Class python in loop

Question

I created a class of matrix with python:

class MatrixOperation:

    ...

    def create_matrix(self):
        some function for creation of matrix.
        return matrix

    def matrix_vector_multiplication(self, x):
        mat = self.create_matrix()
        return numpy.dot(mat, x)

And:

M = MatrixOperation(...)
x = some set of vector

for i in range(n):
   M.matrix_vector_multiplication(x[i])

The problem is, for each iteration, M.matrix_vector_multiplication(x[i]) will recompute mat = self.create_matrix() before calculating numpy.dot(mat, x), that is unnecessary (since it could be computed once at the beginning). How can I avoid this?

Thanks,

Should create_matrix return the matrix, or could it make it an instance attribute? — jonrsharpe
– jonrsharpe, Commented Jul 9, 2014 at 13:46
What do you mean once? by instance? by process like static variable in Java? — emesday
– emesday, Commented Jul 9, 2014 at 13:57
I mean the matrix was created in the first iteration. For the 2, 3... iteration, I just do matrix_vector_multiplication, do not repeat the creation of matrix. — user2863620
– user2863620, Commented Jul 9, 2014 at 14:01

wwii · Accepted Answer · 2014-07-09 13:54:05Z

3

To avoid recreating the matrix each time, create an instance attribute in the class's __init__ method - similar to this.

class Matrix(object):
    def __init__(self, data):
        self.matrix = self.create_matrix(data)
        # or simply
        # self.matrix = np.matrix(data)
    def create_matrix(data):
        # create the_matrix
        return the_matrix
    def do_something(self, x):
        z = some_function(self.matrix, x)
        return z

my_matrix = matrix([[1,2,3,4],[4,3,2,1]])

answered Jul 9, 2014 at 13:54

wwii

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wwii Over a year ago

Here's a good read - docs.python.org/2.7/tutorial/classes.html#class-objects

stoer · Accepted Answer · 2014-07-09 13:48:28Z

1

just making a copy of the matrix should fix your problem.

import copy


class MatrixOperation:
matrix = None
...

def create_matrix(self):
    if self.matrix is not None:
        return copy.copy(self.matrix)
    some function for creation of matrix.
    self.matrix = matrix
    return matrix

def matrix_vector_multiplication(self, x):
    mat = self.create_matrix()
    return numpy.dot(mat, x)

answered Jul 9, 2014 at 13:48

stoer

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