MATLAB: Sum over part of array without knowing array size

subset of arraysumunknown array size

I currently have a function that takes a two dimensional (2D) array as one of its inputs. Part of the function code sums over all but the first column of this input array, i.e.:

function DoSomething( Array2D, OtherInputs)
    ...
    SumOverColumns = sum(Array2D(:, 2:end), 2);
    ...
end

I want to expand the use of the function so that the array can be 3D or more. However, I still want to be able so sum over all columns except the first one. I can't think of a general way to do this because I think that I need to specify ":" for every other dimension.

Any help would be much appreciated.

Best Answer

Or more? How many more dimensions? Really high dimensional arrays will be huge, so it makes little sense to have a 300 dimensional array. Even a 2- dimensional array actually has infinitely many trailing singleton dimensions. So a 5x6 array is actually implicitly of size

[5,6,1,1,1,1,1,1,1,1, ...]

Suppose you are willing to assume your array will have no more than 5 dimensions in the future. Then the trivial solution is:

SumOverColumns = sum(Array2D(:, 2:end, :, :, :), 2);

There are ways to do this with more potential dimensions, but why bother?

Related Solutions

MATLAB: How to sum over specific dimensions in a n-dimensional array

Join the trailing dimensions at first:

x = rand(2,3,4,5,6);
d = 3;
n = ndims(x);
s = num2cell(size(x));
y = reshape(x, s{1:n-d}, []);
result = sum(y, n-d+1)

MATLAB: Vectors of the sums for an array of arbitrary dimensions.

I have overcomplicated a relatively simple problem, which essentially boils down to summing over all dimensions of the array with the exception of one element from the same list

What release of MATLAB are you using? If you're using release R2018b or later, use the vector dimension argument functionality introduced in that release. Let's define a sample 4-dimensional array to start.

sz = [2 3 4 5];
n = numel(sz);
A = reshape(1:prod(sz), sz);

Define a vector containing all the dimensions, and preallocate a cell array to hold the results of summing in all dimensions but one for each of the dimensions of A.

dims = 1:n;
results = cell(1, n);

Now let's do the summation.

for excludedDim = 1:n
    D = dims;
    D(excludedDim) = [];
    results{excludedDim} = sum(A, D);
end

If you look at each element of results, it has the same size as A in the dimension that was excluded from summation and size 1 in all the others. This does mean that some of the elements in results have fewer dimensions than A does due to trailing singleton dimensions being dropped.

Best Answer

Related Solutions

MATLAB: How to sum over specific dimensions in a n-dimensional array

MATLAB: Vectors of the sums for an array of arbitrary dimensions.

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