Error in Following python program

[Peter Otten]

Pramod wrote:

> #/usr/bin/python
> from numpy import matrix
> n=input('Enter matrix range')
> fr=open('mat.txt','r')
> print ('Enter elements into the matrix\n')
> a=matrix([[input()for j in range(n)] for i in range(n)])
> for i in range(n):
>         for j in range(n):
>                 print a[i][j]
>         print '\n'
> 
> When i run the above program the following error is Coming please
> Error is
>                                            Enter matrix range3
> Enter elements into the matrix
> 
> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
> [[1 2 3]]
> Traceback (most recent call last):
>   File "2.py", line 10, in <module>
>     print a[i][j]
>   File "/usr/lib/python2.6/dist-packages/numpy/core/defmatrix.py",
> line 265, in __getitem__
>     out = N.ndarray.__getitem__
> 
> please resolve my problem  Thanks in advance

You can either use an array instead of a matrix and continue to access the 
elements like you did in your code

>>> a = numpy.array([[1,2],[3,4]])
>>> a[1][1]
4

or continue to use the matrix and access its elements with a tuple

>>> b = numpy.matrix([[1,2],[3,4]])
>>> b[1,1]
4

If you pass only one index you get another, smaller matrix:

>>> b[1]
matrix([[3, 4]])

Once you see this printed it should be clear that b[1][1] asks for the non-
existent second row of the above matrix. Hence the error:

>>> b[1][1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/dist-packages/numpy/core/defmatrix.py", line 265, 
in __getitem__
    out = N.ndarray.__getitem__(self, index)
IndexError: index out of bounds

By the way, these matrices are really strange beasts:

>>> b[0][0]
matrix([[1, 2]])
>>> b[0][0][0][0][0][0][0][0][0][0][0][0][0][0][0][0][0][0][0][0]
matrix([[1, 2]])

Peter