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error with interpn function "Grid arrays must have NDGRID structure"

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RR 2022-2-2

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评论： Walter Roberson 2022-2-2

a.mat
b.mat
C.mat

在 MATLAB Online 中打开

Dear all,

I have two matrix a=10x2 and b= 10x2. they are the indipendent variables of an unknown function C(a,b)= 10x3. I would like to evaluate this function in Aq (100x2) and Bq (100x2) variable.

a % 10x2 matrix
b % 10x2 matrix
C % 10x3 matrix
% difine Aq 100x2 and Bq 100x2 
a1=[min(a(:,1)):1:max(a(:,1))]';
a2=[min(a(:,2)):1:max(a(:,2))]';
Aq(:,1)=a1(1:100);
Aq(:,2)=a2(1:100);
b1=[min(b(:,1)):1:max(b(:,1))]';
b2=[min(b(:,2)):1:max(b(:,2))]';
Bq(:,1)=b1(1:100);
Bq(:,2)=b2(1:100);
%interp function C at Aq and Bq;
Cq = interpn(a,b,C,Aq,Bq)

I have this error

"Error using griddedInterpolant

Grid arrays must have NDGRID structure.

Error in interpn (line 151)

F = griddedInterpolant(X{:}, V, method,extrap);"

Why? what is wrong in my code?

thank you very much in advance

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Stephen23 2022-2-2

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"I have two matrix a=10x2 and b= 10x2..."

You have not told us the most important information, which is how the data are arranged within those matrices.

Please upload the matrices a, b, and C in one .mat file by clicking the paperclip button.

KSSV 2022-2-2

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To use interpn a,b,c should be 3D matrices...also note that a, b,c should be of same dimensions. In your case it is 10x3. Also note that, in this case interp2 will work. You need not to use interpn.

Walter Roberson 2022-2-2

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I suggest you consider

Aq = [linspace(min(a(:,1)), max(a(:,1)), 100).',  
      linspace(min(a(:,2)), max(a(:,2)), 100).'];

and similar for Bq .

This solves the problem that the data might not have a span of more than 100, and allows the entire range of values to be queried.

This will not solve the problem you posted about.

Stephen23 2022-2-2

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编辑：Stephen23 2022-2-2

"To use interpn a,b,c should be 3D matrices"

Why?

The NDGRID documentation states on its very first line: "Interpolation for 1-D, 2-D, 3-D, and N-D gridded data in ndgrid format". Do you have any specific reason why you think the MATLAB documentation is incorrect?

RR 2022-2-2

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a.mat
b.mat
C.mat

For further information I have attached the a, b, C matrices.

Stephen23 2022-2-2

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Ran in:

a.mat
b.mat
C.mat

Your data are nothing like an NDGRID or a MESHGRID format:

S = load('a.mat');
a = S.a
a = 10×2
3170  802.7369
4783  816.1367
7446  833.2044
7703  820.0732
1027  805.9781
2049  788.8682
3629  772.0326
0912  785.0601
3600  741.6462
2411  861.2977
S = load('b.mat');
b = S.b
b = 10×2
4879  792.3392
3365  777.2827
8543  791.9391
7148  807.6232
5029  823.4482
8819  807.6869
6655  792.4200
3388  776.6940
4518  732.7923
6856  851.9971
S = load('C.mat');
C = S.C
C = 10×3
   0     0
  10     0
  10     0
   0     0
 -10     0
 -10     0
   -10   -10     0
   -10     0     0
   0    10
   0   -10

How is the data structured within those arrays?

Tip for the future: saving all of the variables into one .mat file as requested is simpler for everyone.

RR 2022-2-2

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I am sorry for the inconvenience for the splitted variables.

Each row of the matrices corresponds to an observation: for example a(1,:) and b(1,:) generates C(1,:).

Stephen23 2022-2-2

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编辑：Stephen23 2022-2-2

在 MATLAB Online 中打开

Ran in:

a.mat
b.mat
C.mat

@RR: Do a and b represent scattered sample points in a four-dimensional space? This is how they look plotted:

S = load('a.mat');

a = S.a;

S = load('b.mat');

b = S.b;

S = load('C.mat');

C = S.C;

plot3(a(:,1),a(:,2))

plot(b(:,1),b(:,2))

If the data are 10 scattered points in some space, how do you want to subdivide that space into 100 points? Do you wish to subdivide the implied line connecting those points (i.e. like a parametric function) ?

Or do you wish to divide up the space in some other way (e.g. gridded, triangulated, etc.) ?

Walter Roberson 2022-2-2

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You cannot use interpn() for data with that structure. You need to use one of the scattered interpolation methods, on a row-by-row basis. scatteredInterpolant() might be most appropriate.

RR 2022-2-2

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Dear @Stephen, thank you for your questions. I wish to divide the space as a grid for a and for b variables so tat the function C can be evaluated in correspondance of some points in a (2D dimensional) and b (2D dimensional). HOw can i do this?

Walter Roberson 2022-2-2

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If you want a to be 2 dimensional and b to be 2 dimensional, and C to work with that, then you are effectively asking that C is a function over 4 dimensions. You would have to use something like https://www.mathworks.com/help/matlab/ref/griddatan.html

It looks to me as if you do not have enough data to produce a robust 4 dimensional fit.

RR 2022-2-2

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Unfortunately griddatan outputs vq with is a vector of length p (p= number observation) while I would like to obtain instead a matrix of size px3.

Walter Roberson 2022-2-2

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