Select subset of data going backwards

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chels19
chels19 2016-6-23
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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rocketboyjka
rocketboyjka 2016-6-23
编辑：rocketboyjka 2016-6-23
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
rocketboyjka 2016-6-24
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

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Thorsten
Thorsten 2016-6-23
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
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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