Help! me set the subscript indices properly so the function can perform while loop iterations

Help! me set the subscript indices properly so the function can perform the req'd iterations. MATLAB errors: Subscript indices must either be real positive Error in ==> connection_analysis_1 at 32

j=0; % rotation angle - assumed THETA1(1)=1; % rotation angle - calculated % THETA2(j)=10; r(1)=0;

while (1.00<r(j) r(j)<0.99)

        M=1;

    T(j)=1/(2*THETA1(j))*(P - ((2*M)/L));

    if T(j)>=Tu
        DELTA(j)=DELTAu;
    else
        DELTA(j)=-4E-10*T(j)^4-4E-08*T(j)^3+0.0003*T(j)^2-0.0255*T(j)+0.4375;
    end

    THETA2(j)=sqrt(2*(T(j)/E*A + DELTA(j)/L) + (T(j)/E*A + DELTA(j)/L)^2);

    r(j)=THETA1(j)/THETA2(j);

    if j==1
        THETA(j+1)=(THETA1(j)+THETA2(j))/2;

    else if r(j)>1 && r(j-1)<1 || r(j)<1 && r(j-1)>1
            THETA1(j+1)=(THETA2(j-1)+THETA2(j))/2;
        else
            THETA1(j+1)=(THETA1(j)+THETA2(j2))/2;
        end
    end

    j=j+1;

end

Answer 1

[Brian B]

Indices start at 1, so

 j=0; 
 ...
 while (1.00<r(j) r(j)<0.99)

is referencing a value that does not exist (the 0th value of r). Another problem with this code is that you need to use a logical operator to combine the conditions in the while statement. For example, if you wanted 1.00<r(j) OR r(j)<0.99 then use:

 while (1.00<r(j) || r(j)<0.99)

[Paul Silano]

I have simply changed the index to k instead of j in the initial post.

The following errors occur when I run the program. What should I do to correct?

??? Attempted to access r(2); index out of bounds because numel(r)=1.

Error in ==> connection_analysis_1 at 34 while (0.99<=r(k)|| r(k)<=0.99) ________________________________________________________________________

k=1; % rotation angle - assumed THETA1(k)=1; % rotation angle - calculated THETA2(k)=10; r(k)=THETA1(k)/THETA2(k);

while (0.99<=r(k)|| r(k)<=0.99)

    % if THETA1(k)>=THETAu;
        M=1;
    % else
    %    M= % M as a function of THETA;
    % end

    T(k)=1/(2*THETA1(k))*(P - ((2*M)/L));

    if T(k)>=Tu
        DELTA(k)=DELTAu;
    else
        DELTA(k)=-4E-10*T(k)^4-4E-08*T(k)^3+0.0003*T(k)^2-0.0255*T(k)+0.4375;
    end

    THETA2(k)=sqrt(2*(T(k)/E*A + DELTA(k)/L) + (T(k)/E*A + DELTA(k)/L)^2);

    r(k)=THETA1(k)/THETA2(k);

    if k==1
        THETA1(k+1)=(THETA1(k)+THETA2(k))/2;

    else if r(k)>1 && r(k-1)<1 || r(k)<1 && r(k-1)>1
            THETA1(k+1)=(THETA2(k-1)+THETA2(k))/2;
        else
            THETA1(k+1)=(THETA1(k)+THETA2(k2))/2;
        end
    end

k=k+1

end

[Brian B]

That error occurs because r is a scalar:

 k=1; % rotation angle - assumed
 ...
 r(k)=THETA1(k)/THETA2(k);

but you are tying to access the (non-existent) second element of r. Perhaps you know all the values of r from the beginning, and you can make r a vector before the while loop. Otherwise, you must fill in values of r as you go. I would expect something like

while ...
   ...
   k=k+1;
   r(k) = ...;
end

Note that your while condition is more baffling than before, because the condition

 (0.99<=r(k)|| r(k)<=0.99)

is true for any finite or infinite numerical value.

[Paul Silano]

Thank you for being so helpful Brian B! I am really struggling to get this while loop working.

I am trying to get the values of THETA1 and THETA2 to converge at the range of 0.99< (r=THETA1/THETA2) <=1.00 through a series of calculations.

I have fixed the non-existent second element of r problem by r(k+1)=... as suggested and the baffling convergence condition (my apologies).

Now the loop only runs once through (ie. k=1), although the ratio r is not in the acceptable range. WHAT IS WRONG?

k=1; % rotation angle - assumed THETA1(k)=1; % rotation angle - calculated THETA2(k)=10; r(k)=THETA1(k)/THETA2(k);

while (0.99<r(k) && r(k)<=1.00)

...

    THETA2(k)=sqrt(2*(T(k)/E*A + DELTA(k)/L) + (T(k)/E*A + DELTA(k)/L)^2)

    r(k+1)=THETA1(k)/THETA2(k)

    if k==1
        THETA1(k+1)=(THETA1(k)+THETA2(k+1))/2

    else if r(k)>1 && r(k-1)<1 || r(k)<1 && r(k-1)>1
            THETA1(k+1)=(THETA2(k-1)+THETA2(k))/2;
        else
            THETA1(k+1)=(THETA1(k)+THETA2(k))/2;
        end
    end

    k=k+1

end

[Brian B]

Are you sure it runs once? With the AND condition in the while statement, it should not run at all given the initial value of r (r(1)==0.1). Do you want to run the loop UNTIL (0.99<r(k) && r(k)<=1.00)? If so, use

 while ~(0.99<r(k) && r(k)<=1.00)

[Brian B]

My suggestion would be to set a breakpoint and step through the program to see where it begins doing things you didn't expect. Try writing down the same steps it should follow on a piece of paper and compare the values at each step. If breakpoints are new, you might find this helpful: http://www.mathworks.com/help/matlab/matlab_prog/debugging-process-and-features.html#brqxeeu-184.