Question

Is there a function in numpy or scipy (or some other library) that generalizes the idea of cumsum and cumprod to arbitrary function. For example, consider the (theoretical) function

cumf( func, array) 

func is a function that accepts two floats, and returns a float. Particular cases

lambda x,y: x+y 

and

lambda x,y: x*y 

are cumsum and cumprod respectively. For example, if

func = lambda x,prev_x: x^2*prev_x 

and I apply it to:

cumf(func, np.array( 1, 2, 3) )

I would like

np.array( 1, 4, 9*4 )

Answer 1

NumPy’s ufuncs have accumulate():

In [22]: np.multiply.accumulate([[1, 2, 3], [4, 5, 6]], axis=1)
Out[22]: 
array([[  1,   2,   6],
       [  4,  20, 120]])

Unfortunately, calling accumulate() on a frompyfunc()‘ed Python function fails with a strange error:

In [32]: uadd = np.frompyfunc(lambda x, y: x + y, 2, 1)

In [33]: uadd.accumulate([1, 2, 3])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

ValueError: could not find a matching type for <lambda> (vectorized).accumulate, 
            requested type has type code 'l'

This is using NumPy 1.6.1 with Python 2.7.3.