Error calculating an integral: ""Input function must return 'double' or 'single' values. Found 'sym'."" How can i get the %% integral(F_potext,0,a) %% done? Thanks for the help :)

Question

Enrique Villa Coronado 2020-12-15

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/694280-error-calculating-an-integral-input-function-must-return-double-or-single-values-found-sym

评论： Enrique Villa Coronado 2020-12-18

a=3; b=(2/3)*a; h=0.01;
q0 = -1000;
syms x y z c 
X_m = [0 0 (x/a)^2 (x/a)^3 (x/a)^4];
Y_n = [0 0 (y/b)^2 (y/b)^3 (y/b)^4];
fn = sym([0 0 0 0 0]);
for i=1:5
    for j=1:5
        fn(i) = fn(i) + X_m(i)*Y_n(j);
    end
end
Pot_ext = [0 0 0 0 0];
for i=1:5
       F_potext = @(x) ((q0.*x)./a)*subs(fn(i),y,2*b/3);
       Pot_ext(i) = integral(F_potext,0,a)
end

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Answer 1

Walter Roberson 2020-12-18

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/694280-error-calculating-an-integral-input-function-must-return-double-or-single-values-found-sym#answer_578865

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a=3; b=(2/3)*a; h=0.01;
q0 = -1000;
syms x y z c 
X_m = [0 0 (x/a)^2 (x/a)^3 (x/a)^4];
Y_n = [0 0 (y/b)^2 (y/b)^3 (y/b)^4];
fn = sym([0 0 0 0 0]);
for i=1:5
    for j=1:5
        fn(i) = fn(i) + X_m(i)*Y_n(j);
    end
end
Pot_ext = [0 0 0 0 0];
for i=1:5
       F_potext = matlabFunction(((q0.*x)./a)*subs(fn(i),y,2*b/3), 'vars', x);
       Pot_ext(i) = integral(F_potext,0,a, 'arrayvalued', true);
end
Pot_ext
Pot_ext = 1×5
         0         0 -703.7037 -562.9630 -469.1358
●

The reason for the 'arrayvalued', true is that the first two entries in fn come out as 0, so the code does a

matlabFunction(sym(0), 'vars', x)

which generates @(x) 0.0 as the anonymous code. But when you use that code in integral() or fplot() you have a problem because those pass in arrays of x values and require that you return back an array of the same size, but @(x) 0.0 returns back a single x not an array.