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Extract certain rows from matrix

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Nathan Paul
Nathan Paul 2016-5-6
评论： Guillaume 2016-5-6
I have a data matrix with 4 columns and 'n' number of rows, I want to extract the rows where the first two columns match certain values from the first two columns.
E.g where they match every possible combination of column 1 = 0.05, 0.1, 0.2, 0.5 and column 2 = 0.05, 0.1, 0.25, 0.5, 1.0, 2.0, 3.0
I am extremely stuck I've managed to find what every possible combination would be for column 1 and 2 but dont know how to apply that to the data matrix
  2 个评论
Azzi Abdelmalek
Azzi Abdelmalek 2016-5-6
You posted an example of two columns with different sizes.
Guillaume
Guillaume 2016-5-6
The set of valid values for column 1 does not have to be the same size as the set of valid values for column 2. I don't see any issue with that.

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回答（2 个）

KSSV
KSSV 2016-5-6
You can subtract the second column with first column. Where ever zero is there, extract that row.
Guillaume
Guillaume 2016-5-6
To test if members of a set belong to another set you use ismember, with the 'rows' option in your case:
%given:
col1set = [0.05, 0.1, 0.2, 0.5];
col2set = [0.05, 0.1, 0.25, 0.5, 1.0, 2.0, 3.0];
%demo data
data = [0.05, 0.5, 1, 1;
0.3, 0.05, 2, 2;
0.2, 0.4, 3, 3;
0.1, 0.1, 4, 4;
0.5, 0.05, 5, 5;
0.05, 0.5, 6, 6]; %demo data
%build every combinations of col1set with col2set
%I'm using ndgrid for this. There are other ways
[c1, c2] = ndgrid(col1set, col2set);
validrows = [c1(:), c2(:)];
%find which rows of data match validrows, only for column 1 and 2:
tokeep = ismember(data(:, [1 2]), validrows, 'rows');
%and use that to filter the original array
filtereddata = data(tokeep, :)

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