Recursive functions and PyMC3 variables

Questions
1

Hi! I’m trying to implement a simple recursive function that works with PyMC3 random variables. Let’s say it sums all numbers from 1 to X. I know that we can’t use if..else when working with random variables.

    def sum (x):
    	return x + sum(x-1) if x > 0 else 0

That produces TypeError: Variables do not support boolean operations.

But pm.math.switch doesn’t work either:

    import pymc3 as pm
    def sum (x):
    	return pm.math.switch(x>0, x + sum(x-1), 0)
    with pm.Model() as model:
    	b = pm.Binomial('b', n=10, p=0.5)
    	y = pm.Deterministic('y', sum(b))
    	result = pm.sample()

The result is RecursionError: maximum recursion depth exceeded while calling a Python object
What am I doing wrong?

Upd.: Using pm.math.gt(x,0) instead of x>0 doesn’t help

2

Try scan; it’s theano’s version of recurrence. Or looping.

http://deeplearning.net/software/theano_versions/dev/cifarSC2011/advanced_theano.html

3

Thanks! Just found a similar question on StackOverflow: https://stackoverflow.com/questions/42480494/python-theano-is-it-possible-to-construct-truly-recursive-theano-functions