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

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