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 2the 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).
Solution Stats
Problem Comments
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6 Comments
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3 older comments
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))
Solution Comments
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2 Comments
Daniel Pereira
on 20 Nov 2015
Don't know why an empty matrix 1-by-0 is not equal to the empty matrix [].
Jan Orwat
on 20 Nov 2015
[] is 0-by-0. mtimes(ones(1,0), ones(0,1)) equals 0, mtimes([], ones(0,1)) is empty 0-by-1. This is one of the reasons.
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1 Comment
Pascal
on 28 Apr 2014
I used exactly the same code as for problem 67.
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1 Comment
Ze Zhang
on 17 Mar 2015
Nice! But I think you made a mistake in if length(m)==size(x,1), according to the instruction, a number that appears more than half of total rows would be okay.
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2 Comments
Marlies
on 25 Nov 2012
My solution fails on the magic, the wilkonson and the [1;2] inputs.
However, on my own computer each input works as expected.
Anyone has a hint on where I am going wrong?
Marlies
on 4 Dec 2012
Found it.
Due to the way the test-suite is set-up (using 'isequal'), MATLAB makes a difference between the values "[]" (size [0 0]) and "Empty matrix: 1-by-0" (size [1 0]), even though they are both considered empty.
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