multidimensional colon operator?

2011 2 4
2 个回答
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3 个投票

I want to do something like that below: copy an N-dimensional array into a N+1-dimensional array at index t in the last dimension. Can this be done efficiently and cleaner than umpteen if-tests or nasty permutes?
function A = insert_x_into_A(A,x,t)
if ndims(A) == 2
A(:,t) = x;
elseif ndims(A) == 3
A(:,:,t) = x;
elseif ndims(A) == 4
A(:,:,:,t) = x;
...
end
I think that something like the pseudo-code below would be neat
function A = insert_x_into_A(A,x,t)
A(colon(ndims(x)),t) = x;

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 采纳的回答

Jan
Jan 2011-2-4

5 个投票

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;

4 个评论
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Kenneth Eaton
Kenneth Eaton 2011-2-4
I vote for the second one. It's often overlooked that you can index a matrix with the character ':'.
Bruno Luong
Bruno Luong 2011-2-4
Just simplify few lines of Jan's code:
function A = insert_x_into_A(A,x,t)
idx(1:ndims(A)-1) = {':'};
A(idx{:},t) = x;
Jan
Jan 2011-2-4
@Bruno: Nicer. I'll insert it in my approach.
Knut
Knut 2013-5-7
编辑：Knut 2013-5-7
Is this documented anywhere? Am I supposed to be able to do stuff like:
ord = 3;
buffN = zeros(5*ones(1,ord));
shiftidx1(1:ord) = {'2:end'};
shiftidx2(1:ord) = {'1:(end-1)'};
buffN(shiftidx1{:}) = buffN(shiftidx2{:});
(circshift seems to be made for this purpose, but it seems slow for my purpose)

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更多回答（1 个）

Matt Fig
Matt Fig 2011-2-4

0 个投票

If you want to get real evil:
function A = insert_x_into_A(A,x,t)
col = repmat(':,',1,ndims(A)-1);
eval(['A(',col,'t) = x;'])
Though I don't know why you would need such a specific function.

2 个评论
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Jan
Jan 2011-2-4
Matt, I can see your EVAL!
Matt Fig
Matt Fig 2011-2-4
I couldn't resist. This seems like a safe application anyway.

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