Bárbara Bárbara - 16 days ago 7
Python Question

Error with arrays/matrix operations

I am trying to run the following code. For those who know it, this is a try of the Ehrenfest Urn simulation.

import numpy as np
import random

C=5
L=2
# Here I create a matrix to be filled with zeros and after with numbers I want
b=np.zeros( (L,C) ) # line x column

A=[] # here I creat 2 lists to put random integer numbers on it
B=[]

i=1
while i<=10: # here I am filling list A (only) with 10 numbers
A.append(i)
i=i+1

for j in range(2):

for i in range(5): #here I want to choose random numbers between 1 and 10,

Sort=random.randint(1,10)

if i==0: # since there is no number on B, the first step, the number goes to B
B.append(Sort)
A.remove(Sort)
print(len(A))

if i>0: # now each list A and B have numbers on it, so I will choose one number and see in which list it is
if Sort in A:
B.append(Sort)
A.remove(Sort)
else:
A.append(Sort)
B.remove(Sort)

i=i+1
b[j,i]=len(A) # here I want to add the lenght of the list A in a matrix, but then I get the error.
j=j+1

print(b)


But I get the following error:

Traceback (most recent call last):

File "<ipython-input-26-4884a3da9648>", line 1, in <module>
runfile('C:/Users/Public/Documents/Python Scripts/33.py', wdir='C:/Users/Public/Documents/Python Scripts')

File "C:\Program Files\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 866, in runfile
execfile(filename, namespace)

File "C:\Program Files\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 102, in execfile
exec(compile(f.read(), filename, 'exec'), namespace)

File "C:/Users/Public/Documents/Python Scripts/33.py", line 42, in <module>
b[j,i]=len(A) # here I want to add the lenght of the list A in a matrix

IndexError: index 5 is out of bounds for axis 1 with size 5


What am I doing wrong with the arrays?
Is there anything to do with the identification of the numbers in the matrix?

Answer

The error implies that b.shape[1] (axis 1) is 5; but i is 5. Remember indexing starts at 0.

In the broader picture:

while i<5:  #here I want to choose random numbers between 1 and 10,
     ...
     i=i+1
     b[j,i] ...

at the last iteration i==4, you add one, and now it is 5, and gives the error.

Typically in Python we iterate with

 for i in range(5):
    b[j,i] ...

range(5) produces [0,1,2,3,4]. You may have good reasons to use the while instead, but it's subject to the same bounds.

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