Ted - 10 days ago 8
Python Question

Conway's Game of Life not counting neighbors correctly

I am doing the standard Conway's Game of Life program using Python. I am having an issue when trying to count neighbors as I iterate through the array. I created print statements that print the succession of the if statement, as well as the value of count for each statement.

Here is my code: ( I have the questions inside the # throughout the code)

``````import random

numrows = 10
numcols = 10
def rnd():
rn = random.randint(0,1)
return rn

def initial():
grid = []
count = 0
for x in range(numrows):
grid.append([])
for y in range(numcols):
rand=random.randrange(1,3)
if(rand == 1):
grid[x].append('O')
else:
grid[x].append('-')
for x in grid:
print(*x, sep=' ',end="\n") #this prints the random 2d array

print("")
print("")

newgrid = []
count = 0

while(answer == 'y'): # I believe I am going through, checking neighbors
# and moving onto the next index inside these for
#loops below
for r in range(0,numrows):
grid.append([])
for c in range(0,numcols):

if(r-1 > -1 and c-1 > -1): #I use this to check out of bound
if(newgrid[r-1][c-1] == 'O'):#if top left location is O
count = count + 1        #should get count += 1
else:
count = count
print("top left check complete")
print(count)
if(r-1 > -1):
if(newgrid[r-1][c] == 'O'):
count = count + 1
else:
count = count
print("top mid check complete")
print(count)
if(r-1 > -1 and c+1 < numcols):
if(newgrid[r-1][c+1] == 'O'):
count = count + 1
else:
count = count
print("top right check complete")
print(count)
if(c-1 > -1 and r-1 > -1):
if(newgrid[r][c-1] == 'O'):
count = count + 1
else:
count = count
print("mid left check complete")
print(count)
if(r-1 > -1 and c+1 < numcols):
if(newgrid[r][c+1] == 'O'):
count = count + 1
else:
count = count
print("mid right check complete")
print(count)
if(r+1 < numrows and c-1 > -1):
if(newgrid[r+1][c-1] == 'O'):
count = count + 1
else:
count = count
print("bot left check complete")
print(count)
if(r+1 < numrows and c-1 > -1):
if(newgrid[r+1][c] == 'O'):
count = count + 1
else:
count = count
print("bot mid check complete")
print(count)
if(r+1 < numrows and c+1 < numcols):
if(newgrid[r+1][c+1] == 'O'):
count = count + 1
else:
count = count
print("bot right check complete")
print(count)

# I am not sure about the formatting of the code below, how do I know that
# the newgrid[r][c] location is changing? should it be according to the for-
# loop above? Or should it get it's own? If so, how could I construct it as
# to not interfere with the other loops and items of them?

if(newgrid[r][c] == '-' and count == 3):
newgrid[r][c] ='O'

elif(newgrid[r][c] == 'O' and count < 2):
newgrid[r][c] = '-'

elif(newgrid[r][c] == 'O' and (count == 2 or count == 3)):
newgrid[r][c] = 'O'

elif(newgrid[r][c] == 'O' and count > 3):
newgrid[r][c] = '-'

# I'm also confused how to go about printing out the 'new' grid after each
# element has been evaluated and changed. I do however know that after the
# new grid prints, that I need to assign it to the old grid, so that it can
# be the 'new' default grid. How do I do this?

for z in newgrid:
print(*z, sep=' ',end="\n")

answer = input("Continue? y or n( lower case only): ")

newgrid = grid

print(" Hope you had a great life! Goodbye!")

initial()
``````

Here is the current output and error message:

``````>>> initial()
- O - - O - - O - -
- O - - O - - - O O
- O - - O - O O - O
O - - O - - O O O O
O - O O - - - O O -
O - O - O - O - O -
O - O O O O - - O -
- - - - O O O - - O
O O - O - - O - - -
- - O O O - O - - -
top left check complete
0
top mid check complete
0
top right check complete
0
mid left check complete
0
mid right check complete
0
bot left check complete
0
bot mid check complete
0
Traceback (most recent call last):
File "<pyshell#68>", line 1, in <module>
initial()
File "C:\Users\Ted\Desktop\combined.py", line 86, in initial
if(newgrid[r+1][c+1] == 'O'):
IndexError: list index out of range
``````

As I iterate through the random array to see what the neighbors are, it seems to be fine up until it moves over to [0][1] while checking the bot right neighbor.

Also, the mid right neighbor should + 1 to count as it is alive. However, even with succession of the if statement, count remains 0?

Question 1: How can I possibly know that my if conditions witll suffice for every instance of [r][c] for all sides of the array?

Question 2: Is my current method of checking out of bounds the best for my situation? Is there a way to make a "check all for out of bounds" before I even check the value?

