MATLAB: How to extract all matrix elements for one index with dynamic number of dimenstions

indexingmatrix

Hello experts,

let A be

A = rand(4, 4, 4); % 
so what I want is:
B = reshape(A(1,:,:), [4 4])

The problem is the unknown dimension of A. So if A has the dimension of 3, I need 2 colon operator; if 4 then 3 and so on…

I am very thankful for any ideas…

Best Answer

ii = repmat({':'},1,ndims(A)-1);
out = squeeze(A(1,ii{:}));

Related Solutions

MATLAB: Index slice of ND array of unknown dimension

If you do not know the size of the array before hand then you can call subsasgn directly. The function subsasgn is the actual function that MATLAB calls whenever you assign to an array using the indexing convenience operators () or {}, e.g. X(1) = 2, in just the same way that 1+2 is just a convenience operator for plus(1,2). We can also call subasgn directly, just like we could call plus if we so wished.

Here is a simple example of removing one row of A without hard-coding how many dimensions it has:

>> A = rand(5,4,3,2);
>> S.subs = repmat({':'},1,ndims(A));
>> S.subs{1} = 3; % the third row
>> S.type = '()';
>> B = subsasgn(A,S,[]);
>> size(B)
ans =
     4     4     3     2

MATLAB: Multidimensional colon operator

Two approaches: 1. Reshape the matrix at first:

function A = insert_x_into_A(A,x,t)
siz = size(A);
A = reshape(A, [], siz(end));
A(:, t) = x(:);
A = reshape(A, siz);

2. Use a cell as index vector: EDITED: Cleaned up with Bruno's suggestion.

function A = insert_x_into_A(A,x,t)
idx(1:ndims(A) - 1) = {':'};
A(idx{:},t) = x;

Best Answer

Related Solutions

MATLAB: Index slice of ND array of unknown dimension

MATLAB: Multidimensional colon operator

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