Problem with Python implementation of Conway's Game of Life

Question

I am working on Conway's Game of Life currently and have gotten stuck. My code doesn't work.

When I run my code in GUI, it says:

 
[[0 0 0 0]
 [0 1 1 0]
 [0 1 0 0]
 [0 0 0 0]]

Traceback (most recent call last):
  File "C:\Users\Documents\Physics\Python\MainProject\conway.py", line 53, in 
    b= apply_rules(a)
  File "C:\Users\Documents\Physics\Python\MainProject\conway.py", line 14, in apply_rules
    neighbours=number_neighbours(universe_array,iy,ix)
  File "C:\Users\Documents\Physics\Python\MainProject\conway.py", line 36, in number_neighbours
    neighbours+=1
UnboundLocalError: local variable 'neighbours' referenced before assignment

Here is my code:

'''If a cell is dead at time T with exactly three live neighbours, the cell will be alive at T+1
If a cell is alive at time T with less than two living neighbours it dies at T+1
If a cell is alive at time T with more than three live neighbours it dies at T+1
If a cell is alive at time T with exactly two or three live neighbours it remains alive at T+1'''
import numpy


def apply_rules (universe_array):
    height, width = universe_array.shape
    # create a new array for t+1
    evolved_array = numpy.zeros((height, width),numpy.uint8)
    for iy in range(1, height-1):
        for ix in range(1,width-1):
            neighbours=number_neighbours(universe_array,iy,ix)
            if universe_array[iy,ix]==0 and neighbours==3:
                evolved_array[iy,ix]==1
            elif universe_array[iy,ix]==1 and neighbours<2:
                evolved_array[iy,ix]==0
            elif universe_array[iy,ix]==1 and neighbours>3:
                evolved_array[iy,ix]==0
            elif universe_array[iy,ix]==1 and neighbours==2 or neighbours==3:
                evolved_array[iy,ix]=universe_array[iy,ix]

    return evolved_array

def number_neighbours(universe_array,iy,ix):
    neighbours=0 #fixed this line,thanks:)
    if universe_array[iy-1,ix-1]==1:
            neighbours+=1
    if universe_array[iy,ix-1]==1:
            neighbours+=1
    if universe_array[iy+1,ix-1]==1:
            neighbours+=1
    if universe_array[iy-1,ix]==1:
            neighbours+=1
    if universe_array[iy+1,ix]==1:
            neighbours+=1
    if universe_array[iy-1,ix+1]==1:
            neighbours+=1
    if universe_array[iy,ix+1]==1:
            neighbours+=1
    if universe_array[iy+1,ix+1]==1:
            neighbours+=1
    else:
        neighbours=neighbours
    return neighbours

if __name__ == "__main__":
    a = numpy.zeros((4,4),numpy.uint8)
    a[1,1]=1
    a[1,2]=1
    a[2,1]=1
    print a
    b= apply_rules(a)
    print b

I am a beginner at Python, and I don't know how to fix the error. I am a little bit confused about import "neighbours" to function "apply_rules", is that right way to do this?

Answer 1

A cursory glance shows that your number_neighbors indices are off.

Also, you never initialize neighbors.

Response to Comment:

def number_neighbours(universe_array,iy,ix):
    if universe_array[iy,ix-1]==1:
        neighbours+=1
    if universe_array[iy,ix-1]==1:
        neighbours+=1
    if universe_array[iy+1,ix-1]==1:
        neighbours+=1

You say, neighbors +=1, which means add 1 to neighbors, but you never told it to start at 0, so it doesn't know what do add 1 to.

Also, notice the first and 3rd lines are exactly the same. I'm pretty sure this is not what you intended. That's what I meant by "your indices are off".

Response to Comment 2: apply_rules has several lines where you want to assign a value to something (which is '='), but you use '==' instead.

Answer 2

Well, I guess that you are also quite new to programming per se, otherwise you should not have any problems in interpreting that simple error message.

I'll help you dissect it:

• First, all "current" line numbers of your project's files are displayed, in calling order.
• Then, it shows you the function in which the error occured: number_neighbours
• Then, it shows you the contents of the line that contains the error: neighbours+=1
• Finally, it tells you what the problem with that line is: UnboundLocalError: local variable 'neighbours' referenced before assignment

Now, what does that mean? Let's see what the += operator does: it adds something to the current value of neighbours. That means that it reads the current value, adds something to it, and finally stores it back. "Reading" is called "reference" with respect to variables.

What is the current value of neighbours? Well, it has never been used before, so it doesn't have any value -- there has never been a value assigned to it. Adding something to 'no value' is not a sensible thing to do. I guess that you expect it to have the value 0, but you have to tell it to your interpreter. To do this, add the following statement before, at the start of your function: neighbours = 0

Answer 3

You're trying to increment a variable that doesn't exist yet. Python can't increment something if it doesn't know what is. Try adding the following line at the top of your def number_neighbours function.

neighbours = 0

Answer 4

This is an extremely low-grade lazy question, but your number_neighbours function is broken, it checks universe_array[iy,ix-1] twice (and hence omits a check it should be doing).