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.. _c-printing:
*****************
Matrix Formatting
*****************
This appendix describes ways to customize the formatting of CVXOPT matrices.
As with other Python objects, the functions :func:`repr` and :func:`str`
return strings with printable representations of matrices. The command
'``print A``' executes '``str(A)``', whereas the command '``A``'
calls '``repr(A)``'. The following example illustrates the default
formatting of dense matrices.
>>> from cvxopt import matrix
>>> A = matrix(range(50), (5,10), 'd')
>>> A
<5x10 matrix, tc='d'>
>>> print(A)
[ 0.00e+00 5.00e+00 1.00e+01 1.50e+01 2.00e+01 2.50e+01 3.00e+01 ... ]
[ 1.00e+00 6.00e+00 1.10e+01 1.60e+01 2.10e+01 2.60e+01 3.10e+01 ... ]
[ 2.00e+00 7.00e+00 1.20e+01 1.70e+01 2.20e+01 2.70e+01 3.20e+01 ... ]
[ 3.00e+00 8.00e+00 1.30e+01 1.80e+01 2.30e+01 2.80e+01 3.30e+01 ... ]
[ 4.00e+00 9.00e+00 1.40e+01 1.90e+01 2.40e+01 2.90e+01 3.40e+01 ... ]
The format is parameterized by the dictionary :data:`options` in the
module :mod:`cvxopt.printing`. The parameters :attr:`options['iformat']`
and :attr:`options['dformat']` determine, respectively, how integer and
double/complex numbers are printed. The entries are Python format strings
with default values :const:`'\% .2e'` for :const:`'d'` and :const:`'z'`
matrices and :const:`\% i'` for :const:`'i'` matrices. The parameters
:attr:`options['width']` and :attr:`options['height']` specify the maximum
number of columns and rows that are shown. If :attr:`options['width']` is
set to a negative value, all columns are displayed. If
:attr:`options['height']` is set to a negative value, all rows are
displayed. The default values of :attr:`options['width']` and
:attr:`options['height']` are 7 and -1, respectively.
>>> from cvxopt import printing
>>> printing.options
{'width': 7, 'dformat': '% .2e', 'iformat': '% i', 'height': -1}
>>> printing.options['dformat'] = '%.1f'
>>> printing.options['width'] = -1
>>> print(A)
[ 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0]
[ 1.0 6.0 11.0 16.0 21.0 26.0 31.0 36.0 41.0 46.0]
[ 2.0 7.0 12.0 17.0 22.0 27.0 32.0 37.0 42.0 47.0]
[ 3.0 8.0 13.0 18.0 23.0 28.0 33.0 38.0 43.0 48.0]
[ 4.0 9.0 14.0 19.0 24.0 29.0 34.0 39.0 44.0 49.0]
In order to make the built-in Python functions :func:`repr` and :func:`str`
accessible for further customization, two functions are provided in
CVXOPT. The function :func:`cvxopt.matrix_repr` is used when
:func:`repr` is called with a matrix argument; and
:func:`cvxopt.matrix_str` is used when :func:`str` is called with a matrix
argument. By default, the functions are set to
:func:`printing.matrix_repr_default` and
:func:`printing.matrix_str_default`, respectively, but they can be
redefined to any other Python functions. For example, if we prefer
``A`` to return the same output as ``print A``, we can simply
redefine :func:`cvxopt.matrix_repr` as shown below.
>>> import cvxopt
>>> from cvxopt import matrix, printing
>>> A = matrix(range(4), (2,2), 'd')
>>> A
<2x2 matrix, tc='d'>
>>> cvxopt.matrix_repr = printing.matrix_str_default
>>> A
[ 0.00e+00 2.00e+00]
[ 1.00e+00 3.00e+00]
The formatting for sparse matrices is similar. The functions :func:`repr`
and :func:`str` for sparse matrices are :func:`cvxopt.spmatrix_repr`
and :func:`cvxopt.spmatrix_str`, respectively. By default, they are set to
:func:`printing.spmatrix_repr_default` and
:func:`printing.spmatrix_repr_str`.
>>> import cvxopt
>>> from cvxopt import printing, spmatrix
>>> A = spmatrix(range(5), range(5), range(5), (5,10))
>>> A
<5x10 sparse matrix, tc='d', nnz=5>
>>> print(A)
[ 0.00e+00 0 0 0 0 0 0 ... ]
[ 0 1.00e+00 0 0 0 0 0 ... ]
[ 0 0 2.00e+00 0 0 0 0 ... ]
[ 0 0 0 3.00e+00 0 0 0 ... ]
[ 0 0 0 0 4.00e+00 0 0 ... ]
>>> cvxopt.spmatrix_repr = printing.spmatrix_str_default
>>> A
[ 0.00e+00 0 0 0 0 0 0 ... ]
[ 0 1.00e+00 0 0 0 0 0 ... ]
[ 0 0 2.00e+00 0 0 0 0 ... ]
[ 0 0 0 3.00e+00 0 0 0 ... ]
[ 0 0 0 0 4.00e+00 0 0 ... ]
As can be seen from the example, the default behaviour is to print the
entire matrix including structural zeros. An alternative triplet printing
style is defined in :func:`printing.spmatrix_str_triplet`.
>>> cvxopt.spmatrix_str = printing.spmatrix_str_triplet
>>> print(A)
(0,0) 0.00e+00
(1,1) 1.00e+00
(2,2) 2.00e+00
(3,3) 3.00e+00
(4,4) 4.00e+00
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