for loop with linspace

Hi all, I'm trying to get the following for loop statement to work. I'm doing shear force calculations for different portions of a beam. I get this as error 'Subscript indices must either be real positive integers or logicals'.

 b =input('Enter the length of the beam  ');
 u =input('Enter the load (N/m)  ');

    R_side = u*b/3;
    R_center = R_side;

    d = 0.17 * b;
    l = 0.33 *b;

   x = linspace(0,b,100);

    for i =length(x)

        if i<= d
            sh(i) = -u*x;
        elseif x<= d+l
            sh(i) = -u*x + R_side;
        elseif x<= d+2*l
            sh(i) = -u*x + R_side + R_center;
        else 
            sh(i) = -u*x + R_center + 2*R_side;

        end

    end

    plot(x,sh(i))
    xlabel(' Length of beam, [m]','FontSize',20);
    ylabel(' Shear force, [N]','FontSize',20);
    title('Shear Force Diagram','FontSize',20);

[Image Analyst]

Not what I get:

Enter the length of the beam  4
Enter the load (N/m)  5
Subscripted assignment dimension mismatch.
Error in test4 (line 21)
            sh(i) = -u*x + R_center + 2*R_side;

What values did you enter?

[Walter Roberson]

Notice that

      for i =length(x)

executes exactly once, with i = 100 (that is, the length of x)

Answer 1

[Sebastian Castro]

I see 2 things here

First, the for-loop should start from the first index and stop at the length of x. Right now, your code is saying that i is only one value ( length(x) ).

for i=1:length(x)

Secondly, just as you're assigning each individual element of the sh vector as sh(i), you want to do the same with x instead of trying to perform operations on the entire array. So, for example:

sh(i) = -u*x(i)

- Sebastian

[Rob]

Okay so now that I followed your corrections I'm getting a blank plot/vectors must be the same length error. Is it because I should make a vector with the output of sh? How could I store every value of sh(i)

as in save_case = 0;

save_case = save_case + 1;

sh_output(save_case) = sh?

[Sebastian Castro]

I think you need to remove the indexing from the plot, since you want the whole sh vector. The following command should do it:

plot(x,sh)

- Sebastian