python and matrices

[15153681]

hey guys
I'm new to python... and I would like to know how do I create a matrix in python, if anyone can help, it would be appreciated. Thanx

[bvdet]

A matrix or array can be represented by a list of lists.
Example:

>>> [[0 for _ in range(3)] for _ in range(3)]
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>>

You can encapsulate an array object as an instance of a class object. Example:

class Matrix(object):
    def __init__(self, rows, cols):
        self.rows = rows
        self.cols = cols
        # initialize matrix and fill with zeroes
        self.data = [[0 for _ in range(cols)] for _ in range(rows)]
        
    def __setitem__(self, pos, v):
        self.data[pos[0]][pos[1]] = v

    def __getitem__(self, pos):
        return self.data[pos[0]][pos[1]]

    def __iter__(self):
        for row in self.data:
            yield row

    def len(self):
        return len(self.data)
    
    def __str__(self):
        return '\n'.join(['Row %s = %s' % (i, self.data[i]) for i in range(self.rows)])

    def __repr__(self):
        return 'Matrix(%d, %d)' % (self.rows, self.cols)

# Example
>>> m0 = Matrix(4,4)
>>> m0
Matrix(4, 4)
>>> print m0
Row 0 = [0, 0, 0, 0]
Row 1 = [0, 0, 0, 0]
Row 2 = [0, 0, 0, 0]
Row 3 = [0, 0, 0, 0]
>>> m0[3,3]='a string'
>>> m0[2,2]=123456
>>> print m0
Row 0 = [0, 0, 0, 0]
Row 1 = [0, 0, 0, 0]
Row 2 = [0, 0, 123456, 0]
Row 3 = [0, 0, 0, 'a string']
>>>

Another example using a dictionary:

class Darray(object):
    
    def __init__(self, rows, cols):
        self.rows = rows
        self.cols = cols
        # initialize array and fill with zeroes
        self.data = {}
        for row in range(rows):
            for col in range(cols):
                self.data.setdefault(row, {})[col] = 0

    def __setitem__(self, rowcol, v):
        self.data[rowcol[0]][rowcol[1]] = v

    def __getitem__(self, rowcol, v):
        return self.data[rowcol[0]][rowcol[1]]

    def __str__(self):
        outputList = ['%s%s%s' % ('Row/Col'.center(11), '|', ''.join(['%6s' % i for i in range(0,self.cols)])),]
        outputList.append('=' * 6 * (2+self.cols))
        for key in self.data:
            outputList.append('%s|%s' % (str(key).center(11), ''.join(['%6s' % self.data[key][i] for i in self.data[key]])))
        return '\n'.join(outputList)
    
if __name__ == '__main__':
    print
    a = Darray(4,6)
    print a
    print
    a[3,4]=19
    a[3,2]=344
    a[0,5]=55
    print a

Output from above example:

>>> 
  Row/Col  |     0     1     2     3     4     5
================================================
     0     |     0     0     0     0     0     0
     1     |     0     0     0     0     0     0
     2     |     0     0     0     0     0     0
     3     |     0     0     0     0     0     0

  Row/Col  |     0     1     2     3     4     5
================================================
     0     |     0     0     0     0     0    55
     1     |     0     0     0     0     0     0
     2     |     0     0     0     0     0     0
     3     |     0     0   344     0    19     0
>>>

If you need more power and efficiency, you can install NumPy to create multi-dimensional arrays. Example:

>>> ar = numpy.array(((0,0,0,0), (1,2,3,4), (5,6,7,8)), float)
>>> ar
array([[ 0.,  0.,  0.,  0.],
       [ 1.,  2.,  3.,  4.],
       [ 5.,  6.,  7.,  8.]])
>>> ar[:2]
array([[ 0.,  0.,  0.,  0.],
       [ 1.,  2.,  3.,  4.]])
>>> ar[1:]
array([[ 1.,  2.,  3.,  4.],
       [ 5.,  6.,  7.,  8.]])
>>> ar[1]
array([ 1.,  2.,  3.,  4.])
>>>