Using Meshgrid for 3 variables

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Divas 2015-4-21

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https://ww2.mathworks.cn/matlabcentral/answers/212532-using-meshgrid-for-3-variables

评论： Michael Haderlein 2015-4-21

I am new to meshing and MATLAB, I have two variables 'a' and 'L' both varying from -sigma to +sigma and a third variable d which results from a matrix evaluation of [a;L] over a function. When I try if true [X,Y] = meshgrid(a,L); mesh(X,Y,d); end I get error saying d must be matrix. where as d is a scalar of same length as 'a' and 'L'. what is wrong here? Thanks in advance

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Answer 1

David Sanchez 2015-4-21

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Make sure the size of d is right:

If length(a) = n and length(L) = m, then, you have to have size(d) = [m x n]

for example:

a = rand(10,1);
L = rand(5,1);
[aa,LL] = meshgrid(a,L);
Z = rand(10,5);
% then, these will work
>> mesh(L,a,Z)   % this will not yield an error
>> mesh(LL,aa,Z')
>> mesh(aa,LL,Z')

while

>> mesh(a,L,Z)
Error using mesh (line 76)
Data dimensions must agree.

from documentation: mesh(X,Y,Z) draws a wireframe mesh with color determined by Z, so color is proportional to surface height. If X and Y are vectors, length(X) = n and length(Y) = m, where [m,n] = size(Z).

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Answer 2

Michael Haderlein 2015-4-21

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For simplicity, let's set a and L to [1 2 3] each, ok? And your function to get d is just summation.

As your code snippet does not include the function, I guess you have done it before. So you have the values

 a=[1 2 3]; %create a, L
 L=[1 2 3];
 d=a+L; %create d, value is [2 4 6];

Obviously, you don't have one d for each combination of a and L. What you acutally need is

 a=[1 2 3];
 L=[1 2 3];
 X=[1 2 3;1 2 3;1 2 3];
 Y=[1 1 1;2 2 2;3 3 3];
 D=X+Y; %create D, value is [2 3 4;3 4 5;4 5 6];

This will then also work with mesh.

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Divas 2015-4-21

I am not looking for 3d plot, rather I need the x and y axis to have a and L with d plotted against them, I tried using plotyy but not quite the result I expected. The only other option I got was meshing. Probably I'll try all combinations for grids.

Michael Haderlein 2015-4-21

So do you know if you want d for all a,L combinations? I mean, the answer is just "yes" or "no". If "yes" you'll need mesh, if "no" you'll need plot3. Your code suggests "no", but it's not clear. In case N is small, you might also just want plot() and use "a" as x value and plot multiple lines for different L. plotyy does not sound like it should help in this context. Given the information I have, I cannot state more.

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