finding neighbor of a position

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Mohammad Golam Kibria
Hi, I have a matrix.
I =
1 0 0
2 5 0
0 0 3
0 0 0
I know the position of 5 in I is 6 linear index.
is there any easy function to have the 8 other neighbors of 5. Thanks

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Oleg Komarov
Oleg Komarov 2011-6-26
EDITED: should be fine now
I =[ 1 0 0
2 5 0
0 0 3
0 0 0];
l = 8;
sz = size(I);
% row, col subs of center
[r,c] = ind2sub(sz,l); % c = ceil(l/4); r = mod(l,4)+ c*sz(1);
% Calculate 8 neighbors
neigh(1:8,1:2) = [r+[-1;0;1;-1;1;-1;0;1] c+[-1;-1;-1;0;0;1;1;1] ];
% Only those in the range
neigh = neigh(all(neigh,2) & neigh(:,1) <= sz(1) & neigh(:,2) <= sz(2),:);
% Convert to position
idx = (neigh(:,2)-1)*sz(1) + neigh(:,1);
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Oleg Komarov
Oleg Komarov 2011-6-27
Hopefuly now is ok. Tested initial and final position.

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更多回答（3 个）

Sean de Wolski
Sean de Wolski 2011-6-27
idx = find(conv2(double(I==5),ones(3),'same'))
%This includes the 6, but that could easily be taken care of with setdiff.
Wolfgang Schwanghart
Hi,
you might find this function useful.
http://www.mathworks.com/matlabcentral/fileexchange/16991-neighbor-indexing
Cheers, W.
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Wolfgang Schwanghart
Wolfgang Schwanghart 2011-6-27
I = [ 1 0 0;
2 5 0;
0 0 3;
0 0 0];
% find the neighbors of the elements where I = 5
I5 = I==5;
[ix,ixn] = ixneighbors(I,I5)
ix =
6
6
6
6
6
6
6
6
ixn =
10
7
2
5
9
1
11
3
% thus, ixn are the linear indices of the neighbors of the indices ix.
% You'll find the values associated with the neighbors by
I(ixn)
ans =
0
0
2
0
0
1
3
0
Mohammad Golam Kibria
Mohammad Golam Kibria 2011-6-28
thanks,this also works fine for me

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Andrei Bobrov
Andrei Bobrov 2011-6-26
idl = 6;
idxs = ...
nonzeros(bsxfun(@plus,idl - [1 0 -1]',size(I,1)*[-1 0 1]).*[1 1 1;1 0 1;1 1 1])
CORRECTED 06/27/2011 10:05 MSK
idl = 6;
s = size(I);
I0 = zeros(s+2);
I0(2:end-1,2:end-1) = reshape(1:numel(I),s);
idxs = nonzeros(I0(bsxfun(@plus,find(I0==idl) - [1 0 -1]',(s(1)+2)*[-1 0 1])).*[1 1 1;1 0 1;1 1 1])
MORE variant (06/27/2011 11:12 MSK)
s = size(I);
[ii jj] = ind2sub(s,idl);
v = [-1 -1 -1;0 0 0;1 1 1];
R=ii+v;
C=jj+v';
loc = (R<=s(1) & R>=1&C<=s(2) & C>=1&[1 1 1;1 0 1;1 1 1])>0;
idxl = sub2ind(s,R(loc),C(loc));
MORE variant 2 (06/27/2011 11:35 MSK) with idea of Oleg
s = size(I);
[ii jj] = ind2sub(s,idl);
R = ii + [-1 0 1 -1 1 -1 0 1];
C = jj + [-1 -1 -1 0 0 1 1 1];
loc = (R<=s(1) & R>=1&C<=s(2) & C >= 1 )>0;
idxl = sub2ind(size(I),R(loc),C(loc));
LAST variant (06/27/2011 13:43 MSK)
I1 = zeros(size(I));
I1(idl)=1;
idx = find(bwdist(I1,'chessboard')==1)
or
idx = find(bwdist(I==5,'chessboard')==1)

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