Error: The size of X must match the size of U or the number of columns of U.
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geometry geometry
am 14 Mai 2017
Kommentiert: geometry geometry
am 15 Mai 2017
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);
2 Kommentare
Akzeptierte Antwort
Stephen23
am 14 Mai 2017
Bearbeitet: Stephen23
am 14 Mai 2017
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);
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/176183/image.png)
3 Kommentare
Star Strider
am 14 Mai 2017
This was a previous attempt by OP (in another Question) that apparently did not work.
OP is using another approach here, so it is best to follow that approach.
Weitere Antworten (1)
Star Strider
am 14 Mai 2017
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);
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