How can I, in python, iterate over multiple 2d lists at once, cleanly?

Question

If I\'m making a simple grid based game, for example, I might have a few 2d lists. One might be for terrain, another might be for objects, etc. Unfortunately, when I need to ite

Answer 1

When you are operating with grids of numbers and want really good performance, you should consider using Numpy. It's surprisingly easy to use and lets you think in terms of operations with grids instead of loops over grids. The performance comes from the fact that the operations are then run over whole grids with optimised SSE code.

For example here is some numpy using code that I wrote that does brute force numerical simulation of charged particles connected by springs. This code calculates a timestep for a 3d system with 100 nodes and 99 edges in 31ms. That is over 10x faster than the best pure python code I could come up with.

from numpy import array, sqrt, float32, newaxis
def evolve(points, velocities, edges, timestep=0.01, charge=0.1, mass=1., edgelen=0.5, dampen=0.95):
    """Evolve a n body system of electrostatically repulsive nodes connected by
       springs by one timestep."""
    velocities *= dampen

    # calculate matrix of distance vectors between all points and their lengths squared
    dists = array([[p2 - p1 for p2 in points] for p1 in points])
    l_2 = (dists*dists).sum(axis=2)

    # make the diagonal 1's to avoid division by zero
    for i in xrange(points.shape[0]):
        l_2[i,i] = 1

    l_2_inv = 1/l_2
    l_3_inv = l_2_inv*sqrt(l_2_inv)

    # repulsive force: distance vectors divided by length cubed, summed and multiplied by scale
    scale = timestep*charge*charge/mass
    velocities -= scale*(l_3_inv[:,:,newaxis].repeat(points.shape[1], axis=2)*dists).sum(axis=1)

    # calculate spring contributions for each point
    for idx, (point, outedges) in enumerate(izip(points, edges)):
        edgevecs = point - points.take(outedges, axis=0)
        edgevec_lens = sqrt((edgevecs*edgevecs).sum(axis=1))
        scale = timestep/mass
        velocities[idx] += (edgevecs*((((edgelen*scale)/edgevec_lens - scale))[:,newaxis].repeat(points.shape[1],axis=1))).sum(axis=0)

    # move points to new positions
    points += velocities*timestep