Error inserting symbolic expression in for-loop

A have some expressions outside a loop and want to use them inside the loop like

p=P(i,j)  %some expressions that must be calculated outside the loop
q=Q(i,j) %some expressions that must be calculated outside the loop
for i=1:10
    for j=1:10
        W(i,j)=P+i+j
        V(i,j)=Q+i-j
    end
end

i keep recieving this error:

The following error occurred converting from sym to double:
Unable to convert expression into double array.

[Rik]

What are the classes and sizes of the variables you're using? And what are you trying to achieve as a result?

Answer 1

[Walter Roberson]

Your P or Q appear to be a symbolic function, and you appear to be invoking them with specific arguments i and j to produce p and q. But then you use the full P and Q functions inside the loop instead of p and q. With P and Q being symbolic functions, it would not be possible to convert the right hand side to double. It might be possible to convert p and q to double.

If it turns out not to be possible to convert p and q to double, then you will need to initialize W and V to symbolic instead of numeric. It could be the case that you had previously used W and V in your calculations and so even though you might be overwriting each entry of W and V, that would have left them lying in memory as numeric.

W = zeros(10, 10, 'sym');
V = zeros(10, 10, 'sym');

[Walter Roberson]

That code attempts to add Q+q(n) etc before Q is defined.

For testing purposes, it would help to know what some inputs should be fore the number of springs and the positions and M and kl^3 values.

[kasra samei]

The i initialized P and Q, the error in inside the second loop. it does not use the expression of P and Q inside second loop. it uses just letters 'P' and 'Q'.

[Walter Roberson]

Your code has

syms i j

so when you calculate sin(i*pi*L(n)/l) and sin(j*pi*L(n)/l) then those are symbolic i and j that are used in the expression. Q will therefore come out including expressions such as

11*sin((pi*i)/5)*sin((pi*j)/5)

where the i and j are unresolved symbolic numbers.

Then when you loop for i and for j, and reference Q, then the i and j inside of Q will not be replaced by the i and j loop variables. Symbolic variables are not "pointers" to variables in the MATLAB workspace. Symbolic variables are references to items that live in a different process (the MuPAD symbolic engine), and no connection is made between the engine and the MATLAB workspace variables unless you request substituation using subs()

syms x l1 l q w_2 i j
format short;
Q = sym(0);
P = sym(0);
nms=input('Enter number of mass-springs attached to the beam: ');
for n=1:nms
    fprintf('Enter the position of mass-spring #%d Ln/L: ',n)
    t(n)=input('\n');
    L(n)=l*t(n);
    fprintf('Enter the mass-spring #%d M/ml: ',n)
    p(n)=input('\n');
    fprintf('Enter the mass-spring #%d kl^3/EI: ',n)
    q(n)=input('\n');
    Q=Q+q(n)*sin(i*pi*L(n)/l)*sin(j*pi*L(n)/l);
    P=P+p(n)*sin(i*pi*L(n)/l)*sin(j*pi*L(n)/l);
end
P
Q
k(1:6,1:6)=0;
m(1:6,1:6)=0;
for i=1:6
    for j=1:6
        if i==j
            k(i,j)=subs(Q)+(0.5*(pi*i)^4);
            m(i,j)=subs(P)+0.5;
        else
        k(i,j)=subs(Q)+(pi^3*(i*j)^2/2)*((sin((i-j)*pi)/(i-j))-(sin((i+j)*pi)/(i+j)));
        m(i,j)=subs(P)+(1/(2*pi))*((sin((i-j)*pi)/(i-j))-(sin((i+j)*pi)/(i+j)));
        end
    end
end

[kasra samei]

that worked, great

THanks