I am at my wit's end at this point. Thanks in advance for the time taken to help answer my questions

You are getting that index error because your `newgrid` only contains a single empty row. And your testing for neighbours in `newgrid` instead of in `grid` (as Blckknght mentions in the comments). I've made a few repairs, but there's a lot more that can be done to improve this code. It looks like it's working now, but it's hard to tell when you're working with random Life forms. :) I suggest giving your program some way of using known Life patterns like blinkers and gliders to see that they behave correctly.

The simplest way to ensure that `newgrid` is valid is to copy it from `grid`. If we just do `newgrid = grid` that simply makes `newgrid` another name for the `grid` object. To copy a list of lists properly we need to make copies of each of the internal lists. My new code does that with the `copy_grid` function.

I've fixed a couple of minor bugs that you had in the `if` tests in the section that counts neighbours, and I've simplified the logic that updates a cell from its neighbour count. I've also condensed the code that makes a random grid, and I've added a simple function that can read a Life pattern from a string and build a grid from it. This lets us test the code with a Glider. I've also added a function that makes an empty grid. The program doesn't currently use that function although I used it during my tests, and I guess it's a useful example. :)

``````import random

# Set a seed so that we get the same random numbers each time we run the program
# This makes it easier to test the program during development
random.seed(42)

numrows = 10
numcols = 10

glider = '''\
----------
--O-------
---O------
-OOO------
----------
----------
----------
----------
----------
----------
'''

# Make an empty grid
def empty_grid():
return [['-' for y in range(numcols)]
for x in range(numrows)]

# Make a random grid
def random_grid():
return [[random.choice('O-') for y in range(numcols)]
for x in range(numrows)]

# Make a grid from a pattern string
def pattern_grid(pattern):
return [list(row) for row in pattern.splitlines()]

# Copy a grid, properly!
def copy_grid(grid):
return [row[:] for row in grid]

# Print a grid
def show_grid(grid):
for row in grid:
print(*row)
print()

def run(grid):
show_grid(grid)

# Copy the grid to newgrid.
newgrid = copy_grid(grid)
while True:
for r in range(numrows):
for c in range(numcols):
# Count the neighbours, making sure that they are in bounds
count = 0
# Above this row
if(r-1 > -1 and c-1 > -1):
if(grid[r-1][c-1] == 'O'):
count += 1
if(r-1 > -1):
if(grid[r-1][c] == 'O'):
count += 1
if(r-1 > -1 and c+1 < numcols):
if(grid[r-1][c+1] == 'O'):
count += 1

# On this row
if(c-1 > -1):
if(grid[r][c-1] == 'O'):
count += 1
if(c+1 < numcols):
if(grid[r][c+1] == 'O'):
count += 1

# Below this row
if(r+1 < numrows and c-1 > -1):
if(grid[r+1][c-1] == 'O'):
count += 1
if(r+1 < numrows):
if(grid[r+1][c] == 'O'):
count += 1
if(r+1 < numrows and c+1 < numcols):
if(grid[r+1][c+1] == 'O'):
count += 1

# Update the cell in the new grid
if grid[r][c] == '-':
if count == 3:
newgrid[r][c] ='O'
else:
if count < 2 or count> 3:
newgrid[r][c] = '-'

# Copy the newgrid to grid
grid = copy_grid(newgrid)
show_grid(grid)

answer = input("Continue? [Y/n]: ")
if not answer in 'yY':
print(" Hope you had a great life! Goodbye!")
break

#grid = random_grid()
grid = pattern_grid(glider)
run(grid)
``````

This code does work correctly, but there is still plenty of room for improvement. For example, here's an improved version of `run()` that condenses the neighbour counting section by using a couple of loops.

``````def run(grid):
show_grid(grid)

# Copy the grid to newgrid.
newgrid = copy_grid(grid)
while True:
for r in range(numrows):
for c in range(numcols):
# Count the neighbours, making sure that they are in bounds
# This includes the cell itself in the count
count = 0
for y in range(max(0, r - 1), min(r + 2, numrows)):
for x in range(max(0, c - 1), min(c + 2, numcols)):
count += grid[y][x] == 'O'

# Update the cell in the new grid
if grid[r][c] == '-':
if count == 3:
newgrid[r][c] ='O'
else:
# Remember, this count includes the cell itself
if count < 3 or count > 4:
newgrid[r][c] = '-'

# Copy the newgrid to grid
grid = copy_grid(newgrid)
show_grid(grid)

answer = input("Continue? [Y/n]: ")
if not answer in 'yY':
print(" Hope you had a great life! Goodbye!")
break
``````