How to index first element of an arbitary sized matrix?

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Joseph Hansen-Shearer
评论： Stephen23 2023-11-8
I have a matrix and I need to index the first dimension but I don't know the size of the matrix. Is there a way to do this in a generic way rather than hard code each case. I am aware I could do this with eval but I try and steer clear of using this function where possible.
Example:
ind_left = 25;
ind_right = 66;
% For a 2-D matrix it would be this
A = rand(100,100); % sample 3-D input matrix
B = A(ind_left:ind_right,:);
% For a 3-D matrix it would
A = rand(100,100,100); % sample 3-D input matrix
B = A(ind_left:ind_right,:,:);
% For a 4-D matrix it would be ...
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Joseph Hansen-Shearer
Joseph Hansen-Shearer 2023-11-8
This is an even neater solution. Although shouldn't it be
C = repmat({':'},ndims(A),1);

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Dyuman Joshi
Dyuman Joshi 2023-11-8
编辑：Dyuman Joshi 2023-11-8
Ran in:
%Example
ind_left = 25;
ind_right = 45;
vec = ind_left:ind_right;
n = 5;
A = rand(50*ones(1,n)); % sample n-D input matrix
size(A)
ans = 1×5
50 50 50 50 50
%Manually
B = A(vec,:,:,:,:);
size(B)
ans = 1×5
21 50 50 50 50
%Generalised method
C = reshape(A(vec,:),[numel(vec) size(A, 2:ndims(A))]);
size(C)
%Check for equality
isequal(B,C)
ans = logical
1

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