python error : ''TypeError: only length-1 arrays can be converted to Python scalars''

Question

I am writing a simulation of the monty hall problem and I can't for the life of me understand what is causing this error. If you are not familiar with the monty hall problem, it is a hypothetical game show where there are 3 doors, there is a prize behind one door and 2 doors with nothing. A contestant picks a door and then the host opens a non-winning door and gives the contestant the option to switch or stay with their original pick. The original pick has a 1/3 chance of being right and the switch strategy has a 2/3 chance of being right.

My first function there takes 2 arrays which are randomly chosen doors and then creates a third array which is the door

import numpy as np
import pandas as pd


def reveal_and_switch(win_door,first_pick):
    '''Create arrays for the door to be revealed by the host and the switch door'''
    #Take in arrays for the winning door and the contestant's first pick
    doors = [1,2,3]
    switch_door = np.array([0]*len(win_door))
    for i in range(len(switch_door)):
        if first_pick[i] != win_door[i]:
            switch_door[i] = win_door[i]
        else:
            del doors[np.searchsorted(doors,first_pick[i])]
            switch_door[i] = np.random.choice(doors)

    #print switch_door
    return switch_door


def create_doors(iterations):
    '''Create a DataFrame with columns representing the winning doors,
    the picked doors and the doors picked if the player switches and the
    accumulating probabilities'''
    win_door = np.random.random_integers(1,3,iterations)
    first_pick = np.random.random_integers(1,3,iterations)
    switch_door = reveal_and_switch(win_door,first_pick)
    #allocate memory for 
    denom = np.array([0]*len(win_door))
    first_win = np.array([0]*len(win_door))
    switch_win = np.array([0]*len(win_door))
    switch_prob = np.array([0]*len(win_door))
    stay_prob = np.array([0]*len(win_door))

    for i in len(range(switch_door)):
        denom[i] = i + 1
        if switch_door[i] == win_door[i]:
            switch_win[i] = 1
            first_win[i] = 0
        elif first_pick[i] == win_door[i]:
            switch_win[i] = 0
            first_win[i] = 1



    switch_prob = np.cumsum(switch_win)/denom
    stay_prob = np.cumsum(first_win)/denom
    df = pd.DataFrame({'iterations': iterations,
                     'Stubborn Win': first_win,
                     'Switch Win': switch_win,
                     'stubborn probability': stay_prob,
                     'switch probability': switch_prob})
    print df
    return df

and when I call create_doors(10), I get this:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 14, in create_doors
TypeError: only length-1 arrays can be converted to Python scalars

zhangxaochen · Accepted Answer · 2014-02-08 14:22:12Z

1

reproduce such an error:

In [32]: a
Out[32]: array([0, 1, 2])

In [33]: range(a)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-33-5515275ab580> in <module>()
----> 1 range(a)

TypeError: only length-1 arrays can be converted to Python scalars

In your code range(switch_door), it's just like my range(a).

BTW, in your code,

denom = np.array([0]*len(win_door))
first_win = np.array([0]*len(win_door))

could just be simplified:

denom=np.zeros_like(win_door)
first_win = denom.copy()

answered Feb 8, 2014 at 14:22

zhangxaochen

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