[Edu-sig] Introducing Python to Engineering Students

[Rob Malouf]

On Mar 11, 2008, at 5:11 PM, David MacQuigg wrote:
> It would make a nice improvement in this Mandelbrot demo if you  
> could show me a way to significantly improve the speed of the Python  
> I already have, perhaps avoiding the need for C.

Actually, I don't see a clean way to vectorize that inner loop, so  
maybe numpy isn't a good fit here.  Which means weave is.

First, running the program as-is, I get:

     1a) Python speed = 776 points/sec
     1b) C speed      = 103200 points/sec
     2a) Python speed = 833 points/sec
     2b) C      speed = 108600 points/sec

With this added to the beginning of the program to load and invoke  
psyco:

	import psyco
	psyco.full()

I get:

     1a) Python speed = 2700 points/sec
     1b) C speed      = 101600 points/sec
     2a) Python speed = 2800 points/sec
     2b) C      speed = 110100 points/sec

Or, instead, replacing your getpts with the weave-d version:

		from numpy import zeros
		from scipy import weave
		...
     vals = zeros(100,int)

     code = r"""
         double cx = cx0 - dx;
         for (int p=0; p<100; p++) {
             double zx = 0.0;
             double zy = 0.0;
             int i;
             cx += dx;
             for (i=0; i<999; i++) {
                 double zx2 = zx*zx;
                 double zy2 = zy*zy;
                 if ((zx2 + zy2) > 4.0) break;
                 zy = 2*zx*zy + cy0;
                 zx = zx2 - zy2 + cx;
             }
             vals[p] = i;
         }
     """
     weave.inline(code,['cx0','cy0','dx','vals'])

I get:

     1a) Python speed = 102200 points/sec
     1b) C speed      = 103300 points/sec
     2a) Python speed = 108400 points/sec
     2b) C      speed = 110700 points/sec

Not bad! There's probably a more pythonic way to write that, but  
this'll do.  And after all, this is a realistic situation: you've got  
the inner loop of a program written in C, but you'd rather write all  
the supporting code around it in something like Python.  And note that  
weave automatically compiles and links (if necessary) and then loads  
the C part when you run mandel.py.  You (or your students) don't need  
to remember where the python headers are or how to build a shared  
library on whatever platform they're using.

---
Rob Malouf <rmalouf at mail.sdsu.edu>
Department of Linguistics and Asian/Middle Eastern Languages
San Diego State University