MATLAB: How to get all the outputs of the ind2sub function

matrix manipulationvariable-dimensions

Say I have a variable dimension matrix A (it could have 2,3 ..n dimensions) so I could access it by A(x1,x2,…,xn). When I use a linear index 'lin_ind' how can get the matrix coordinates for that point ?

I could use

ind2sub(size(A),lin_ind)

but the problem is how to get 'all' the outputs for a variable dimension?

Best Answer

You can preallocate a cell array to the same size as the siz input, and use this to collect all of the outputs. This will expand to whatever size siz has:

>> idx = [3 4;5 6];
>> siz = [2,2,2];
>> out = cell(size(siz));
>> [out{:}] = ind2sub(siz,idx)
out = 
    [2x2 double]    [2x2 double]    [2x2 double]

Related Solutions

MATLAB: N-D matrix accumarray

Yes, accumarray will do it easily. You just need to generate the correct subscripts for it, which is the cartesian product of all the indices in all the dimension, replacing the appropriate dimensions with your grouping variables:

A = randi(100, [3 4 2 7]);  %demo data
%generate indices in all the dimension, and replace appropriate one with grouping variables:
indices = arrayfun(@(s) 1:s, size(A), 'UniformOutput', false); %vector of indices in each dimension
indices{2} = [2 1 2 3];  %grouping in 2nd dimension
indices{4} = [3 3 2 1 3 2 4];  %grouping in 4th dimension
%calculate cartesian product of indices
[indices{:}] = ndgrid(indices{:});
indices = cell2mat(cellfun(@(v) v(:), indices, 'UniformOutput', false));
%apply accumarray with default sum function
B = accumarray(indices, A(:));

Edit: note that the above will work with arrays of any dimension (within reason, ndgrid will run out of memory for too many dimensions).

MATLAB: How to modify the assignment of an array in a specific dimension

Indexing out of variable using () and {} are just shorthand for the function subsref, in exactly the same way that * is shorthand for mtimes. You can also call subsref directly, which offers one solution to your question, although not quite in one line:

>> f = ones(10,30,5);
>> dim = 2;

Now lets get the data you want:

>> C = repmat({':'},1,max(dim,ndims(f)));
>> C{dim} = 3:2:size(f,dim)-2;
>> out = subsref(f,substruct('()',C));

and checking:

>> size(out)
ans =
   10   13    5

This is likely more efficient than calling arrayfun, and has the advantage that it does not generate lists of all indices for all dimensions. You may possibly find some efficient solution with sub2ind. You could even put the code into a small function:

function out = mysub(f,dim)
C = repmat({':'},1,max(dim,ndims(f)));
C{dim} = 3:2:size(f,dim)-2;
out = subsref(f,substruct('()',C));
end

Best Answer

Related Solutions

MATLAB: N-D matrix accumarray

MATLAB: How to modify the assignment of an array in a specific dimension

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