Accessing the nth dimension in a variable sized multidimensional array
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I have a large array of values, and before the execution I don't know how many dimension I need.
How can I extract a single column from this? Or even a specific value?
I tried things like writing a vector of the indices, but that didn't return a single value.
Example: I have a matrix A, how do I get A(1,1,...,1,:) when I don't know how many dimensions A has?
And what if I want to set a single dimension to a specific value like A(1,:,:,...,:)
Alternatively: is there another good way of storing high-dimensional scalar fields? I tried a list of index vectors like in a sparse matrix, but finding a specific value becomes very inefficient that way right?
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Accepted Answer
Stephen23
on 13 Jul 2020
Edited: Stephen23
on 13 Jul 2020
"...how do I get A(1,1,...,1,:) when I don't know how many dimensions A has?"
You use a comma-separated list, which you can easily generate from a cell array:
C = {1,1,1,...1,,':'};
A(C{:})
You can create the cell array using repmat or num2cell or similar. The function ndims will also be useful here.
"...what if I want to set a single dimension to a specific value like A(1,:,:,...,:)"
Use a comma-separated list. Lets try a more practical example, where we define the size of C automatically:
C = repmat({':'},1,ndims(A));
C{1} = 1; % C{dim} = index
val = A(C{:})
4 Comments
Walter Roberson
on 14 Jul 2020
subsref twice? That does not sound right. end calls the end method; https://www.mathworks.com/help/matlab/matlab_oop/object-end-indexing.html
More Answers (1)
Walter Roberson
on 13 Jul 2020
nd = ndims(YourArray);
dim_wanted = randi(nd);
proto = repmat({1}, 1, nd);
proto{dim_wanted} = ':';
just_that_dim = YourArray(proto{:});
dim_one = randi(nd);
proto = repmat({':'}, 1, nd);
proto{dim_one} = 1;
all_except_that_dim = YourArray(proto{:});
proto = arrayfun(@randi, size(YourArray), 'uniform', 0);
dim_wanted = randi(nd);
proto{dim_wanted} = ':';
random_vector_in_N_space = YourArray(proto{:});
1 Comment
Walter Roberson
on 15 Jul 2020
My Answer showed how to index by any arbitrary dimension: repmat() the most common index, and replace the specific choices, then use cell expansion as the index expression. You could make a helper function, such as
IndexDim = @(A,n,index) A(struct('t',[repmat({':'},1,n-1),index,repmat({':'},1,ndims(A)-2+(n==1))]).t)
Note: doing this on one line requires a fairly new version of MATLAB; I do not recall at the moment if it needs R2020a or R2019b, but R2019a would be expected to fail for this.
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