How to take elements from a multidimensional array along the page axis?
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Hello,
I want to map a 3d array to a 2d array by taking a single value along the page axis at each coordinate.
For example, in python I would do the following:
% x = np.linspace(0, 1, 50)
% X, Y = np.meshgrid(x, x)
% Z = np.exp(-X**2/2) + np.exp(-Y**2/2)
% A = np.arange(10)[:,np.newaxis,np.newaxis] * Z
%
% I, J = np.mgrid[0:50, 0:50]
% choice_idx = np.randint(10, size=(50, 50))
% choice_arr = A[choice_idx,I,J]
However, applying this naivly to matlab gives me "Requested array exceeds the maximum possible variable size.":
x = linspace(0, 1, 50);
[X, Y] = meshgrid(x, x);
Z = exp(-X.^2/2) + exp(-Y.^2/2);
A = reshape(0:9, 1, 1, []) .* Z;
[I, J] = meshgrid(1:50, 1:50);
choice_idx = randi(10, 50, 50);
choice_arr = A(I,J,choice_idx)
How can I achieve this in matlab? Apologies, if this question was asked before.
Accepted Answer
By a single page do you mean all rows, all columns of one index in the 3rd dimension (see here)? In MALAB you would use the colon operator to specify all rows/columns, and then specify the index of the 3rd dimension
x = linspace(0, 1, 50);
[X, Y] = meshgrid(x, x);
Z = exp(-X.^2/2) + exp(-Y.^2/2);
A = reshape(0:9, 1, 1, []) .* Z;
choice_arr = A(:,:,10)
3 Comments
jer lueb
on 1 Jan 2022
Cris LaPierre
on 1 Jan 2022
Edited: Cris LaPierre
on 1 Jan 2022
There may be other ways to do this, but I think I would use linear indexing to do what you want. See here and the relevant section here. I use the sub2ind function below to convert subscripts to linear indexing.
x = linspace(0, 1, 50);
[X, Y] = meshgrid(x, x);
Z = exp(-X.^2/2) + exp(-Y.^2/2);
A = reshape(0:9, 1, 1, []) .* Z;
[I, J] = meshgrid(1:50, 1:50);
choice_idx = randi(10, 50, 50);
% obrain linear indeces of desired elements
ind = sub2ind(size(A),I,J,choice_idx);
% extract and reshape back to 50x50
choice_arr = reshape(A(ind),50,50)
jer lueb
on 1 Jan 2022
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