how to extract elements along specified dimension of array

when e.g. selecting all elements in the 2nd dimension and the first element of all remaining dimensions of an array, for a 3-dimensional array one would write: A(1,:,1)

how to program this elegantly when the dimension over which I want the elements (2 in the example) is a variable itself, whose value is unknown until runtime ?

thanks!!

Answer 1

[Jonathan Sullivan]

A = rand(100,100,10,10);
dim = 4;
sz = size(A);
inds = repmat({1},1,ndims(A));
inds{dim} = 1:sz(dim);
A(inds{:})

[Peter]

thanks a lot. exactly what i needed.

just for my understanding: inds is a cell array. what exactly does inds{:} give? looks like seperate doubles.

thanks again!

Answer 2

[per isakson]

A(ixi,ixj,ixk)

where ixi,ixj,and ixk are numerical vectors (/scalar) of integers. ":" would correspond to (1:number_of_element_in_dimension). Or with logical indexing

A(isi,isj,isk)

where isi,isj,isk are logical vectors all with the length "number_of_element_in_dimension". ":" would be true(1,number_of_element_in_dimension).

The elegance will be in the calculation of ixi,ixj,and ixk or isi,isj,isk.

Answer 3

[Walter Roberson]

Something like this,

DIM = 2;   %changed at runtime

idxexpr = { repmat({1}, 1, DIM - 1), {':'}, repmat({1}, 1, ndim(A)-DIM) };

A(idxexpr{:})

[Peter]

hi Walter, thanks! but idxexpr becomes a cell array of cells, resulting in the error:

"Function 'subsindex' is not defined for values of class 'cell'"

ran across this myself which made me decide to construct a characterstring similar to your idxexpr and than use 'eval'.

thought however that something more elegant than eval should be possible here.

[Walter Roberson]

I have done it successfully before, my post is either on the newsgroup or somewhere in Answers. _Finding_ the post would take longer than reinventing it!

Try

idxexpr = [num2mat(ones(1,DIM-1)), {':'}, num2mat(ones(1,ndim(A)-DIM))];

[Peter]

num2mat doesn't seem to exist. can you maybe comment on what class idxexpr should be to make it work as a subscript expression? that would give some direction to my own attempts. it seems like a cell array doesn't work. ('ndim' should be 'ndims' btw.)

thanks again!