MATLAB: access to an element of a multidimensional array with a list

Question

I have a d-dimensional array, A and a vector inds with a length equal to d. I would like to access the value of A in inds .

Ideally, I would do something like A (* inds) (borrowing the decompression syntax from Python). I am not sure how to do this in MATLAB.

If I do A (inds) , I actually get d separate values ​​from A , which I don't want. I want the element i inds to be the ith parameter in the call to the A () function.

Answer 1

One solution is to create a comma-separated list from your indexed index vector inds . You can do this by converting it to an array of cells using NUM2CELL , then using the {:} syntax when indexing A :

 inds = num2cell(inds); value = A(inds{:}); 

Answer 2

I think this tool can help you:

if you have an ND matrix such as R = rand (5,10,15,20) and you want to access the elements in a specific template, you can use spindex to access the output in the same form as the input access Therefore, if you have size (i1) = [5.5.5], size (i2) = [5.5.5], etc. Then the size (spindex (R, i1, i2, i3, i4)) is also equal to [5,5,5].

 %#example: z = reshape(1:(5^4),[5,5,5,5]); zid1 = [1,1,5]; zid2 = [1,2,5]; zid3 = [1,3,5]; zid4 = [1,4,5]; zOut = spindex(z,zid1,zid2,zid3,zid4) %# should be like [1,431,625] zid1 = [1,2;3,4]; zid2 = [1,1;1,1]; zid3 = [1,1;1,1]; zid4 = [1,1;1,1]; zOut = spindex(z,zid1,zid2,zid3,zid4) %%# should be like [[1,2];[3,4]] 

you will need to add the code below as spindex.m to a place in your MATLAB path.

  function outM = spindex(inM,varargin) %function outM = spindex(inM,varargin) % %returns a matrix indexed from inM via index variables contained in varargin %useful for retreiving multiple values from a large multidimensional matrix % % %inM is an Nd matrix %the index variables stored in varargin must be as numerous as the number of dimensions in inM %each index variable must be identical in size % %example: % %z = reshape(1:(5^4),[5,5,5,5]); %zid1 = [1,1,5]; %zid2 = [1,2,5]; %zid3 = [1,3,5]; %zid4 = [1,4,5]; %zOut = spindex(z,zid1,zid2,zid3,zid4) %% should be like [1,431,625] %zid1 = [1,2;3,4]; %zid2 = [1,1;1,1]; %zid3 = [1,1;1,1]; %zid4 = [1,1;1,1]; %zOut = spindex(z,zid1,zid2,zid3,zid4) %% should be like [[1,2];[3,4]] sz = size(inM); ndim = length(sz); if((ndim == 2) & (sz(2) ==1)) % ndim always returns at least 2 ndim =1; end if(nargin ~= (ndim +1)) extraDims = setdiff(1:(nargin - 1),1:ndim); for iExtraDim = extraDims if(any(varargin{iExtraDim}~=1)) error('must have as many indicies as dimensions\n'); end end end szid = size(varargin{1}); for i = 1:ndim szid2 = size(varargin{i}); if(any(szid2 ~= szid)) error('indicies must have identical shape'); end ndIdxs(:,i) = varargin{i}(:); end if(ndim == 1) idxs = ndIdxs(:,1); else idxs = myNDsub2ind(size(inM),ndIdxs); end outM = nan(1,length(idxs)); outM(find(not(isnan(idxs)))) = inM(idxs(find(not(isnan(idxs))))); outM = reshape(outM,size(varargin{1})); function ndx = myNDsub2ind(siz,subs) %function ndx = NDsub2ind(siz,subs) %------------------------------- %works more smoothly when the dimensionality of the mtrx is unknown %siz should be like [10 10 4 5] if subs is like % 9 8 3 5 % 1 1 1 1 % 10 10 4 5 % 5 8 3 3 % % siz will be rotated for you if submit a row vec instead a col vector % example: NDsub2ind([10 10 4 5],[[9,8,3,5];[1,1,1,1]]) %---------------------------------------------- if(size(siz,1) > 1) && (size(siz,2) > 1) error('the siz variable must be a vector'); end if((size(subs,1) ~= 1) && (size(subs,2) == 1)) subs = subs'; end siz = siz(:)'; if length(siz)<2 error('MATLAB:sub2ind:InvalidSize',... 'Size vector must have at least 2 elements.'); end if ((length(siz) ~= size(subs,2))) error('NDsub2ind: length(siz) must = size(subs,2)'); end nPoints = size(subs,1); %Compute linear indices k = [1 cumprod(siz(1:end-1))]; ndx = ones(nPoints,1); s = size(subs); %For size comparison for i = 1:length(siz), v = subs; fNaN = find( (v(:,i) < 1) | (v(:,i) > siz(i)) ); %Verify subscripts are within range v(fNaN,i) = nan; ndx = ndx + (v(:,i)-1)*k(i); end