Cython random roll does not produce the expected output

Question

I am following this tutorial by Hans Petter Langtangen, in order to understand Cython better for the purpose of fast generation of random numbers.

The author has the following line:

r = 1 + int(rand()/(RAND_MAX*6.0))

in his tutorial, which claims to produce a random integer between 1 and 6. To me, it looks like a mistake, as rand() (cimported from libc.stdlib ) is generating a random integer from 0 to RAND_MAX , so I guess the line

r = 1 + int(6*rand()/(RAND_MAX(1.0))

should be more appropriate there. So I have created a little Cython script that should roll a random integer between 1 and n , printing debug messages while doing so. Here's the script:

    from libc.stdlib cimport rand, RAND_MAX

    def print_rand(int n):
        cdef int r
        print "max", RAND_MAX
        cdef int roll
        roll = rand()
        print "roll", roll
        r = 1 + int(n*roll/(RAND_MAX*1.0))
        print r

Then I have compiled the script using the following setup.py script:

    from distutils.core import setup
    from distutils.extension import Extension
    from Cython.Distutils import build_ext

    setup(name='Random print',
          ext_modules=[Extension('_rand', ['rand.pyx'],)],
          cmdclass={'build_ext': build_ext},)

running it via

python setup.py build_ext --inplace

For testing, I opened IPython , and the following mystery occurred:

In [1]: import _rand

In [2]: _rand.print_rand(1000)
max 2147483647
roll 1804289383
1

but that doesn't make sense, as

In [3]: 1 + int(1000*1804289383/(2147483647*1.0))
Out[3]: 841

What am I missing here?

Answer 1

The multiplication

n*roll

has a result too large to fit into a Cython int. Unlike with Python ints, which automatically switch to arbitrary-precision representation when such a thing happens, Cython handles overflow like C. That generally means the high bits that don't fit are discarded; I am not sure whether it is undefined behavior in Cython (in which case much worse things can happen), or whether Cython guarantees specific overflow handling.