L=10; %given
EI=1900; %given
q=-0.6; %given
dx=0.05;%desired grid spacing
N = 1+L/dx
X = linspace(0,10,N)';
Y=X*0;%inital condistion
for n = 1 : length(X)-2 % the # of interrior points
%this creates B bar which is all known at every time step
B_bar(n,:) = (((q.*X(n).*(X(n)-L))*(dx^2))/(EI*2))
A=gallery('tridiag' , 4 , 1,-2,1)
class(i)
whos i
disp(i)
Y(i)=A'.*B_bar
end
You are storing into Y(i) but i has value complex(0,1) .
You did not assign a value to i, so it has its default value, which is sqrt(-1) . The default value of j is also sqrt(-1)