CODEWITHSUNDEEP
matlab
siegel
siegel

Reputation: 829

Keep only rows of matrix with largest elements within intervals

I have a matrix in which the right-most elements are repeated YYYYMMDD dates in descending order, for example:

      40        1630        1711    20140326
     169         700         950    20140326
      40        1630        1711    20140326
     169         700         950    20140327
      40        1630        1711    20140327
     169         700         950    20140327
      40        1630        1711    20140327
     169         700         950    20140328
      40        1630        1711    20140328
     169         700         950    20140328
      40        1630        1711    20140328
    3049         700         950    20140331
      40        1630        1711    20140331
    3049         700         950    20140331
      40        1630        1711    20140331
     169         700         950    20140401
      40        1630        1711    20140401
     169         700         950    20140401
      40        1630        1711    20140401
     169         700         950    20140402
      40        1630        1711    20140402

Within each date, I want to keep only the row that corresponds to the largest element in the leftmost column. So I would like to produce a new matrix:

     169         700         950    20140326
     169         700         950    20140327
     169         700         950    20140328
    3049         700         950    20140331
     169         700         950    20140402

What is the best way to do this?

Ideas: use diff to find indices of when date changes, and use a for loop to cycle through intervals using max to find the correct rows. any solution without a for loop?

Upvotes: 2

Views: 86

Answers (1)

Stewie Griffin
Stewie Griffin

Reputation: 14939

Try to first sort the rows based on the first column, thereafter find the first unique dates based on the last:

B = sortrows(A)
[~, ia] = unique(B(:,4))
B(ia,:)
ans =
        169        700        950   20140326
        169        700        950   20140327
        169        700        950   20140328
       3049        700        950   20140331
        169        700        950   20140401
        169        700        950   20140402

Upvotes: 5

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