finding duplicates

A=[ 1 1 2 2 3 3 3];

unique(A)=[1 2 3]; but I want to find the duplicates that are not the first occurrence. i.e x=[2 4 6 7]; I typed help unique but I couldn't figure out if I and J reported by this function helps with my purpose.I know that I can program it but i want to be as efficient as possible in my codes to reduce the running time.

Answer 1

[the cyclist]

Here is one way:

[uniqueA i j] = unique(A,'first');
indexToDupes = find(not(ismember(1:numel(A),i)))

Answer 2

[Jan]

Another solution:

A = [1 1 2 2 3 3 3];
[U, I] = unique(A, 'first');
x = 1:length(A);
x(I) = [];

[Oleg Komarov]

Clever and simple.

[Jan]

I'm inspired by Marsaglia's KISS random number generator: "Keep It Simple Stupid".