Re: Setting parts of matrices
- Subject: [mg2996] Re: Setting parts of matrices
- From: lk3a at kelvin.seas.virginia.edu (Count Dracula)
- Date: 19 Jan 1996 11:04:51 -0600
- Approved: usenet@wri.com
- Distribution: local
- Newsgroups: wri.mathgroup
- Organization: University of Virginia
- Sender: mj at wri.com
In article <4cqk0q$ig5 at dragonfly.wri.com>, jnygaard at math.uio.no writes:
>
>Can somebody tell me how to set a submatrix of a larger matrix?
>Assume a is a larger matrix than b. I would like to do something
>like
>
> a[[Range[i1,i2],Range[j1,j2]]]=b;
>
>Unfortunately, this does not work
A solution to this was posted some time ago by Alan Hayes.
It was similar to the MatrixInsert module below.
MatrixInsert[A, B, rowbeg, colbeg] inserts matrix B into matrix A
with the (1, 1) element of B replacing the (rowbeg, colbeg) element
of A. The module returns Null but changes A.
Attributes[MatrixInsert] = {HoldFirst}
MatrixInsert[A_?MatrixQ, B_?MatrixQ, rowbeg_Integer, colbeg_Integer] :=
Module[ {rowend, colend, aux},
{rowend, colend} = {rowbeg, colbeg} + Dimensions[B] - 1;
aux = Take[A, {rowbeg, rowend}];
A = Join[
Take[A, rowbeg - 1],
MapThread[Join, {Map[Take[#1, colbeg - 1] &, aux], B, Map[Drop[#1, colend] &, aux]}],
Take[A, rowend - Length[A]]
];
]
A = IdentityMatrix[5];
B = {{a, b}, {c, d}};
test := (MatrixInsert[A, B, 2, 4]; MatrixForm[A]);
The test run gives this:
In[2]:= test
Out[2]//MatrixForm= 1 0 0 0 0
0 1 0 a b
0 0 1 c d
0 0 0 1 0
0 0 0 0 1
--
___________________________________________________________________________________
Levent Kitis lk3a at cars.mech.virginia.edu lk3a at kelvin.seas.virginia.edu
University of Virginia Department of Mechanical, Aerospace, and Nuclear Engineering