Using vector to access multidimensional array or passing vector as multiple inputs

Hello,

I would like to access multidimensional array using vector like:

n=2
arraySizes=[n*ones(1,2),3*ones(1,3)]
A=ones(arraySizes)
ind=[3,2,1,1,2]
A(ind) % I would like it, if output was a scalar. 

or to convert that vector of indices to linear indice:

indN= sub2ind(arraySizes,ind)
A(indN)

I know that it is possible to use cell and {:}, but num2cell and mat2cell aren't supported in MEX creation, so it's also not a solution I look for (and for loop copying num array to cell is very slow).

indCell=num2cell(ind)
indN= sub2ind(arraySizes,indCell{:})
A(indN)

I don't want just use hard indexing, because n isn't constant, and switch case with hundreds hard coded options doesn't seem optimal (but maybe it would be the fastest method then I will).

A(ind(1),ind(2),ind(3),ind(4),ind(5))

My current solution is showed below, but I`m looking for faster approch.

indNum=(ind-ones(1,length(arraySizes)))*((cumprod(arraySizes))./arraySizes).'+1
A(indNum)

Thank you for your time,

Matt

[Rik]

How is ind created? It might be easier to make sure it is a cell from the start. Then you can use A(ind{:}).

[Matt Tejer]

It's an output from another function, and it is used in different places, where I do math operations, which don't work for cells ("Operator '-'/'+'/'*' is not supported for operands of type 'cell'.").

But I can duplicate output, one as vector, one as cell. Thanks for sugestion.

Answer 1

[Chetan]

Hello @Matt Tejer,

I understand you're looking to access elements in a multidimensional array for MEX file creation without using `num2cell` or `mat2cell`. Your current method of converting subscript indices to a linear index is correct and efficient for this purpose.

Here's a concise way to calculate the linear index:

n = 2;
arraySizes = [n*ones(1,2), 3*ones(1,3)];
A = ones(arraySizes);
ind = [3, 2, 1, 1, 2];

% Calculate cumulative product of array sizes
cumprodSizes = [1, cumprod(arraySizes(1:end-1))];

% Compute linear index and access the element
linearIndex = sum((ind - 1) .* cumprodSizes) + 1;
elementValue = A(linearIndex);

This code calculates the linear index by considering the array's dimensions and then accesses the element directly. It's a MEX-friendly approach and scales with different array dimensions.

Make sure the subscript indices in `ind` are valid for the array's size before indexing.

For more information about array indexing and linear indexing in MATLAB, you might find these resources helpful:

• Array Indexing: https://www.mathworks.com/help/matlab/math/array-indexing.html
• Linear Indexing: https://www.mathworks.com/help/matlab/math/matrix-indexing.html

Hope it helps

Best regards,

Chetan Verma