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Convert list of indices to array

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Hello,
I have a 27x4 matrix with symbolic entries, say X, as seen below.
[ 1, 1, 1, 0]
[ 2, 1, 1, 0]
[ 3, 1, 1, e31]
[ 1, 2, 1, 0]
[ 2, 2, 1, 0]
[ 3, 2, 1, 0]
[ 1, 3, 1, e15]
[ 2, 3, 1, 0]
[ 3, 3, 1, 0]
[ 1, 1, 2, 0]
[ 2, 1, 2, 0]
[ 3, 1, 2, 0]
[ 1, 2, 2, 0]
[ 2, 2, 2, 0]
[ 3, 2, 2, e32]
[ 1, 3, 2, 0]
[ 2, 3, 2, e24]
[ 3, 3, 2, 0]
[ 1, 1, 3, e15]
[ 2, 1, 3, 0]
[ 3, 1, 3, 0]
[ 1, 2, 3, 0]
[ 2, 2, 3, e24]
[ 3, 2, 3, 0]
[ 1, 3, 3, 0]
[ 2, 3, 3, 0]
[ 3, 3, 3, e33]
From this, I would like to create a 3D array, say x, in which the columns are the index values that map to the fourth column in X. For example, once x is created, I would be able to make the call x(2,2,3) and it would return the symbolic 'e24'; calling x(2,1,1) returns 0; and so forth. Thank you for your help!

Accepted Answer

Cris LaPierre
Cris LaPierre on 20 Sep 2021
You should be able to use the reshape command on the 4th column
% Setup your matrix
syms e31 e15 e32 e24 e33
a= [1, 1, 1, 0;
2, 1, 1, 0;
3, 1, 1, e31;
1, 2, 1, 0;
2, 2, 1, 0;
3, 2, 1, 0;
1, 3, 1, e15;
2, 3, 1, 0;
3, 3, 1, 0;
1, 1, 2, 0;
2, 1, 2, 0;
3, 1, 2, 0;
1, 2, 2, 0;
2, 2, 2, 0;
3, 2, 2, e32;
1, 3, 2, 0;
2, 3, 2, e24;
3, 3, 2, 0;
1, 1, 3, e15;
2, 1, 3, 0;
3, 1, 3, 0;
1, 2, 3, 0;
2, 2, 3, e24;
3, 2, 3, 0;
1, 3, 3, 0;
2, 3, 3, 0;
3, 3, 3, e33];
% reshape to a 3x3x3
x = reshape(a(:,4),3,3,3)
x(:,:,1) = 
x(:,:,2) = 
x(:,:,3) = 
x(2,2,3)
ans = 
x(2,1,1)
ans = 
0
  1 Comment
Christopher Kube
Christopher Kube on 20 Sep 2021
Perfect! Thank you. I think this can easily be generalized as well with N being the dimension of the matrix.
x = reshape(a(:,end),ones(1,N)*3);

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More Answers (1)

David Hill
David Hill on 20 Sep 2021
x=reshape(yourMatrix(:,4),[3],[3],[3]);

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