Trying to multiply Matrices

Hey there,

I'm trying to pultiple some matricies to output a new matrix or array of Y but seem to be getting an error. "Array indices must be positive integers or logical values." I only need values at 4 evenly spaced interior points but cant seem to tell the equation to only look at 4 values instead of all 199. Anything help thank you.

  close all;
    clear all;
    clear;
    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)

       Y(i)=A'.*B_bar 
   

Answer 1

[Walter Roberson]

    L=10; %given
    EI=1900; %given
    q=-0.6; %given
    dx=0.05;%desired grid spacing
    N = 1+L/dx

N = 201

    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

B_bar = 0

A = 
   (1,1)       -2
   (2,1)        1
   (1,2)        1
   (2,2)       -2
   (3,2)        1
   (2,3)        1
   (3,3)       -2
   (4,3)        1
   (3,4)        1
   (4,4)       -2

ans = 'double'

   0.0000 + 1.0000i

Array indices must be positive integers or logical values.

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)