Problem 36. Find relatively common elements in matrix rows

Created by Cody Team

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You want to find all elements that exist in greater than 50% of the rows in the matrix.
For example, given
 A =
     1 2 3 5
     9 2 5 9
     3 2 5 8
     1 2 1 5
     5 1 3 2
the answer would be the row vector
 [1  2  3  5]
since each of these appears three or more times in the rows of matrix A. Elements should be returned sorted ascending.
Note there is no guarantee that there are any such elements; if there are none, the routine should return the empty set. Also, the matrix might contain repeats, very large or very small numbers, or NaNs (though NaN should not be treated as a number to be returned in the output).

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Last Solution submitted on Nov 18, 2024

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Jean-Marie Sainthillier on 2 Aug 2012

Tests aren't specifically adapted to this problem. The code for problem 67 (Find common elements in matrix works here).

Sergey on 7 Aug 2012

Absolutely agree with Jean-Marie.
Tests is not good enough.

Marlies on 4 Dec 2012

Not sure if the previous commenters have the same issue that I have, but I dislike that this test-suite makes a difference in how the empty-matrix is defined (See also comments for solution 167825).
I would suggest checking isempty' for the relevant solutions.

Heather on 5 Sep 2013

Ironically, simply including the example from the problem statement in the test suite would reject the current smallest ("best") solutions.

Peter on 18 Nov 2014

Indeed, the current leading solutions with size 29 yield only elements that exist in ALL of the rows in the matrix!

Simon Thomas on 20 Nov 2017

This needs another test case.
Many would fail this:

%%
x = [1 1 1 1;2 3 4 5;3 6 7 8];
y_correct = [3];
assert(isequal(common_by_row(x),y_correct))

arsenic 陈 on 10 May 2021

https://blog.csdn.net/qq_44846756/article/details/116567963
i thought different

Dyuman Joshi on 25 Aug 2021

The test suit has been updated to check for miscellaneous cases and a reference solution is added.

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Problem 36. Find relatively common elements in matrix rows

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