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I have a huge panel data. I want to create a new data keeping only first observation of each group. How can I achieve it? For illustrative purpose;

mydata = {{1, a, aa}, {1, b, bb}, {1, c, cc}, {2, d, dd}, {2, e, 
    ee}, {3, f, ff}, {3, g, gg}, {3, h, hh}, {4, i, ii}, {4, j, 
    jj}, {4, k, kk}, {4, l, ll}, {4, m, mm}, {5, n, nn}, {5, o, 
    oo}, {7, p, pp}};

My final result should be:

myanswer = {{1, a, aa}, {2, d, dd}, {3, f, ff}, {4, i, ii}, {5, n, 
    nn}, {7, p, pp}};

Any help is greatly appreciated.

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2 Answers 2

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This is a straightforward application of GatherBy and Map (/@):

First /@ GatherBy[mydata, First]

(* {{1, a, aa}, {2, d, dd}, {3, f, ff}, {4, i, ii}, {5, n, nn}, {7, p, pp}} *)

You could also use Part ([[]]) to get the first element of each group.

GatherBy[mydata, First][[All,1]]
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    $\begingroup$ Oops! Typing too fast – thanks bel! $\endgroup$
    – Verbeia
    Nov 22, 2014 at 4:55
  • $\begingroup$ @ Verbeia thank you for your answer. I did not know it was that simple. $\endgroup$
    – ramesh
    Nov 22, 2014 at 4:58
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 First /@  Split[mydata, #1[[1]] == #2[[1]] &]
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  • $\begingroup$ @ belisarius, thank you as always. Its that simple. $\endgroup$
    – ramesh
    Nov 22, 2014 at 4:57

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