Error: The size of X must match the size of U or the number of columns of U.

What's the problem with this code? when I run it the following Error occurs:

Error using quiver

The size of X must match the size of U or the number of columns of U.

syms x y;
f=input('enter function: ','s');
f = symfun(eval(f), [x y]);
u=-diff(f,x);
v=-diff(f,y);
[X,Y]=meshgrid(-10:.5:10);
quiver(X,Y,u,v);

[KSSV]

Dimensions of X,Y and u,v should be same.

[geometry geometry]

could you please fix the code? (I'm new to matlab) thanks;

Answer 1

[Stephen23]

There is no need to make the code slower and more complicated by using syms and eval. Exactly as I wrote in my answer to your related question, simple numeric calculations will do everything you need:

% str = input('enter a function in x and y: ','s');
str = 'log(x^2+y^2)';
fun = str2func(['@(x,y)',str]);
[X,Y] = meshgrid(-10:.51:10);
[DX,DY] = gradient(arrayfun(fun,X,Y));
quiver(X,Y,DX,DY,5);

[Andrei Bobrov]

+1

str = 'log(x^2+y^2)';
fun = str2func(['@(x,y)',vectorize(str)]);
[X,Y] = meshgrid(-10:.51:10);
[DX,DY] = gradient(fun(X,Y);
quiver(X,Y,DX,DY,5);

[Star Strider]

This was a previous attempt by OP (in another Question) that apparently did not work.

See How to use gradient (link) for context.

OP is using another approach here, so it is best to follow that approach.

[geometry geometry]

Thanks; sorry to make a similar post.

after you changed the name of that post I thought someone deleted that post! hence I made this post.

Answer 2

[Star Strider]

Remember that ‘u’ and ‘v’ are also functions of (x,y), so use them as ‘u(x,y)’ and ‘v(x,y)’. With one change (changing the step in the meshgrid vector to 0.51 to prevent ‘division by zero’ errors), this works:

syms x y;
% f=input('enter function: ','s');
f = 'log(x^2+y^2)';
f = symfun(eval(f), [x y]);
u=-diff(f,x);
v=-diff(f,y);
[X,Y]=meshgrid(-10:.51:10);
quiver(X,Y,u(X,Y),v(X,Y),5);