spInner.m

function c = spInner(varargin)
%SPINNER Inner (scalar) product of two sparse array structures.
%
%   c = spInner(varargin): Inner (scalar) product of two full arrays, each
%   represented as a sparse array structure or a full array. There are
%   alternative definitions of 'inner product' for tensors/arrays. Here, it is
%   their scalar product -- the sum of entries resulting from their entrywise
%   (Hadamard) product. The sparse arrays can entered as a comma separated list
%   or as a members of a cell.
%
%   By Andrew J. Milne, The MARCS Institute, Western Sydney University

% Check whether varargin is a comma separated list or a cell array
if ~iscell(varargin{1})
    spA = varargin;
elseif iscell(varargin{1})
    spA = varargin{:};
end
nSpA = size(spA,2); % count the number of arrays

if nSpA ~= 2
    error(['There must be exactly two arguments (arrays or sparse array ', ...
           'structures).'])
end

% Convert full array arguments to sparse array structures
for i = 1:nSpA
    if ~isstruct(spA{i})
        spA{i} = array2SpArray(spA{i});
    end
end

% Check both sparse array strucures have the same numbers of elements
if prod(spA{1}.Size) ~= prod(spA{2}.Size)
    error('Both full arrays must have the same number of entries.')
end

spC = spTimes(spA{1},spA{2});
c = sum(spC.Val);

end