MATLAB: How to access array element from vector of its entries

arrayindexing

Is there a function that takes an array A and a vector [v1 … vn] as inputs and returns A(v1,…,vn), provided A has `size' m1 x … x mn, and 1 ≤ vi ≤ mi for all i between 1 and n?

Best Answer

One way:

c = num2cell(v);
result = A(sub2ind(size(A),c{:}));

Related Solutions

MATLAB: Dimensions of arrays being concatenated are not consistent

You included definitions for every variable except for the one that likely causes the problem, I0.

Running your code gives an error that I0 is undefined.

Defining I0 as a scalar double lets your code run without error.

Defining I0 as anything with more than one column will result in your code throwing that "Dimensions of arrays being concatenated are not consistent" error. The cause of the error is quite simple, it is because you are doing this:

[I0 + scalar; scalar]

Clearly the 1st row will have whatever size I0 has, the 2nd row is scalar.

Note that while using subscript indexing like this x(2,:) is semantically correct, the colon implies that there are multiple columns, which is not the case (indeed, x only has one column). It would be clearer to use linear indexing x(2) or specify the column explicitly x(2,1).

MATLAB: Indexing an array with a vector

%setup
As = size(A);
Aoff = cumprod([1 As(1:end-1)]);
%calculation at run-time
vidx = (v-1) * Aoff.' + 1;
A(vidx)

This will work when v is an M x N array of indices, producing a column array of M values. If your array has trailing singular dimensions being indexed then the code will not work as-is (the last entry of Aoff needs to be duplicated for all trailing singular dimensions.)

The calculation here converts the index components into linear indices using some trivial matrix multiplication.

Best Answer

Related Solutions

MATLAB: Dimensions of arrays being concatenated are not consistent

MATLAB: Indexing an array with a vector

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