detect same value

how do I detect on the elements of a (:, 1) there is the same value?

a=[1 34;
   2 33;
   2 45;
   3 32;
   4 65;
   4 99;
   5 11;
   1 33;
   6 22]

Answer 1

[Walter Roberson]

u = unique(a(:,1));
if length(u) ~= size(a,1)
  %there were duplicates
end

If you want to find out which are equal to which,

bsxfun(@eq, a(:,1), a(:,1).'))

and that will return a logical matrix in which for row r, column c is set if a(r) == a(c)

[M M]

How would you isolate the duplicated values? So I would know which values are duplicated and use them for another purpose? (The opposite of the unique function)

[Walter Roberson]

setdiff(V, unique(V))  %for vector V, results are in sorted order

or

T = bsxfun(@eq, V(:), V(:).');  %for vector V
sum(T) > 1  %will be true at index K if V(K) is not unique