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

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?

Answer 1

[Thorsten]

if size(M,1) > 1 & size(M,2) > 1
  res = M(2,2);
else
  res = [];
end

[Guillaume]

if ~isvector(M)

would be a simpler test

[Thorsten]

Right. Thank you, Guillaume.

[DJ V]

Thanks but I can't use "if" statements. No tests.

[Image Analyst]

Huh? Why on earth not??? What a bizarre limitation you're putting on yourself!

http://www.mathworks.com/matlabcentral/answers/38787-what-can-be-programmed-without-any-built-in-functions

Answer 2

[Guillaume]

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, [])

[DJ V]

Thanks! I