Linear Congruential Generator in Python

Question

I am writing a LCG function in Python that I will use for a Monte Carlo type simulation for coin flips and generating runs. The problem I am facing is that when I generate a list of random numbers, the numbers are patterned such that odds and evens alternate. I don't know whether that is a property of the LCG function itself or a mistake in how I am generating the numbers.

Here is my code:

def seedLCG(initVal):
    global rand
    rand = initVal

def lcg():
    a = 1140671485
    c = 128201163
    m = 2**24
    global rand
    rand = (a*rand + c) % m
    return rand

seedLCG(1)

for i in range(10):
    print lcg()

Values Returned:

10581448
11595891
1502322
14136437
11348076
1403015
9622582
11013417
11529808
15836891

I'm assuming I don't need to worry about overflow and size because int and long are interchanged as needed by Python.

Answer 1

a*rand multiplies rand by an odd number, so the result is always odd when rand is odd, and even when rand is even. You then add in the odd number c, which changes odd to even and vice versa. The modulo has no effect on the last bit. So, every call to lcg flips rand from odd to even or from even to odd.

If you're serious about random numbers (but you don't need crypto-strength ones), consider using numpy.random.