Unique Function based on 2 columns [Instead of rows]

Question

RDG 2013-4-7

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/70535-unique-function-based-on-2-columns-instead-of-rows

Suppose, I have a cell array, a, with contents as such:

a{1}=[1 3 4 5;
3 4 5;
5 4 5
4 2 6;
5 2 6
2 3 1;]

How can I apply the 'unique' function on 2 columns [column 3 and 4] such that they will return the value '3' and '2'. [Since there are 3 duplicates for the pair 4,5 and 2 duplicates for the pair 2,6.]

Any hint is greatly appreciated.

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Cedric 2013-4-7

See the edit in my answer.

Cedric 2013-4-8

See 2nd edit.

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Answer 1

Cedric 2013-4-7

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/70535-unique-function-based-on-2-columns-instead-of-rows#answer_81337

编辑：Cedric 2013-4-8

在 MATLAB Online 中打开

NEW ANSWER AFTER 2ND EDIT

You could do it as follows (you might want to fine tune it):

diff(find([true; any(diff(a{1}(:,3:4)),2); true]))

To understand this solution, evaluate it by part:

 >> a{1}(:,3:4)                   % Index col 3 and 4.
 ans =
     4     5
     4     5
     4     5
     2     6
     2     6
     3     1
 >> diff(a{1}(:,3:4))             % Difference between rows in col 3 and 4.
 ans =
     0     0
     0     0
    -2     1
     0     0
     1    -5
 >> any(diff(a{1}(:,3:4)),2)      % Flag rows were there is a diff in either col.
 ans =
     0
     0
     1
     0
     1
 >> [true; any(diff(a{1}(:,3:4)),2); true]        % Add flags for boundaries.
 ans =
     1
     0
     0
     1
     0
     1
     1
 >> find([true; any(diff(a{1}(:,3:4)),2); true])  % Get positions of all flags.
 ans =
     1
     4
     6
     7

Finally, compute positions differences.

 >> diff(find([true; any(diff(a{1}(:,3:4)),2); true]))
 ans =
     3
     2
     1

which give you the size (in terms of number of rows) of each block of similar cols 3 and 4.

FORMER ANSWER

 >> [u, ia] = unique(a{1}(:,3:4), 'rows', 'stable')
 u =
     4     5
 ia =
     1

The flag 'rows' tests rows uniqueness (taking all columns into account). So here it seems that you want to pass columns 3 and 4 of array a{1} to UNIQUE, using the 'rows' flag. The stable flag makes unique return the first occurrence instead of the last in this case.

If you wanted to operate on columns (for all rows), you could pass a subset of the transpose of a{1} to UNIQUE.

EDIT (after you edited the question): remove the 'stable' flag if you want 4.