runge_kutta_4 With C++ Speed: Example and Test

runge_kutta_4_cpp.py

@(@\newcommand{\B}[1]{{\bf #1}} \newcommand{\R}[1]{{\rm #1}}@)@ runge_kutta_4 With C++ Speed: Example and Test
Purpose
Discussion
Source Code

Purpose
In this example we demonstrate how a Python function can be recorded in a pycppad function object and then evaluated at much higher speeds than the Python evaluation.

Discussion
Define @(@ y : \B{R}^2 \times \B{R} \rightarrow \B{R}^n @)@ by @[@ \begin{array}{rcl} y(x, 0) & = & x_0 \\ \partial_t y(x, t) & = & x_1 y(x, t) \end{array} @]@ It follows that @[@ y(x, t) = x_0 \exp ( x_1 t ) @]@ Suppose we want to compute values for the function @(@ g : \B{R}^2 \rightarrow \B{R} @)@ defined by @[@ g(x) = y(x, 1) @]@ In this example we compare the execution time for doing this in pure Python with using a pycppad function object to compute @(@ g(x) @)@ in C++.

Source Code




 
from pycppad import *
import time
def pycppad_test_runge_kutta_4_cpp() :
	x_1 = 0;   # use this variable to switch x_1 between float and ad(float)
	def fun(t , y) :
		f     = x_1 * y
		return f
	# Number of Runge-Kutta times steps to include in the function object
	M = 100 

	# Start time for recording the pycppad function object
	s0  = time.time()
	# Declare three independent variables. The operation sequence does not
	# depend on x, so we could use any value here.
	x    = numpy.array( [.1, .1, .1] )
	a_x  = independent( numpy.array( x ) )
	# First independent variables, x[0], is the value of y(0)
	a_y = numpy.array( [ a_x[0] ] )
	# Make x_1 a variable so can use rk4 with various coefficients.
	x_1 = a_x[1]
	# Make dt a variable so can use rk4 with various step sizes.
	dt  = a_x[2]
	# f(t, y) does not depend on t, so no need to make t a variable.
	t   = ad(0.)
	# Record the operations for 10 time step
	for k in range(M) :
		a_y = runge_kutta_4(fun, t, a_y, dt)
		t   = t + dt
	# define the AD function rk4 : x -> y
	rk4 = adfun(a_x, a_y)
	# amount of time it took to tape this function object
	tape_sec = time.time() - s0
	# make the fucntion object more efficient
	s0       = time.time()
	rk4.optimize()
	opt_sec  = time.time() - s0

	ti  = 0.              # initial time
	tf  = 1.              # final time
	N   = M * 100         # number of time steps 
	dt  = (tf - ti) / N   # size of time step
	x_0 = 2.              # use this for initial value of y(t)
	x_1 = .5              # use this for coefficient in ODE

	# python version of integrator with float values
	s0  = time.time()
	t   = ti
	y   = numpy.array( [ x_0 ] ); 
	for k in range(N) :
		y = runge_kutta_4(fun, t, y, dt)
		t = t + dt
	# number of seconds to solve the ODE using python float
	python_sec =  time.time() - s0
	# check solution is correct
	assert( abs( y[0] - x_0 * exp( x_1 * tf ) ) < 1e-10 ) 

	# pycppad function object version of integrator 
	s0  = time.time()
	t   = ti
	x   = numpy.array( [ x_0 , x_1 , dt ] ) 
	for k in range(N/M) :
		y    = rk4.forward(0, x);
		x[0] = y[0];
	# number of seconds to solve the ODE using pycppad function object
	cpp_sec =  time.time() - s0
	# check solution is correct
	assert( abs( y[0] - x_0 * exp( x_1 * tf ) ) < 1e-10 ) 

	# Uncomment the print statement below to see actual times on your machine
	format =          'cpp_sec            = %8f,\n'
	format = format + 'python_sec/cpp_sec = %5.1f\n'
	format = format + 'tape_sec/cpp_sec   = %5.1f\n'
	format = format + 'opt_sec/cpp_sec    = %5.1f'
	s      = cpp_sec
	# print format % (s, python_sec/s, tape_sec/s, opt_sec/s )
	
	# check that C++ is always more than 30 times faster (on all systems)
	assert( 30. * cpp_sec <= python_sec )

Input File: example/runge_kutta_4_cpp.py