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

Question

geometry geometry 2017-5-14

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/340207-error-the-size-of-x-must-match-the-size-of-u-or-the-number-of-columns-of-u

评论： geometry geometry 2017-5-15

采纳的回答： Stephen23

在 MATLAB Online 中打开

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 个评论
显示无隐藏无

KSSV 2017-5-14

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

geometry geometry 2017-5-14

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

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Stephen23 2017-5-14

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/340207-error-the-size-of-x-must-match-the-size-of-u-or-the-number-of-columns-of-u#answer_266977

编辑：Stephen23 2017-5-14

在 MATLAB Online 中打开

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);

3 个评论
显示 1更早的评论隐藏 1更早的评论

Star Strider 2017-5-14

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 2017-5-15

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.

请先登录，再进行评论。

Answer 2

Star Strider 2017-5-14

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/340207-error-the-size-of-x-must-match-the-size-of-u-or-the-number-of-columns-of-u#answer_266970

在 MATLAB Online 中打开

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);