CODEWITHSUNDEEP
python
Teddy Haley
Teddy Haley

Reputation: 93

Repeating if statement

I am having a problem with my code mapping a random walk in 3D space. The purpose of this code is to simulate N steps of a random walk in 3 dimensions. At each step, a random direction is chosen (north, south, east, west, up, down) and a step of size 1 is taken in that direction. Here is my code:

import random  # this helps us generate random numbers

N = 30  # number of steps

n = random.random()  # generate a random number

x = 0
y = 0
z = 0
count = 0 
while count <= N:

    if n < 1/6:
        x = x + 1           # move east
        n = random.random() # generate a new random number
    if n >= 1/6 and n < 2/6:
        y = y + 1           # move north
        n = random.random() # generate a new random number
    if n >= 2/6 and n < 3/6:
        z = z + 1           # move up
        n = random.random() # generate a new random number
    if n >= 3/6 and n < 4/6:
        x = x - 1           # move west
        n = random.random() # generate a new random number
    if n >= 4/6 and n < 5/6:
        y = y - 1           # move south
        n = random.random() # generate a new random number
    if n >= 5/6:
        z = z - 1           # move down
        n = random.random() # generate a new random number

    print("(%d,%d,%d)" % (x,y,z))
    count = count + 1

print("squared distance = %d" % (x*x + y*y + z*z))

The problem is I am getting more than a single step between each iteration. I've added comments showing the difference in steps between iterations.

Here are the first 10 lines of the output:

(0,-1,0) #1 step  
(0,-2,0) #1 step  
(1,-3,1) #4 steps  
(1,-4,1) #1 step  
(1,-3,1) #1 step  
(1,-2,1) #1 step  
(2,-2,0) #2 steps  
(2,-2,0) #0 steps  
(2,-2,0) #0 steps  
(2,-1,0) #1 step

Upvotes: 1

Views: 1256

Answers (4)

u8129
u8129

Reputation: 177

If you remove the multiple n = random.random() from within the if statements and replace by a single n = random.random() at start of the while loop then there will be only one step per loop.

Upvotes: 2

Rahul Verma
Rahul Verma

Reputation: 3089

This is because you are setting n in each if-block which is satisfying next if blocks as well.

Just take out n = random.random() from all if blocks and place it just before end of while loop.

Tip:
To debug this I would've printed the value of n in each if case.

Upvotes: 1

Yaroslav Surzhikov
Yaroslav Surzhikov

Reputation: 1608

Replacing multiple if statements with chain of if-elif-else solved the problem. Also, you could iterate using for instead of while. And in addition - you don't need to regenerate n under each if.

import random  # this helps us generate random numbers

N = 30  # number of steps

x = 0
y = 0
z = 0

for _ in range(N):

    n = random.random()   # generate a new random number

    if n < 1/6:
        x += 1            # move east
    elif 1/6 <= n < 2/6:
        y += 1            # move north
    elif 2/6 <= n < 3/6:
        z += 1            # move up
    elif 3/6 <= n < 4/6:
        x -= 1            # move west
    elif 4/6 <= n < 5/6:
        y -= 1            # move south
    else:
        z -= 1            # move down

    print("(%d,%d,%d)" % (x, y, z))

print("squared distance = %d" % (x*x + y*y + z*z))

Upvotes: 1

user8624339
user8624339

Reputation:

The issues lies with this line in each of your if statement : n = random.random(). Basically you should had used if and else if so if in a single iteration one if has been executed then it does not get executed again. Also you can add this line at the end of the all if statements instead of repeating it.n = random.random()

Upvotes: 1

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