select rows based on the value of the last column of the matrix

Question

Alberto Acri 2023-11-3

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https://ww2.mathworks.cn/matlabcentral/answers/2042511-select-rows-based-on-the-value-of-the-last-column-of-the-matrix

编辑： Jon 2023-11-3

I have a matrix 'A'. As can be seen in the figure (red rectangles), some rows have the first and second columns the same but not the third column. Considering one of the red rectangles, I would like to select the entire row (green rectangle) that has a greater value in the fourth column. The end result is the matrix 'A_out'.

A = [334	1358	175	3
1359	176	3
1359	175	1
1360	176	2
1361	176	6
1361	177	1
1362	177	5
1363	177	10
1364	178	1
1364	177	10];
A_out = [334	1358	175
1359	176
1360	176
1361	176
1362	177
1363	177
1364	177];

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Answer 1

Matt J 2023-11-3

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https://ww2.mathworks.cn/matlabcentral/answers/2042511-select-rows-based-on-the-value-of-the-last-column-of-the-matrix#answer_1346251

编辑：Matt J 2023-11-3

在 MATLAB Online 中打开

This solution requires only 1 sorting operation.

A = [334	1358	175	3
335	1359	176	3
335	1359	175	1
336	1360	176	2
337	1361	176	6
337	1361	177	1
338	1362	177	5
339	1363	177	10
340	1364	178	1
340	1364	177	10];
B=sortrows(A,[1,2,4]);
I=diff([findgroups(B(:,1), B(:,2));0])~=0;
A_out=B(I,1:3)
A_out = 7×3
         334        1358         175
         335        1359         176
         336        1360         176
         337        1361         176
         338        1362         177
         339        1363         177
         340        1364         177

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Jon 2023-11-3

This is a neat approach. I haven't used splitapply very much, but I can see here how powerful it is.

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Answer 2

Jon 2023-11-3

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https://ww2.mathworks.cn/matlabcentral/answers/2042511-select-rows-based-on-the-value-of-the-last-column-of-the-matrix#answer_1346236

编辑：Jon 2023-11-3

在 MATLAB Online 中打开

A = [334	1358	175	3
335	1359	176	3
335	1359	175	1
336	1360	176	2
337	1361	176	6
337	1361	177	1
338	1362	177	5
339	1363	177	10
340	1364	178	1
340	1364	177	10];
% Sort by first and then third column
% use descending order so that when first column values are the same
% rows with bigger 3rd column values
% will occur first
Asrt = sortrows(A,[1,3],"descend")
Asrt = 10×4
         340        1364         178           1
         340        1364         177          10
         339        1363         177          10
         338        1362         177           5
         337        1361         177           1
         337        1361         176           6
         336        1360         176           2
         335        1359         176           3
         335        1359         175           1
         334        1358         175           3
% Get location of first instance of each unique first column value (by default sorted
% in increasing order), that is location for 334,335,336...
[~,ia] = unique(Asrt(:,1))
ia = 7×1
    10
     8
     7
     5
     4
     3
     1
% Choose rows with largest 3rd column value
A_out = Asrt(ia,(1:3))
A_out = 7×3
         334        1358         175
         335        1359         176
         336        1360         176
         337        1361         177
         338        1362         177
         339        1363         177
         340        1364         178

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Voss 2023-11-3

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I like this answer, but I think to get the OP's intent ("first and second columns the same ... select the entire row that has a greater value in the fourth column"), a couple of modifications are needed:

A = [334	1358	175	3
335	1359	176	3
335	1359	175	1
336	1360	176	2
337	1361	176	6
337	1361	177	1
338	1362	177	5
339	1363	177	10
340	1364	178	1
340	1364	177	10];
Asrt = sortrows(A,[1 2 4],"descend");  % sort by column 1, then 2, then 4
[~,ia] = unique(Asrt(:,[1 2]),'rows'); % unique according to first two columns
A_out = Asrt(ia,1:3)
A_out = 7×3
         334        1358         175
         335        1359         176
         336        1360         176
         337        1361         176
         338        1362         177
         339        1363         177
         340        1364         177

Jon 2023-11-3

编辑：Jon 2023-11-3

@Voss, Oh, I see now that in the OP's written description they say to select the "entire row", but in the example output A_out, they only show the first three columns. I was just looking at the example. Anyhow the OP can now see how to do it either way.

Oops, you're right my code selected the one with the larger value in the third column, I didn't read the problem description carefully enough.

Also, good catch on the first two columns being unique rather than just the first as I had. For this example I don't think it makes any difference, but your correction follows the OP's specification, thanks.

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