Stuck in infinite loop trying to swap words to the right place

[lhr0909]

Error code snippet:

#!/usr/bin/env python2.7

from __future__ import division
import time

grid_shape = (1024, 1024)

def evolve(grid, dt, D=1.0):
    xmax, ymax = grid_shape
    new_grid = []
    for i in xrange(xmax):
        cur_row = []
        for j in xrange(ymax):
            grid_xx = grid[(i+1)%xmax][j] + grid[(i-1)%xmax][j] - 2.0 * grid[i][j]
            grid_yy = grid[i][(j+1)%ymax] + grid[i][(j-1)%ymax] - 2.0 * grid[i][j]
            cur_row.append(grid[i][j] + D * (grid_xx + grid_yy) * dt)
        new_grid.append(cur_row)
    return grid

def run_experiment(num_iterations):
    # setting up initial conditions
    grid = [[0.0 for x in xrange(grid_shape[1])] for x in xrange(grid_shape[0])]

    block_low = int(grid_shape[0] * .4)
    block_high = int(grid_shape[0] * .5)
    for i in xrange(block_low, block_high):
        for j in xrange(block_low, block_high):
            grid[i][j] = 0.005

    start = time.time()
    for i in range(num_iterations):
        grid = evolve(grid, 0.1)
    return time.time() - start

[lhr0909]

Should be fixed.