How can I get a first value of each index from the other column?

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Jaehwi Bong 2019-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/481824-how-can-i-get-a-first-value-of-each-index-from-the-other-column

评论： Rik 2019-9-24

I have this kind of data. First column and seconds column refer to an index and x value relatively.

data = [1 200; 1 201; 1 202; 1 201; 2 301; 2 313; 2 311; 3 401; 3 452; 3 433; 3 405; 4 504; 4 303; 4 604; 4 703; 5 600; 5 700; 5 606; 5 703; 5 905; 5 444;];

I want to have first value of 2nd colum of each index.

I'd like to have this kind of data that has half length (or tird whatever ratio) of index.

data = [1 200; 2 301;3 401;4 604;5 600;];

If anyone can help, it would be greatly appreciated.

Thank you!

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Answer 1

Rik 2019-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/481824-how-can-i-get-a-first-value-of-each-index-from-the-other-column#answer_393235

Use unique to get the list of first indices, the use either find or min in a loop.

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Jaehwi Bong 2019-9-24

编辑：Jaehwi Bong 2019-9-24

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Thank you for your answer!

After I use the unique function, I'd like to get the corresponding values of second colum to the first value of first column. I don't want to use min.

for i = 1: max(col1)
    col2(col1==i); %how can I get the 1st value of every iteration col2?
end

Could you give me advice again,please?

Rik 2019-9-24

在 MATLAB Online 中打开

This doesn't exactly match your output, but I suspect that could be a typo.

data = [1 200; 1 201; 1 202; 1 201; 2 301; 2 313; 2 311; 3 401; 3 452; 3 433; 3 405; 4 504; 4 303; 4 604; 4 703; 5 600; 5 700; 5 606; 5 703; 5 905; 5 444;];
col1=unique(data(:,1));
output=col1;
for n=1:numel(col1)
    output(n,2)=data(find(data(:,1)==col1(n),1),2);
end
clc,disp(output)