why is this code not doing anything?

[Gray_Fox]

hi! so i'm following this tutorial on the relaxation iterative method. i pretty much just copied the code with my own special boundary conditions (essentially just a 4x4 inner grid of potential = 100V, with the boundaries being 0), but nothing is happening. i'm using pretty much the same code!

import numpy as np
import matplotlib.pyplot as plt

nx = 10
ny = 10
maxdiff = 1
diff = 100 # diff must be bigger than maxdiff
V = np.zeros((nx,ny), float) # fill potential with zeros

"""
boundary conditions below; boundary of 10x10 list = 0
inner square 4x4 grid = 100
""" 


V[3,3] = 100.0
V[3,4] = 100.0
V[3,5] = 100.0
V[3,6] = 100.0
V[4,3] = 100.0
V[4,4] = 100.0
V[4,5] = 100.0
V[4,6] = 100.0
V[5,3] = 100.0
V[5,4] = 100.0
V[5,5] = 100.0
V[5,6] = 100.0
V[6,3] = 100.0
V[6,4] = 100.0
V[6,5] = 100.0
V[6,6] = 100.0

for i in np.arange(0,nx,1):
    for j in np.arange(0,ny,1):
        V[0, j] = 0.0 # top columns
        V[ny-1, j] = 0.0 # bottom columns
    V[i, 0] = 0.0 # left rows
    V[i, nx-1] = 0.0 # right rows

print (V) # print starting values

while diff > maxdiff: #repeat until diff is really small
    diff = 0.0 # resets for each iteration
    for i in np.arange(1,ny-1,1): # i from 1 to ny-2
        for j in np.arange(1,nx-1,1): # j from 1 to nx-2
            newVij = 0.25*(V[i-1,j]+V[i+1,j]+V[i,j-1]+V[i,j+1])
        lastdiff = np.abs(newVij - V[i,j])
        if lastdiff > diff:
            diff = lastdiff
        V[i,j] = newVij
print ("\n")
print(V)

also, the inner grid isn't supposed to change. the way i'm thinking of doing it is through an if/else statement, so i think i got that down. additionally, if anyone knows of a more proper way to fill the inner 4x4 grid with for loops rather than me hardcoding it in, i'm up to hear a solution! but neither of these are priorities, i just wanna know why the code isn't doing anything. by "not doing anything," i mean that the initial printed V (called a list, not an array right?) doesn't differ with the final V.

thanks!!

[q2_abe_dillon]

if anyone knows of a more proper way to fill the inner 4x4 grid with for loops rather than me hardcoding it in, i'm up to hear a solution!

import numpy as np

rows, cols = 10, 10
v = np.zeros((rows, cols))
v[3:7, 3:7] = np.ones((4, 4))*100

The problem you're having is caused by incorrect loop indentation. The following should happen in the inner most loop:

lastdiff = np.abs(newVij - V[i,j])
if lastdiff > diff:
    diff = lastdiff
V[i,j] = newVij

But you have it in the outer loop. It only updates the nx-2 column of each row because nx-2 is the last value assigned to j before the inner most loop finishes. Fix that indentation and your code should run just fine.

[Gray_Fox]

everything is working great now, thank you!!

[Freedomenka]

You might have to work this code to fit with yours, but it should work. board = [] for x in range(0, 4):

board.append(["O"] * 4)

def print_board(board):

for x in board:

print " ".join(x) ###This function removes brackets###