Matrix dimensions must agree. error .

%***** CALCULATE STRESS *****
stress1 = ones(1,6);
stress2 = ones(1,6);
direct = ones(1,6);
for i = 1:n_elem
    lambda = [l_ij(i)  m_ij(i)  0     0     0     0      ;...
              -m_ij(i)  l_ij(i)   0     0     0     0      ;...
              0     0      1     0     0     0      ;...
              0     0      0     l_ij(i)  m_ij(i) 0      ;...
              0     0      0     -m_ij(i)  l_ij(i)  0      ;...
              0     0      0     0     0     1];  
    B1Direct = [-1/l(i) 0 0 1/l(i) 0 0];
    B1 = [-1/l(i) 6*t/l(i)^2 4*t/l(i) 1/l(i) (-6)*(t)/l(i)^2 (t)*(2)/l(i)];
    B2 = [-1/l(i) -6*t/l(i)^2 -2*t/l(i) ...
    1/l(i) 6*t/l(i)^2 -4*t/l(i)];
    q = [u(connect(i,1)*3 - 2);...
        u(connect(i,1)*3 - 1);
        u(connect(i,1)*3);...
        u(connect(i,2)*3 - 2);...
        u(connect(i,2)*3 - 1);...
        u(connect(i,2)*3)];
    stress1(i) = (E(i)* B1) .* lambda .* q ;
    stress2(i) = (E(i) * B2) .* lambda .* q ;
    direct(i) = (E(i) * B1Direct) .* lambda .* q ;
end
%%Displays results as table format for the stresses node i of element
di = cell(1,n_elem); %Node in x Direction
for i=1:n_elem % Defines the number of the node
    di{i} = ['elem' num2str(i)];
end
dirc = (reshape((di), size(di,1), (di))).'; %Concatenates node
stress_i_nodeMPa=table(dirc,round(stress1',4)); % Defines the values to show in table
stress_i_nodeMPa.Properties.VariableNames = {'Element','Stress'}; %Table column names
%%Displays results as table format for the stresses node j of element
sj = cell(1,n_elem); %Node in x Direction
for i=1:n_elem % Defines the number of the node
 sj{i} = ['elem' num2str(i)];
end
stjc = (reshape((sj), size(sj,1), [])).'; %Concatenates node
stress_j_nodeMPa=table(stjc,round(stress2',4)); % Defines the values to show in table
stress_j_nodeMPa.Properties.VariableNames = {'Element','Stress'}; %Table column names
%%Displays results as table format for the direct stresses
di = cell(1,n_elem); %Node in x Direction
for i=1:n_elem % Defines the number of the node
 di{i} = ['elem' num2str(i)];
end
dirc = (reshape((di), size(di,1), [])).'; %Concatenates node
DirStressMPa=table(dirc,round(direct',4)); % Defines the values to show in table
DirStressMPa.Properties.VariableNames = {'Element','Stress'}; %Table column names

[Geoff Hayes]

karthik - what line is throwing this error? What is the the full error message? We cannot run your code because there are some missing variables. Please clarify,

[karthik velkuru venkata ramana murthy]

%***** MESH *** 
X = [0 254 457.2 787.4 812.8 914.4 ]; % node x coordinates (mm) 
Y = [0 381 0 381 304.8 0]; % node y coordinates (mm)
n_node = length (X); % number of nodes 
n_elem = 7; % number of elements
connect = [1 2;2 3;1 3;2 4;3 5;4 5;5 6]; % node to element assignment
%***** MATERIAL & GEOMETRY PROPERTIES *** 
l = sqrt((X(connect(:,2))-X(connect(:,1))).^2 +... 
    (Y(connect(:,2))-Y(connect(:,1))).^2); % element length (mm) 
l_ij = (X(connect(:,2))-X(connect(:,1)))./l(1,:); % direction cosine cos(alfa) 
m_ij = (Y(connect(:,2))-Y(connect(:,1)))./l(1,:); % direction cosine cos(beta)
D = [25.4 25.4 25.4 25.4 25.4 25.4 25.4]; 
d = [23.8 23.8 23.8 23.8 23.8 23.8 23.8]; 
A = pi*(D.^2-d.^2)/4; % cross-sectional area (mm^2) Q2 
I = pi*(D.^4-d.^4)/64; % Izz moment of inertia (mm^4) t = 1.57 ; E = [200e3 200e3 200e3 200e3 200e3 200e3 200e3]; % Young's modulus of elasticity (N/mm^2) Q2
%***** BOUNDARY CONDITIONS ASSIGNMENT ** 
fixed_bc = [1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1]; % 1 fixed node % 0 free node 
F = [0 0 0 0 -70.09 0 0 0 0 0 -12.09 0 0 0 0 0 0 0]; % 0 either unknown reaction force or % no force on node (N)

[Geoff Hayes]

what is u? what is the full error message?