Python: Drunkard's walk

Question

I have to write a code to calculate route and length of a drunkard's walk.

Excersise: A drunkard begins walking aimlessly, starting at a lamp post. At each time step he takes one step at random, either north, east, south, or west. How far will the drunkard be from the lamp post after N steps? In order to emulate drunkard's steps we can encode each direction with the number so that when the random variable is equal to 0 the drunkard moves north, if random variable is equal to 1 the drunkard moves east and so on.

Write a program that takes an integer argument N and simulates the motion of a random walker for N steps. After each step, print the location of the random walker, treating the lamp post as the origin (0, 0). Also, print the final squared distance from the origin.

So far I've come up with:

import random
x = 0
y = 0
def randomWalk(N):
    for i in range (1, (N)):
        a = random.randint
        if a == 0:
            x = x+0
            y = y+1
            return (x, y)
            print (x, y)
        if a == 1:
            x = x+1
            y = y+0
            return (x, y)
            print (x, y)
        if a == 3:
            x = x+0
            y = y-1
            return (x, y)
            print (x, y)
        if a == 3:
            x = x-1
            y = y+0
            return (x, y)
            print (x, y)
print(randomWalk(input()))

But I get None as output, when I test this code.

I would be thankful for any help with this excersise.

You have forgotton to call `random.randint`: Put instead `random.randint(0, 4)` — matsjoyce, Oct 04 '14 at 18:25
@matsjoyce: That `4` should be `3` (somewhat inconsistently, `randint` includes the upper bound as part of the range). — NPE, Oct 04 '14 at 18:28
Who downvoted this? This is actually well written and a valid question. At least they show a good attempt at solving the problem. — SethMMorton, Oct 04 '14 at 18:29

NPE · Accepted Answer · 2014-10-04T18:36:10.757

It's a good start.

The main problem is that you are failing to call randint:

    a = random.randint

This just turns a into an alias for random.randint. Instead, it should be

    a = random.randint(0, 3)

Also, you repeat if a == 3: twice.

Also, setting x and y to zero should be done inside the function, not outside.

Finally, your loop (which, by the way, is one iteration too short) doesn't really function as a loop, since you always return during the first iteration.

P.S. Here is a little parting puzzle for you. Figure out how the following works:

dx, dy = random.choice([(-1, 0), (1, 0), (0, -1), (0, 1)])
x += dx
y += dy

score 0 · Answer 2 · answered Oct 04 '14 at 18:31

def randomWalk(steps):
    x = 0  # Make sure you initialize the position to 0,0 each time the function is called
    y = 0
    directions = ['N', 'E', 'S', 'W']  # To keep track of directions, you could use strings instead of 0, 1, 2, 3.
    for i in range(steps):
        a = random.choice(directions)  # You can use random.choice to choose a dir
        if a == 'N':
            y += 1
            print('Current position: ({},{})'.format(x,y))  # You can print the position using format
        elif a == 'S':
            y -= 1
            print('Current position: ({},{})'.format(x,y))
        elif a == 'E':
            x += 1
            print('Current position: ({},{})'.format(x,y))
        else:
            x -= 1
            print('Current position: ({},{})'.format(x,y))

Testing

>>> randomWalk(8)
Current position: (0,-1)
Current position: (1,-1)
Current position: (1,0)
Current position: (1,-1)
Current position: (0,-1)
Current position: (-1,-1)
Current position: (-1,0)
Current position: (0,0)

Python: Drunkard's walk

2 Answers2