Is Diagonal |2|
~~~~~~~~~~~~~~~

In mathematics, a diagonal matrix is a matrix in which all non-diagonal
entries are zero. That is, :math:`\mathbf{A_{i,j} = 0}` if
:math:`\mathbf{i\neq j}`. Since it is easier to work with diagonal
matrices, we should check them if they are diagonal or not.

Write a function called *is_diagonal* that takes an
:math:`\mathbf{n\times n}` matrix as a *list of list of integers* and
returns a *Boolean* value stating whether the matrix is diagonal or not.

Note that in a diagonal matrix, diagonal entries can be zero as well,
but in our case diagonal entries are non-zero integers.

*Hint:* There is a list method that returns the number of occurrences of
a given element in that list.

.. container:: sampleio

   Sample I/O:

.. |2| image:: ../../figures/difficulty_three.png
   :class: difficulty

.. code:: default

   >>> is_diagonal([[1,0,0],[0,1,0],[0,0,1]])
   True

   >>> is_diagonal([[1,1,1],[2,4,0],[3,3,3]])
   False

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Show the Answer

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.. code:: python

   def is_diagonal(matrix):
       n = len(matrix)

       for i in range(n):
           zeros = matrix[i].count(0)
           if(zeros != n-1 or matrix[i][i] == 0):      
               return False

       return True

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