How to extract elements in an array that have multiple entries?

Question

I have a large vector that contains monotonically increasing data or a duplicate, looking something like this:

data = [0 1.1 2.2 3.3 4.4 4.4 4.4 4.4 5.5 6.6 6.6 6.6 7.7];

In this data set, I'm interested in the duplicate entries (in this case, 4.4 and 6.6). I have a sorta clunky solution to extract these values, but I feel like MATLAB should have a one-liner solution to extract a result like

result = [4.4 6.6];

Answer 1

The combination of unique and diff is enough, find is actually not necessary.

out = unique(data(~diff(data)))

Answer 2

Here's another option using just hist and indexing:

result = data(hist(data, data) > 1);

Answer 3

My one liner:

unique(data(find(diff(data)==0)))

Test it here!

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Explanation:

1. with diff you get the differences from one element to the next.

2. As the array is previously ordered, the result of the above line will output zero on the repeated ones.

3. With find(result_from_above == 0) I get where they live (indexes for zeroes)

4. With data(result_from_above) I get the repeated elements, and then apply unique to get them.

Update:

You can use logical indexing, as @rayryeng said, you dont need the find, then it becomes:

unique(data(diff(data)==0)); 

Then test it here