <module>
    if isprime(number) == True:
  File "./primal.py", line 24, in isprime
    for x in range(3, int(n**0.5)+1, 2):
MemoryError

Answer 1

Use xrange instead of range, most importantly in isprime, here:

for x in xrange(3, int(n**0.5)+1, 2):

xrange doesn’t create the whole list in memory, while range does, but you are not using the results after you iterated over them.

Another tip: just test for isprime(n), there is no need to see if it is equal to True, that is what if does. 🙂

if isprime(number):  # Only works if isprime(number) is boolean True
    ...