Remove the rows/columns with single values of a matrix

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Dani Tormo
Dani Tormo 2012-11-28
Hello,
I have a matrix quite big (A < 23 x 250000 >). I want to evaluate the first row A(1,:) and eliminate the columns where there are values only repeated once. The number in this first row is an integer and always is increasing.
Lets say:
B = [1 1 3 5 5 5 7 9 9
0.1 0.5 0.2 0.4 0.3 0.9 0.1 0.6 0.5]
The result should be:
B = [1 1 5 5 5 9 9
0.1 0.5 0.4 0.3 0.9 0.6 0.5]
I know how to do it with for and if statements but I think it won't be efficient because the size of the matrix.
Many thanks!

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采纳的回答

Dani Tormo
Dani Tormo 2012-11-28
This is the code I have and it works, but maybe it is not efficient:
for i = 2:length(A) - 1;
if A(1,i) ~= A(1,i-1) && A(1,i) ~= A(1,i+1)
A = A(:,[1:i-1 i+1:end]);
i = i - 1;
end
end
  4 个评论
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Dani Tormo
Dani Tormo 2012-12-3
编辑：Dani Tormo 2012-12-3
Yes, finally I had to do it with the while expression to avoid the for fail. The rush isn't good ever.
finished = 0; i = 2;
while finished == 0
if A(1,i) ~= A(1,i-1) && A(1,i) ~= A(1,i+1)
simulation_data = simulation_data(:,[1:i-1 i+1:end]);
elseif i == length(A)-1
finished = 1;
else
i = i + 1;
end
end
Regards.
Jan
Jan 2012-12-3
编辑：Jan 2012-12-3
Collect the indices to be removed at first:
toDelete = false(size(A, 2));
for i = 2:size(A, 2) - 1
if A(1,i) ~= A(1,i-1) && A(1,i) ~= A(1,i+1)
toDelete(ii) = true;
end
end
A(:, toDelete) = [];

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更多回答（1 个）

per isakson
per isakson 2012-11-28
编辑：per isakson 2012-11-28
A start:
B = [1 1 3 5 5 5 7 9 9
0.1 0.5 0.2 0.4 0.3 0.9 0.1 0.6 0.5];
[ uniqueB, ix ] = unique( B(1,:) );
sB = B(1,:);
sB( ix ) = [];
singleB = setdiff( uniqueB, sB );
[ ~, ixm ] = ismember( singleB, B(1,:) );
B( :, ixm ) = []
prints
B =
1.0000 1.0000 5.0000 5.0000 5.0000 9.0000 9.0000
0.1000 0.5000 0.4000 0.3000 0.9000 0.6000 0.5000
>>
your turn.
I'm wouldn't be surprise if the for-loop is faster :)

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