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

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)

[Yulissa Gallardo]

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]

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

[Yulissa Gallardo]

how can I do that?

[Walter Roberson]

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]

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]

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]

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]

I've already done it, I just need to graph it, my friend, thank you so much, you don't know how much you helped me today!