topic Set submatrix values within a matrix in Discussions

Set submatrix values within a matrix

MathStatChem — Mon, 22 Sep 2025 15:02:40 GMT

I have a matrix of missing values, and I want to set the upper left portion of that matrix to values in another matrix.

initial_matrix=J(3,3,.);
update_matrix=[1 2, 3 4];

The desired result is

[
1 2 .,
3 4 .,
. . . 
]

I know I could iterate on the indices of the matrix like this


for(i=1, i<=N rows(update_matrix), i++,
	for(k=1, k<=N cols(update_matrix), k++,
		initial_matrix[i,k]=update_matrix[i,k]
	)
);

Show(initial_matrix)

but is there an easier more compact way to do this using direct referencing of the matrix indices?

Re: Set submatrix values within a matrix

SDF1 — Mon, 22 Sep 2025 16:42:00 GMT

Hi @MathStatChem ,

This might not be as elegant as what you're hoping for, but it should do it:

initial_matrix = J(3,3,.);
update_matrix=[1 2, 3 4];

initial_matrix[1,1::2]=update_matrix[1,1::2];
initial_matrix[2,1::2]=update_matrix[2,1::2];

Which gives you the following output:

[1 2 .,
3 4 .,
. . .]

I believe this is what you might be looking for. The indexing goes [row,n1::n2], so [1,1::2] is row 1 elements 1 to 2. The only drawback is that you have to know what submatrix you're assigning the values to ahead of time. If you don't know that information, your loop would be a better approach.

Hope this helps!,

Re: Set submatrix values within a matrix

mmarchandFSLR — Mon, 22 Sep 2025 17:14:37 GMT

This should work for you. The Try() will simply not assign anything if the update matrix doesn't contain the row and column.

Names Default To Here( 1 );
im = J( 3, 3, . );
um = [1 2, 3 4];
im = Transform Each( {e, {r, c}}, im, Try( um[r, c] ) );
////  [1 2 .,
////  3 4 .,
////  . . .]

Re: Set submatrix values within a matrix

Craige_Hales — Mon, 22 Sep 2025 19:17:03 GMT

m = j(7,8,99);
m[2::5,3::4] = [
	41 42,
	43 44,
	45 46,
	47 48
];
show(m)

m = 
[	99 99 99 99 99 99 99 99, 
	99 99 41 42 99 99 99 99, 
	99 99 43 44 99 99 99 99, 
	99 99 45 46 99 99 99 99, 
	99 99 47 48 99 99 99 99, 
	99 99 99 99 99 99 99 99, 
	99 99 99 99 99 99 99 99];

Re: Set submatrix values within a matrix

MathStatChem — Tue, 23 Sep 2025 13:07:03 GMT

Thanks @Craige_Hales , that's what I was looking for. To make it more general, here is what I am doing

m=J(7,8,99);

update=[41 42,
	43 44,
	45 46,
	47 48
];

m[1::nrows(update), 1::ncols(update)]=update;
show(m);

result

m = 
[	41 42 99 99 99 99 99 99, 
	43 44 99 99 99 99 99 99, 
	45 46 99 99 99 99 99 99, 
	47 48 99 99 99 99 99 99, 
	99 99 99 99 99 99 99 99, 
	99 99 99 99 99 99 99 99, 
	99 99 99 99 99 99 99 99];

Re: Set submatrix values within a matrix

MathStatChem — Tue, 23 Sep 2025 13:10:19 GMT

Thanks. I've never used Transform Each( ), look pretty useful, and would not of thought using that on a matrix.

Re: Set submatrix values within a matrix

mmarchandFSLR — Wed, 24 Sep 2025 11:35:28 GMT

My code should be changed just a bit. The way it's written, any row/column combination not in the update matrix will replace the value in the initial matrix with a missing value. The Try() should have a Catch Expression to prevent overwriting the value, in case the original value isn't missing.

Names Default To Here( 1 );
im = J( 3, 3, 5 );
um = [1 2, 3 4];
im = Transform Each( {e, {r, c}}, im, Try( um[r, c], e ) );