How can I use a symbolic variable in a for-loop?

Hello, I try to simulate the wave acceleration of irregular waves, which is defined as: a = ut/dt So I created a symbolic variable for the derivation (diff), my problem is to involve the symbolic variable in the for loop. I get the following error:

Error using mupadmex
Error in MuPAD command: Dimensions do not match. [(Dom::Matrix(Dom::ExpressionField()))::_mult2]

Error in sym/privBinaryOp (line 1694)
            Csym = mupadmex(op,args{1}.s, args{2}.s, varargin{:});

Error in sym/mtimes (line 184)
        X = privBinaryOp(A, B, 'symobj::mtimes');

%% calculate irregular waves

syms time;

t = 0:dt:180;    %modeling with dt = .1 s                         

u3 = zeros(50,length(t));

phase = 2*pi*rand(50);

for j = 1:50

         u3(j,:)  = u2(j)*sin(omega(j).*time + phase(j));

end

u4=sum(u3);

a4 = diff(u4,time);

-----------------------------------

u2 and omega are 1x50 matrices

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Thanks in advance!

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/13205-tutorial-how-to-format-your-question-with-markup

Answer 1

[Walter Roberson]

You have

    u3(j,:) = u2(j)*sin(omega(j).*time + phase(j))

where u3 has been assigned zeros(), so it can only store numeric values and not symbolic values.

Your omega(j) is scalar, your time is symbolic scalar, your phase is scalar, your u2(j) is scalar, and you assign the whole result to a row vector. Are you sure that is what you want to do?

You sum() the u3 array. sum() without a dimension number sums along the first dimension, so it sums by columns giving a row result. But your columns are all the same, so all the row entries will be the same.

[Germer DA]

Hello Walter, thanks for your quick reply!

First , I was wrong the error I get is:

| _The following error occurred converting from sym to double: Error using mupadmex Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use the VPA function instead._ |

Second : Previously, I defined the time = 0:.1:90 and that worked.

What I try to do here is to define u as a function of time ( u=f(time) ), so I can get my acceleration.

But I am not sure, how to do that!

[Walter Roberson]

u3 = sym(zeros(50,1));