pythonrandomnumpycython

# Correct way to generate random numbers in Cython?

What is the most efficient and portable way to generate a random random in `[0,1]` in Cython? One approach is to use `INT_MAX` and `rand()` from the C library:

``````from libc.stdlib cimport rand
cdef extern from "limits.h":
int INT_MAX
cdef float randnum = rand() / float(INT_MAX)
``````

Is it OK to use `INT_MAX` in this way? I noticed that it's quite different from the constant you get from Python's max int:

``````import sys
print INT_MAX
print sys.maxint
``````

yields:

``````2147483647  (C max int)
9223372036854775807  (python max int)
``````

Which is the right "normalization" number for `rand()`? EDIT additionally, how can the random seed be set (e.g. seeded based on current time) if one uses the C approach of calling `rand()` from libc?

Solution

• The C standard says `rand` returns an `int` in the range 0 to RAND_MAX inclusive, so dividing it by RAND_MAX (from `stdlib.h`) is the proper way to normalise it. In practice, RAND_MAX will almost always be equal to MAX_INT, but don't rely on that.

Because `rand` has been part of ISO C since C89, it's guaranteed to be available everywhere, but no guarantees are made regarding the quality of its random numbers. If portability is your main concern, though, it's your best option, unless you're willing to use Python's `random` module.

Python's `sys.maxint` is a different concept entirely; it's just the largest positive number Python can represent in its own int type; larger ones will have to be longs. Python's ints and longs aren't particularly related to C's.