retrieve value from specific location of matrix

Hi suppose I have a matrix as follows:

m =

     1     1
     1     3
     1     5
     2     2
     2     4
     3     1
     3     3
     3     5

each row of m is the row,column index respectively.

And I have another matrix I.

I need to find the values of I of location m. How can I do that?

Is there anybody to help me? Thanks

Answer 1

[Oleg Komarov]

use sub2ind to create from coordinates an index that points to the position.

m = [1     1
     1     3
     1     5
     2     2
     2     4
     3     1
     3     3
     3     5]
I = rand(5);
sub2ind(size(I),m(:,1),m(:,2))

To skip the overhead you can apply the engine of sub2ind directly (2d adaptation):

(m(:,2)-1)*size(I,1) + m(:,1)

[Mohammad Golam Kibria]

If

I =

1 2 1 2 1 3

2 1 2 1 2 3

1 2 1 2 1 3

and

m =

1 1

1 3

1 5

2 2

2 4

3 1

3 3

3 5

then sub2ind(size(I),m(:,1),m(:,2))

shows the following output:

ans =

1

7

13

5

11

3

9

15

But it should be

1

1

1

1

1

1

1

1

[Matt Fig]

You are using SUB2IND incorrectly. SUB2IND returns the indices for use into an array, not the indexed array. So, use it like this:

IDX = sub2ind(size(I),m(:,1),m(:,2)); % Create index vector

I(IDX) % Index into I with IDX.

[Mohammad Golam Kibria]

yes this is correct.

Thanks.