Select subset of data going backwards

Question

chels19 2016-6-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/291886-select-subset-of-data-going-backwards

评论： chels19 2016-6-24

Hi, I've got a matrix of data and need to start at a specific row within the dataset and, based on a condition, select the previous 5 rows. For example:

if column 6 > 0

a = iterate through and get the value of column 4 that corresponds to column 6 > 0 (i.e. 694)
starting at position a, get the previous 5 rows of data from column 4 where column 3 = 1

iterate through the whole file

In this case I would expect 361, 360, 349, 343, 331 to be returned (yellow).

What I really need to do is get the mean of these numbers but I need to select them first.

I hope I've explained it well. I'm just really stuck on how to go backwards in Matlab.

Thanks

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Answer 1

rocketboyjka 2016-6-23

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https://ww2.mathworks.cn/matlabcentral/answers/291886-select-subset-of-data-going-backwards#answer_226494

编辑：rocketboyjka 2016-6-23

在 MATLAB Online 中打开

Try this:

 targetIndex = find(myMatrix(:,6) > 0);
 desiredData = myMatrix(targetIndex-5:targetIndex-1,4);

If there is more than one location in your matrix where the condition is met, then targetIndex will be a vector of indices where the condition occurs and you could adapt it like this:

 targetIndex = find(myMatrix(:,6) > 0);
 for ii = 1:length(targetIndex)
     desiredData(:,ii) = myMatrix(targetIndex(ii)-5:targetIndex(ii)-1,4);
 end

Then each column will be the numbers that met your condition.

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rocketboyjka 2016-6-24

在 MATLAB Online 中打开

I missed your part about sorting by column 3. I tested this on your data. Should work if you're still looking.

 targetIndex = find(myMatrix(:,6) > 0);
 for ii = 1:length(targetIndex)
     candiateRows = find(myMatrix(1:targetIndex(ii)-1,3) ~= 0);
     if length(candiateRows) < 5
          startIndex = 1;
          numRows = length(candiateRows);
     else
          startIndex = length(candiateRows)-4;
          numRows = 5;
     end
     desiredData(:,ii) = myMatrix(candiateRows(startIndex:end),4);
 end

chels19 2016-6-24

That works perfectly, thank you.

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Answer 2

Thorsten 2016-6-23

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https://ww2.mathworks.cn/matlabcentral/answers/291886-select-subset-of-data-going-backwards#answer_226493

在 MATLAB Online 中打开

i1 = find(a(:,6) > 0, 1, 'first')
i2 = find(a(1:i1, 3) == 1, 5, 'last')
res = flipud(a(i2, 4))

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chels19 2016-6-23

Thanks. It is returning a number but I'm having trouble looping it. I tried to combine yours with the answer below but it's not working. With the data I have, I would expect 4 subsets of data.

What I really need to do is get the mean of these numbers but I want to check first that the correct 5 are being selected.

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