I can't make A<B, it gives me 'lt' error

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María 2013-4-22

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采纳的回答： Walter Roberson

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Hello! I am trying to make a loop with for inside a for, inside another for, but Matlab gives me this error:

Undefined function or method 'lt' for input arguments of type 'sym'

I've heard it's something about 'less than'.

I tried the program yesterday and it 'worked' (it run the program but my computer is very slow so I cancelled it before it ended, but it worked), but now I am doing the same and it gives me the error.

My loop is that:

 for q1=(-185:10:185)*pi/180
    for q2=(-135:10:35)*pi/180
        for q3=(-120:10:158)*pi/180
        J=jacobiana(q1,q2,q3,0,0,0);
        D=det(J);
            if abs(D)<0.05
            P=tcd([q1,q2,q3,0,0,0]);
            G=P*[0 0 0 1]';
            x(i)=G(1);
            y(i)=G(2);
            z(i)=G(3);
            i=i+1;
            end   
        end
    end
end

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Walter Roberson 2013-4-22

The "<" operator is implemented by a function named "lt".

http://www.mathworks.com/help/matlab/ref/lt.html

Azzi Abdelmalek 2013-4-22

Ok I see

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Walter Roberson 2013-4-22

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What is "jacobiana" ? There is no function of that name in MATLAB or any of the toolboxes.

There is a function "jacobian", which is part of the Symbolic toolbox, and produces a symbolic output, but it uses a different calling sequence and needs to be provided with a variable name; see http://www.mathworks.com/help/symbolic/jacobian.html

You seem to be working with numeric values (unless somehow your 'pi' has become symbolic); the numeric routine most similar to "jacobian" is "gradient"

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Walter Roberson 2013-4-22

编辑：Walter Roberson 2013-4-22

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Why does that code not make use of the passed parameters, q1, q2, q3 ?

Possibly what you want is

J = double( subs( [fx1 fx2 fx3;fy1 fy2 fy3;fz1 fz2 fz3], {q_1, q_2, q_3, q_4, q_5, q_6}, {q1, q2, q3, q4, q5, q6}) );

Note that if that is the case, then for efficiency I would suggest calculating the symbolic matrix only once and just doing the double() of subs() the other times.

persistent J_precalc
if isempty(J_precalc)
  .... your current code here, except assign to J_precalc instead of to J ...
end
J = double( subs(J_precalc, {q_1, q_2, q_3, q_4, q_5, q_6}, {q1, q2, q3, q4, q5, q6}) );