Python 'spread' simulation

Question

So, I have a list in the form of

[['0','0','0','0','0'],
['0','0','0','0','0'],
['1','0','0','0','0'],
['1','0','0','0','0'],
['0','0','0','0','0']]

and I want the '0' surrounding the '1' to change to a '1' after a step , like this.

[['0','0','0','0','0'],
['1','0','0','0','0'],
['1','1','0','0','0'],
['1','1','0','0','0'],
['1','0','0','0','0']]

and after enough steps, all the '0' become '1'.

The code I have is as follows

def simulate_bushfire(list, steps):
    for _ in range(steps):# iterates through the number of steps 
        for y in range(len(initial_bushfire[0])):
            for x in range(len(initial_bushfire)): #for all y coordinates possible in the file   
                if initial_bushfire[y][x] =='1':# looks for cells that has '1' in it 
                    for a in range(x-1,x+2): #looks at the neighbour of the cell that has'1' in it (x coordinates)
                        for b in range(y-1,y+2):#looks at the neighbour of the cell that has'1' in it (y coordinates)                           
                            if a<0 or b<0 or b>=len(initial_bushfire[0]) or a>=len(initial_bushfire):# if neighbour is outside the border of the map, 
                                #code will ignore to avoid errors like list out of range 
                                continue
                            if initial_bushfire[b][a]=='':# if there's an empty string (no tree)
                                continue    # ignore this as well (no trees to burn )
                            if initial_bushfire[b][a]=='0': #if there is a '0' in the file (there is a tree)
                                initial_bushfire[b][a]='1'# change the '0' to a '1' (tree on fire)
    return (initial_bushfire)

but it seems the 'spread' far too much for 1 step. I can't seem to understand why but I guess it's due to this line

for a in range(x-1,x+2): #looks at the neighbour of the cell that has'1' in it (x coordinates)
    for b in range(y-1,y+2):#looks at the neighbour of the cell that has'1' in it (y coordinates)

Would really appreciate if someone could guide me regarding this code.

Answer 1

The problem is that you're making the newly-spreaded fire part of the original fire in the same matrix and so they would further spread into other bushes as you continue searching for fire. You should use a separate list to keep track of the new fires and only apply them in the end when you have finished scanning the entire bushfire matrix:

def simulate_bushfire(initial_bushfire, steps):
    for _ in range(steps):# iterates through the number of steps
        new_fire = []
        for y in range(len(initial_bushfire[0])):
            for x in range(len(initial_bushfire)): #for all y coordinates possible in the file
                if initial_bushfire[y][x] =='1':# looks for cells that has '1' in it
                    for a, b in (x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1): #looks at the neighbouring cells
                        if a<0 or b<0 or b>=len(initial_bushfire[0]) or a>=len(initial_bushfire):# if neighbour is outside the border of the map,
                            #code will ignore to avoid errors like list out of range
                            continue
                        if initial_bushfire[b][a]=='':# if there's an empty string (no tree)
                            continue    # ignore this as well (no trees to burn )
                        if initial_bushfire[b][a]=='0': #if there is a '0' in the file (there is a tree)
                            new_fire.append((b, a))# change the '0' to a '1' (tree on fire)
        for y, x in new_fire:
            initial_bushfire[y][x] = '1'
    return (initial_bushfire)

so that:

simulate_bushfire(l, 1)

would return:

[['0', '0', '0', '0', '0'],
 ['1', '0', '0', '0', '0'],
 ['1', '1', '0', '0', '0'],
 ['1', '1', '0', '0', '0'],
 ['1', '0', '0', '0', '0']]