How do I find unique points in an array in a certain context?

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FsC 2023-4-28

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https://ww2.mathworks.cn/matlabcentral/answers/1954764-how-do-i-find-unique-points-in-an-array-in-a-certain-context

评论： FsC 2023-5-1

采纳的回答： Image Analyst

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Hello,

I am trying to find unique points in an array but within a unique context.

Say I have the following arrays consisting of x and y coordinates at spcific time points:

time = [ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19]';
x_coord = [ 1 1 1 rand(1,6) 1 1 1 rand(1,5) 1 1]';
y_coord = [[ 1 1 1 rand(1,6) 1 1 1 rand(1,5) 1 1]';
    mat = [x_coord y_coord];

I want to find the unique x,y coordinates so that the first value of the '1's is preserved. Specifically, the unique X/Y point indices would be:

ia = 1, 4, 5, 6, 7, 8, 9,10,13,14,15,16,17,18; (essentially preserves the 1s that occur later on in the matrix)

when using the unique() function, it removes the 1s later on in the matrix.

is there a way to find the unique points but preserve the same points that occur later in time?

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Walter Roberson 2023-4-28

Are you sure that you want unique() ? unique() is talking about the global situation -- so for example [x1, y1; x2, y2; x1, y1] the second x1, y1 would be considered duplicate .

It sounds to me as if it is more you are concerned whether you are in a run of duplicate values, which is a different task that can be determined with local information.

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Answer 1

Image Analyst 2023-4-28

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https://ww2.mathworks.cn/matlabcentral/answers/1954764-how-do-i-find-unique-points-in-an-array-in-a-certain-context#answer_1225509

编辑：Image Analyst 2023-4-28

在 MATLAB Online 中打开

Try this:

time = [ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19]';
x_coord = [ 1 1 1 rand(1,6) 1 1 1 rand(1,5) 1 1]';
y_coord = [ 1 1 1 rand(1,6) 1 1 1 rand(1,5) 1 1]';
mat = [x_coord, y_coord]
mat = 19×2
    1.0000    1.0000
    1.0000    1.0000
    1.0000    1.0000
    0.2902    0.7648
    0.9712    0.7973
    0.9503    0.5313
    0.5635    0.9937
    0.1493    0.0628
    0.1966    0.1561
    1.0000    1.0000
all1rows = all(mat == 1, 2); % Find out which rows have 1 in every column.
% Throw out row if the prior row is all 1s.
% There is a vectorized way but I think you might find the for loop more intuitive.
rowsToKeep = true(numel(x_coord), 1);
for row = 2 : numel(all1rows)
    if (all1rows(row) == 1) && (all1rows(row) == all1rows(row-1))
        rowsToKeep(row) = false;
    end
end
ia = find(rowsToKeep)'
ia = 1×14
     1     4     5     6     7     8     9    10    13    14    15    16    17    18
mat = mat(rowsToKeep, :)
mat = 14×2
    1.0000    1.0000
    0.2902    0.7648
    0.9712    0.7973
    0.9503    0.5313
    0.5635    0.9937
    0.1493    0.0628
    0.1966    0.1561
    1.0000    1.0000
    0.5499    0.2234
    0.0796    0.7463