Matrix Formatting¶
This appendix describes ways to customize the formatting of CVXOPT matrices.
As with other Python objects, the functions repr and 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 options in the
module cvxopt.printing. The parameters options['iformat']
and options['dformat'] determine, respectively, how integer and
double/complex numbers are printed. The entries are Python format strings
with default values '% .2e' for 'd' and 'z'
matrices and % i' for 'i' matrices. The parameters
options['width'] and options['height'] specify the maximum
number of columns and rows that are shown. If options['width'] is
set to a negative value, all columns are displayed. If
options['height'] is set to a negative value, all rows are
displayed. The default values of options['width'] and
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 repr and str
accessible for further customization, two functions are provided in
CVXOPT. The function cvxopt.matrix_repr is used when
repr is called with a matrix argument; and
cvxopt.matrix_str is used when str is called with a matrix
argument. By default, the functions are set to
printing.matrix_repr_default and
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 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 repr
and str for sparse matrices are cvxopt.spmatrix_repr
and cvxopt.spmatrix_str, respectively. By default, they are set to
printing.spmatrix_repr_default and
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 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