Is it possible to use 'solve' on a vector of functions?

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Harshvardhan 2012-11-6

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https://ww2.mathworks.cn/matlabcentral/answers/52996-is-it-possible-to-use-solve-on-a-vector-of-functions

Is it possible to use 'solve' function on a column/row vector of functions where each row/column is a function of just one variable?

 syms t
 funcVec = [3*t+2; 5t-3]
 solve(funcVec)

For example, running the above lines of code should give me a vector

[-2/3; 3/5].

回答（1 个）

Walter Roberson 2012-11-7

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https://ww2.mathworks.cn/matlabcentral/answers/52996-is-it-possible-to-use-solve-on-a-vector-of-functions#answer_64554

在 MATLAB Online 中打开

No.

You can use

arrayfun(@solve, funcVec)

Or you can use the symbolic map() function. Note that map() is not exposed at the MATLAB level, so you would need evalin() or feval()

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Harshvardhan 2012-11-8

在 MATLAB Online 中打开

Can you please elaborate on how I can use the above statement in the following code to avoid the for loop?

 BOX = randi(1000, [240 9]);
 boxCOM = zeros(size(BOX,1), 3);
 P1 = BOX(:,1:3)/1000;
 P2 = BOX(:,4:6)/1000;
 P3 = BOX(:,7:9)/1000;
 normal = cross(P1 - P2, P1 - P3);
 d = dot(-P1, normal,2);
 x = 0.1;
 d2 = d - x*sqrt(sum(normal.^2, 2));
 syms x y z t;
 P = [x.*ones(size(BOX,1), 1) y.*ones(size(BOX, 1), 1)... z.*ones(size(BOX,1), 1)];
 planefunction2 = ((normal(:,1).*x)+(normal(:,2).*y)+(normal(:,3).*z)-(d2));
 line = P1 + t*normal;
 for k = 1 : size(BOX,1)
    newfunction = subs(planefunction2(k), P(k,:), line(k,:));
    t0 = solve(newfunction);
    boxCOM(k,:) = double(subs(line(k,:),t,t0));
 end

I understand that instead of placing 'solve' inside the loop I can use

arrayfun(@solve, newfunction, 'Uniform', 0)

But, I am not sure how I can use the statement

arrayfun( @(A, B, C) solve(subs(A, B, C)), 'Uniform', 0)

to eliminate the need for 'subs' in the first and third lines of the 'for' loop?

Harshvardhan 2012-11-8

Walter. That's just awesome.

But, you are right. arrayfun() is just another way of writing a 'for' loop. As a matter of fact, arrayfun() saved me only a few milliseconds and nothing more. I think it's better I stop pursuing vector functionality for symbolic math functions like 'subs' and 'solve' as I understand that MATLAB calls altogether a different engine, to solve such problems, which I think doesn't possess the speed.

Walter Roberson 2012-11-8

Yup. You would probably get better speed by evalin(symengine, ...) or feval(symengine, ...) some code that did the work for you all at the MuPAD level, such as by using MuPAD's map() . At the very least that would remove the overhead of talking to the symbolic engine for each operation.

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