Access Elements of a Matrix, via an array of varying size

Hello, I would like to access elements of an N-D matrix, by inputting an Nx1 vector as it's element.

Currently, I have an array that contains the indices that I would like, for example:

idx = [1,2,3,4,5];
a = data.set(1,2,3,4,5);

Howevever, the size of idx is variable based on the dimensions of the matrix, and I can't seem to figure out how to do this without hard-coding it up. I've looked into the sub2ind function, but don't believe that it will serve the purpose I need.

Can anybody help?

[Adam Danz]

Terminology: a matrix is always 2D. If a variable is N-D where N>2, it's called a multidimensional array or, if you'd like, an N-D array or more simply, an array.

What is the intention of this line? data.set(1,2,3,4,5)

[David Hill]

You need to explain more using better examples.

[Ayden Clay]

Sorry about that, I’m using the mathematical terminology rather than the programming terms.

Data is an N dimensional array, and I’m doing some maths to work out where to look and this line will access the specific element of data and assign it to a variable.

The idea is that I have an array of length N, and I would like to access the element of data governed by that:

For example: Idx=[1,2,3,4,...]; Data.set(1,2,3,4,...); And this must work for varying length idx and can dynamically update.

[Adam Danz]

Is Idx a vector of linear indices? If so,

Data.set(Idx)

[Ayden Clay]

No it is a vector of numbers.

Is it a simple process to change them from numbers to linear indices? Is that what the sub2ind function does?

Pardon my idiocy!

[Ameer Hamza]

I guess OP is using idx to represent subscripts. You can use sub2ind, but It will be similar to my solution, it will just add one extra step.

[Adam Danz]

Ahhhh I see. To be fair, I sometimes use idx a variable name for subscripts. In that case, sub2ind is the way to go exactly as Ameer has shown.

[Ayden Clay]

Perfect, thank you very much!

[Ameer Hamza]

Adam, agree. I, too, use idx too commonly for subscripts. I realized this while writing the answer that it can create confusion sometimes.

Answer 1

[Ameer Hamza]

I believe you should be able to use sub2ind for this purpose, but here is a simpler solution with cell array

A = rand(4,5,10,20); % a 4x5x10x20 matrix
Subs = {1, 3, 5, 10};
required_element = A(Subs{:});

For you example

Subs = [1,2,3,4,5];
Subs = num2cell(idx);
a = data.set(Subs{:});

[Ayden Clay]

I will give this a go next time I’m at a machine, thank you!

[Ameer Hamza]

Glad to be of help.

[Ayden Clay]

Worked flawlessly, thank you so much for your help!