Suppose I have a matrix A which is m x n in order. How do I extract a submatrix of specified rows and colums from A. Like I want a p x q submatrix of A. Will the expanded_submatrix method help in this regard? Thanks beforehand.
The method matrix_from_rows_and_columns will do just this; it is especially helpful if you don't want to use consecutive rows and/or columns:
sage: M = matrix(4, 4, range(16))
sage: M
[ 0 1 2 3]
[ 4 5 6 7]
[ 8 9 10 11]
[12 13 14 15]
sage: M.matrix_from_rows_and_columns([0,3], [1])
[ 1]
[13]
sage: M.matrix_from_rows_and_columns([0,3], [1,2])
[ 1 2]
[13 14]
The arguments are a pair: a list of rows to use and a list of columns to use. The submatrix method can be used (in addition to the answers already provided) if you want consecutive rows and columns.
Slice indexing works for both dense and sparse matrices :
sage: M=matrix(SR, 5, sparse=True) ; M[2, 1]=3 ; M[3,2]=5 ; M
[0 0 0 0 0]
[0 0 0 0 0]
[0 3 0 0 0]
[0 0 5 0 0]
[0 0 0 0 0]
sage: M[2:5, 1:4]
[3 0 0]
[0 5 0]
[0 0 0]
HTH,
The syntax was unexpectedly easy. It is similar to the one used in Matlab/Octave. We just use the syntax A[i-1:p,j-1:q], wherei,j are the positions from which we want to extract the p x q matrix, to get the desired answer. So, basically matrix is just storing a list of list as an array data structure.
That's true for dense matrix. Try this on a sparse matrix...
Asked: 2023-08-16 00:46:36 +0100
Seen: 1,883 times
Last updated: Aug 17 '23
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