Integer Overflow

[Gareth McCaughan]

"Ursus Horibilis" wrote:

> unsigned int Lcprng(unsigned int *seed)
> {
>      *seed = 29 * (*seed) + 13;
>      return (*seed);
> }
> 
> How do you do this in Python?

def Lcprng(state):
  """Pass this a list containing just the seed. It will be updated,
  and the value inside returned."""
  state[0] = (29*seed[0]+13) & 0xFFFFFFFFL
  return state[0]

Points to note:

  - using a 1-element list is about the nearest equivalent
    we have to passing in a pointer as your C code does.
    It's not necessarily good style (but then, neither is
    the pointer-passing thing).

  - After one iteration, the value in "state" will be a
    long integer and there will therefore be no danger
    of overflow. But you should make sure you start it
    off with a long anyway.

By the way, that's not a very good random number generator. :-)

-- 
Gareth McCaughan  Gareth.McCaughan at pobox.com
.sig under construc