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I was asked to calculate the gradient vector of a function entered by the user WITHOUT USING THE GRADIENT COMMAND. This is what I have, but is wrong I also added how to grap

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Yulissa Gallardo 2022-2-17

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评论： Yulissa Gallardo 2022-2-17

在 MATLAB Online 中打开

f = input('enter function ','s');
sf = str2sym(f);
v = symvar(sf);
g = diff(sf,v);
[x,y] = meshgrid(-5:0.2:5,-5:0.2:5);
u=eval(g(1));
v=eval(g(2));
quiver3(x,y,u,v)

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Yulissa Gallardo 2022-2-17

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se me pidió calcular el vector gradiente de una función introducida por el usuario SIN USAR EN COMANDO GRADIENTE, esto es lo que tengo, tambien añadí la opción de graficar

Walter Roberson 2022-2-17

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Never eval() a symbolic expression; doing that is not defined. Use subs() instead.

Yulissa Gallardo 2022-2-17

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how can I do that?

Walter Roberson 2022-2-17

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在 MATLAB Online 中打开

f = input('enter function ','s');
sf = str2sym(f);
v = symvar(sf);
g = diff(sf,v);
[x,y] = meshgrid(-5:0.2:5,-5:0.2:5);
u = double(subs(g(1)));
v = double(subs(g(2)));
quiver3(x,y,u,v)

Walter Roberson 2022-2-17

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https://ww2.mathworks.cn/matlabcentral/answers/1652215-i-was-asked-to-calculate-the-gradient-vector-of-a-function-entered-by-the-user-without-using-the-gra#comment_1991800

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Why are you expecting two results for g? That implies either that you expect that the user will enter a vector of functions or that you expect the user to enter a function of two variables and you think that diff(sf, symvar(sf)) will differentiate with respect to the first variable and then the second variable giving a vector of values.

But... when you diff() with a vector of variables, that means to differentiate with respect to each of the variables in turn. For example, diff(f, [x, x]) would be double differentiation of f, like diff(diff(f,x),x) . diff(f,[x y]) would be diff(diff(f,x),y) which gives one result, not two.

Yulissa Gallardo 2022-2-17

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thank you, now I did this

f = input('Digite la función ','s');

sf = str2sym(f);

v = symvar(sf);

syms v;

g = diff(sf,v);

subs(g,sf,v)

disp(g)

but the results are 0 and 0, do you know how can I fix it?

Walter Roberson 2022-2-17

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在 MATLAB Online 中打开

What is the idea behind doing the

syms v

Consider instead doing diff(sf, v(1)) and diff(sf, v(2)) as two calls.

Yulissa Gallardo 2022-2-17

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