Logical Indexing a spefic column in a matrix
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Patrick
el 17 de Dic. de 2014
Comentada: Guillaume
el 20 de Dic. de 2014
Hi,
I have a matrix with 5 columns and a variable number of rows (usually around 100). Column 4 of the matrix is a set of angles that can range anywhere from -180 to 180. I want to be able to create a set of smaller matrixes that splits up the larger matrix based on whether the angles fall in a 30 degree segment or not. For example a matrix that has 0 to 30 degrees, 30 to 60 degrees, 60 to 90 degrees, etc.
Can I use something like below. I don't think that will work though.
Zero_to_Thirty = A(4,:) < 30
Any help is appreciated!
Thanks,
Pat
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Guillaume
el 17 de Dic. de 2014
Editada: Guillaume
el 17 de Dic. de 2014
Don't store these submatrices in individual variables with different names.Rather store them in a cell array.
Use histcounts to find the distribution of the indices of the rows you want to split and arrayfun or a for loop to do the splitting:
m = rand(150, 6); m(:, 4) = randi([-180 180], 150, 1); %random matrix for demo, size 150*6
[~, ~, bins] = histcounts(m(:, 4), -180:30:180);
subm = arrayfun(@(bin) m(bins == bin, :), 1:max(bins), 'UniformOutput', false)
That last line with the arrayfun is equivalent to
subm = cell(1, max(bins));
for bin = 1:max(bins)
subm{bin} = m(bins == bin, :);
end
5 comentarios
Guillaume
el 20 de Dic. de 2014
All the columns are present in the matrices of the subm cell array, so I'm not sure what you're asking.
If you want the value of column 1 that correspond to the maximum, use the second return value of max:
max_subm = zeros(numel(subm), 2)
lastmax = -Inf;
lastval = -Inf;
for idx = 1:numel(subm)
if ~isempty(subm{idx})
[lastmax row] = max(subm{idx}(:, 5));
lastval = subm{idx}(row, 1);
end
max_subm(idx, :) = [lastmax lastval];
end
Más respuestas (1)
Roger Stafford
el 17 de Dic. de 2014
If M is the original matrix,
c4 = M(:,4);
M1 = M(0<=c4 & c4<30,:); % <-- Having the angles between 0 and 30
M2 = M(30<=c4 & c4<60,:); % <-- Having the angles between 30 and 60
etc.
2 comentarios
Stephen23
el 18 de Dic. de 2014
Surely it would be best not to split the array up into numbered variables. Doing this seems to invite the next question "how do I evaluate sequentially-numbered variable names?" which inevitably leads on to eval...
Roger Stafford
el 18 de Dic. de 2014
Not necessarily. It depends on how many there are and how they are to be used.
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