How do I take a matrix of x,y values and insert in a value 1 in a square matrix? For instance in a Mat = 4 x 4 matrix of zeros and the matrix XY = (1,2) (3,4). How do I enter a 1 into (1,2) and (3,4)?

How do I take a matrix of x,y values and insert in a value 1 in a square matrix? For instance in a Mat = 4 x 4 matrix of zeros and the matrix XY = (1,2) (3,4). How do I enter a 1 into the positions (1,2) and (3,4)?

Answer 1

[KSSV]

A = zeros(4) ;
A(1,2) = 1 ;
A(3,4) = 1 ;

Read about matlab matrix indexing.

[MP]

Thank you, what if my A matrix is huge 2213 X 2213 and my XY matrix is 2387 X 2? How can I use MATLAB commands to substitute a 1 into the proper XY positions on the 2213 x 2213 matrix (initially all zeros)?

[KSSV]

In such case, you need to find out the respective indices for give subindices and replace those indices with 1. Check the below code:

A = zeros(4) ;
XY = [ 1 2 ; 3 4] ;
idx = sub2ind(size(A),XY(:,1),XY(:,2)) ;
A(idx) = 1 ;

Read about sub2ind and ind2sub.

[KSSV]

I guess, you are working on FEM and you want to apply some boundary conditions at XY to given matrix A. is it?

[MP]

Thank you! That worked for me