How to construct a matrix values by its row or column indexes?

Hi all,

I need your help on this. Suppose we have a 4x3 matrix and we need to construct a metrix with its row and column indexes as follows,

R = 1 1 1; 2 2 2; 3 3 3; 4 4 4

C= 1 2 3; 1 2 3; 1 2 3; 1 2 3

Thx in advance.

Answer 1

[Evan]

A(sub2ind(size(A),R,C))

Example:

>>A = magic(4);
>>R = [1 1 1; 2 2 2; 3 3 3; 4 4 4];
>>C = [1 2 3; 1 2 3; 1 2 3; 1 2 3];
>>B = A(sub2ind(size(A),R,C))

B =
    16     2     3
     5    11    10
     9     7     6
     4    14    15

If you're not talking about a generalized case and just want that particular indexing of any matrix, though, it looks like you just need to trim off part of the matrix:

>>B = A(1:4,1:3);

B =
    16     2     3
     5    11    10
     9     7     6
     4    14    15

[Antonio Trujillo-Ortiz]

Hi Evan,

What I need is to get from any matrix, two new ones with its row and column indexes. I mean,

Eg, given

A = [2 5 6;12 7 0;2 9 7;4 17 1 0]

we have 4 rows and 3 columns, and we need to get matrices

R = [1 1 1;2 2 2;3 3 3;4 4 4]

and

C = [1 2 3;1 2 3;1 2 3;1 2 3]

Thx

[Evan]

Ah, okay. So the row and column indices of every element in your array? I have submitted a new answer below that does this. Note that it looks like your example is for a 3x4 matrix, though, not a 4x3.

Answer 2

[Evan]

[R C] = find(A);

Answer 3

[Matt J]

 [R,C]=ndgrid(1:size(A,1),  1:size(A,2))

[Antonio Trujillo-Ortiz]

Thanks Matt J. Its just what I need.

[Matt J]

Clicking on "Accept this answer" is all the thanks I need ;)