用matlab / octave'去旋转'矩阵

Question

I want to convert a matrix M into "tall-skinny" format. The resulting matrix would have rows like [r, c, M(r,c)] for every r and c in the rows and columns of M. Is there a function which does this? Alternatively, is there a function which does the reverse?

Answer 1

Is this what you want?

[ii jj] = ndgrid(1:size(M,1), 1:size(M,2));
T = [ii(:) jj(:) M(:)];

Reverse:

M = full(sparse(T(:,1), T(:,2), T(:,3)));

or more simply, as noted by Jumppy89,

M = full(spconvert(T));

Answer 2

I am curious too if this would satisfy your needs -

Normal to Tall-skinny (based on meshgrid ) -

[x1,x2] = meshgrid(1:size(M,1),1:size(M,2))
TSM = [x2(:) x1(:) M(:)] %// Tall-Skinny M

Tall-skinny to Normal (based on sub2ind and reshaping) -

m1 = max(TSM(:,2))
n1 = max(TSM(:,1))
TSM3 = TSM(:,3)
M_out = reshape(TSM3(sub2ind([n1 m1],TSM(:,1),TSM(:,2))),m1,n1)

Answer 3

You can do it in a different way to Luis and Divakar, using sub2ind and ind2sub for the reverse. I doubt this is a better solution than theirs though.

To convert to the "tall-skinny" format:

M = [1     1     1
     1     2     2
     2     1     3
     2     2     4]
S=zeros([max(M(:,1)) max(M(:,2))])
I=sub2ind([max(M(:,1)) max(M(:,2))],M(:,1),M(:,2))
S(I)=M(:,3)

And the reverse:

[r,c]=ind2sub(size(S),1:numel(S))
M=[r.' c.' S(:)]