How to find even positioned numbers in a vector or matrix.

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DJ V
DJ V 2016-11-8
评论： Image Analyst 2016-11-8
Hi, if I have a matrix, say, [1 2 3; 4 5 6] and I want the 2,2 position and all even number positions within it, I can write a line of code that will produce "5" as the answer. for a single row vector [1 2 3 4 5 6] there are no even numbered row and column positions, so nothing should be returned. I can send it a matrix M, and ask for M(2,2:2:end) but that won't work for the single row vector, I get an error, and it should return nothing. How do I do this?

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Thorsten
Thorsten 2016-11-8
编辑：Thorsten 2016-11-8
if size(M,1) > 1 & size(M,2) > 1
res = M(2,2);
else
res = [];
end
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Guillaume
Guillaume 2016-11-8
M(2:2:end, 2:2:end)
will work for both vectors and matrices (and of course return empty for vectors).
To extend to any number of dimensions:
dimidx = arrayfun(@(dimsize) 2:2:dimsize, size(M), 'UniformOutput', false);
[dimidx{:}] = ndgrid(dimidx{:});
out = reshape(M(sub2ind(size(M), dimidx{:})), 1, [])

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