# Weird MemoryError issue

Discussion in 'Python' started by jedi200581@yahoo.co.uk, Jul 19, 2006.

1. ### Guest

Hi,

I'm new at python as I just started to learn it, but I found out
something weird. I have wrote a little program to compute Mersenne
number:

# Snipet on
def is_prime n:
for i in range(2, n):
if (n % i) == 0:
return 0
else:
return 1

for a in range(2, 10000000):
if (is_prime(a) and is_prime(2**a-1))
print 2**a-1, " is a prime number"
# Snipet off

This program raise MemoryError. But this one:

# Snipet on
def is_prime n:
for i in range(2, n):
if (n % i) == 0:
return 0
return 1 # the change is here

for a in range(2, 10000000):
if (is_prime(a) and is_prime(2**a-1))
print 2**a-1, " is a prime number"
# Snipet off

Does not! Why ??

Thx.

, Jul 19, 2006

2. ### Diez B. RoggischGuest

wrote:

> Hi,
>
> I'm new at python as I just started to learn it, but I found out
> something weird. I have wrote a little program to compute Mersenne
> number:
>
> # Snipet on
> def is_prime n:
> for i in range(2, n):
> if (n % i) == 0:
> return 0
> else:
> return 1
>
> for a in range(2, 10000000):
> if (is_prime(a) and is_prime(2**a-1))
> print 2**a-1, " is a prime number"
> # Snipet off
>
> This program raise MemoryError. But this one:
>
> # Snipet on
> def is_prime n:
> for i in range(2, n):
> if (n % i) == 0:
> return 0
> return 1 # the change is here
>
> for a in range(2, 10000000):
> if (is_prime(a) and is_prime(2**a-1))
> print 2**a-1, " is a prime number"
> # Snipet off
>
> Does not! Why ??

Don't use range, use xrange. The former will generate an actual list in
memory, which you usually don't need - especially in for-loops as you
don't even have a reference on the list itself.

And I don't think that your change afflicts memory consumption at all. Must
be a coincidence.

Diez

Diez B. Roggisch, Jul 19, 2006

3. ### John MachinGuest

On 20/07/2006 1:58 AM, wrote:
> Hi,
>
> I'm new at python as I just started to learn it, but I found out
> something weird. I have wrote a little program to compute Mersenne
> number:
>
> # Snipet on
> def is_prime n:

Syntax error. Should be:
def is_prime n:

> for i in range(2, n):
> if (n % i) == 0:
> return 0
> else:
> return 1
>
> for a in range(2, 10000000):
> if (is_prime(a) and is_prime(2**a-1))

Syntax error (missing

> print 2**a-1, " is a prime number"
> # Snipet off
>
> This program raise MemoryError. But this one:
>
> # Snipet on
> def is_prime n:
> for i in range(2, n):
> if (n % i) == 0:
> return 0
> return 1 # the change is here
>
> for a in range(2, 10000000):
> if (is_prime(a) and is_prime(2**a-1))
> print 2**a-1, " is a prime number"
> # Snipet off
>
> Does not! Why ??

Neither of the above compiles without error. You may regard this as a
novel suggestion (it's not), but try pasting the actual code that you ran.

When corrected, the versions are functionally equivalent. They give the
same result: on my box, they fell over trying to do
is_prime(536870911), in particular trying to do range(536870911). If
successful, that would produce a list of approx 0.5G elements. On a
32-bit box, that's about 2GB of pointers to objects. Each object has a
reference count, a pointer to its type, and its value -- another 6GB.
Total about 8GB. Use xrange() to avoid the memory grab.

HTH,
John

John Machin, Jul 19, 2006
4. ### John MachinGuest

On 20/07/2006 6:05 AM, John Machin wrote:
> On 20/07/2006 1:58 AM, wrote:
>> def is_prime n:

>
> Syntax error. Should be:
> def is_prime n:

Whoops! Take 2:

Should be:

def is_prime(n):

John Machin, Jul 19, 2006
5. ### Guest

John Machin wrote:
> On 20/07/2006 6:05 AM, John Machin wrote:
> > On 20/07/2006 1:58 AM, wrote:
> >> def is_prime n:

> >
> > Syntax error. Should be:
> > def is_prime n:

>
> Whoops! Take 2:
>
> Should be:
>
> def is_prime(n):

Sorry for that, I was not able to cut-paste the code at the moment, and
was in a hurry, I know it's bad.

Thanks for "xrange", I tried and it works fine.

, Jul 20, 2006