Use a vector as an index

I have a matrix of variable dimension and would like to access it with an index stored in a variable, something like this:

matrix = [1, 2; 3, 4; 5, 6]
idx = [3, 1]
elem = matrix(idx) % should result in access to element (3, 1) => 5

Of course, this does not work and will instead get element numbers 3 and 1 (which have values 3 and 1 in that case).

I know sub2ind, but that does not help either, since the number of dimensions is variable. Is there a built-in solution for my problem or will I have to write an access method by myself? (It's not too difficult, but a built-in solution would be preferable.)

[Mikhail]

Yeah, I don't know simple solution too.

Answer 1

[Guillaume]

You can use cell expansion to comma separated list to index your matrix. So first convert your index matrix into a cell array, then convert the cell array into a list:

c = num2cell(idx);
matrix(c{:}); %expand c into comma separated list

[Michael B.]

I think this is what I'm looking for. However, it would be nice if Matlab would provide a more smoothly integrated solution with no need for an extra line of code.

[Mikhail]

matrix(num2cell(idx){:}) - 1 line

[Mikhail]

doesn't work this way, sorry

[Guillaume]

I agree. Unfortunately, the way matlab is designed prevents any simpler method.

Saying that, you can always write your own helper function based on this:

function v = matindex(matrix, vecidx)
  %MATINDEX use vector vecidx as an nD index intro matrix to return value v
  cidx = num2cell(vecidx);
  v = matrix(cidx{:});
end

[Michael B.]

And that's exactly why I wonder even more about the lack of a built-in way to do it - it would be so simple.