Trying to make a 2-D random walk function, why am I getting "unsupported operand type(s) for +: 'float' and 'NoneType'" error?

Question

Here I made a 2-D random-walk where the "character" can only move straight up, down, left or right:

import random 
import numpy as np
import matplotlib.pyplot as plt
# I define the possible moves at each step of the 2D walk
dirs = np.array( [ [1,0], [-1,0], [0,1], [0,-1] ] 
# I define the number of steps to take
numSteps = 50

# I set up a 2D array to store the locations visited
locations = np.zeros( (numSteps, 2) )   # numSteps rows, 2 columns

# take steps
for i in range(1, numSteps):
  r = random.randrange(4)  # random integer from {0,1,2,3}
  move = dirs[r]           # direction to move
  locations[i] = locations[i-1] + move  # store the next location

locations

My output works well and I get arrays of the random walk. Now here I'm trying the same where this time I want my random walk character to go in a direction with the angle theta, hence [cos(theta), sin(theta)]:

import random
import numpy as np
import matplotlib.pyplot as plt
import math
import decimal

# I again define the possible moves at each step of the 2D walk, I make it  function this time.
# The function works and returns an array (I tested)
def dirs(x):
  print(np.array( [math.cos(x), math.sin(x)] ))

# I define the number of steps to take
numSteps = 50

# I set up a 2D array to store the locations visited
locations = np.zeros( (numSteps, 2) )   # numSteps rows, 2 columns

# take steps
for i in range(1, numSteps):
  r = random.randrange(314159)/100000  # random integer from {0 to an approximation of pi}
  move = dirs(r)          # direction to move
  locations[i] = locations[i-1] + move  # This is where the issue is! Why is this array not considered valid?

locations

Here this code seems to fail, I'm getting the issue:

[0.8116997 0.584075 ]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-c81f4b47e64d> in <module>()
     14   r = random.randrange(314159)/100000  # random integer from {0 to an approximation of pi}
     15   move = dirs(r)          # direction to move
---> 16   locations[i] = locations[i-1] + move  # This is where the issue is! Why is this array not considered valid?
     17 
     18 locations

TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'

So I have defined the array in the same way as before, and my new function (dirs(r)) returns an array, so what could be the reason I am getting this error? Thank you!

Answer 1

Your function:

def dirs(x):
    print(np.array( [math.cos(x), math.sin(x)] ))

is only printing the array, not returning it. That is why move is None, and you get an error when you try to add locations[i-1], of type float, and move, of type NoneType. Change it to:

def dirs(x):
    return np.array( [math.cos(x), math.sin(x)] )