Saving values from a for loop into an existing matrix

Hello,

I am trying to define the boundary conditions in a FEM truss problem. I am having trouble saving what I did to the matrix. For example if BC(1) = 0 I want to make the first row equal to 0 then make the value in that row along the diagnol equal to 1 and save it to the original matrix.

here is my code:

%number of nodes / number of degrees of freedom
num_nd = 4;
num_dof = 2;

% Boundary Conditions
n_u = ones(num_nd, num_dof);
n_u(1,1) = 0; % Boundary conditions for node 1 x
n_u(1,2) = 0; % Boundary conditions for node 1 y
n_u(3,1) = 0; % Boundary condition for node 3 y

% Global matrix
kg = [1.3536 .3536 -1 0 0 0 -.3536 -.3536; 
    .3536 1.3536 0 0 0 -1 -.3536 -.3536; 
    -1 0 1 0 0 0 0 0; 0 0 0 1 0 0 0 -1; 
    0 0 0 0 1 0 -1 0; 0 -1 0 0 0 1 0 0; 
    -.3536 -.3536 0 0 -1 0 1.3536 .3536; 
    -.3536 -.3536 0 -1 0 0 .3536 1.3536];

%Apply boundary conditions
for i = 1:num_nd
    if n_u(i) == 0 
        kg(i,:) = 0;
        if kg(i,:) == 0 
            kg(i,i) = 1;
        end
    end
end

% What I want kg to look like
% kg = [1 0 0 0 0 0 0 0; 
%       0 1 0 0 0 0 0 0; 
%      -1 0 1 0 0 0 0 0; 
%      0 0 0 1 0 0 0 -1; 
%       0 0 0 0 1 0 0 0;
%      0 -1 0 0 0 1 0 0; 
%    -.3536 -.3536 0 0 -1 0 1.3536 .3536; 
%    -.3536 -.3536 0 -1 0 0 .3536 1.3536];

Answer 1

[Spencer Chen]

This might be your problem:

for i = num_nd ...

This code does not give you a proper for-loop.

I'll leave you pondering on the problem yourself.

Blessings,

Spencer

[Allen Love]

even for i = 1:num_nd I still get the original matrix. I understand for loops in the basic sense. My question is how to save values to an existing matrix.

[Allen Love]

Any ideas beyond the for loop syntax?