How to keep first observation and delete other observations of each group in unbalanced Panel data

Question

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.

score 5 · Accepted Answer · edited Nov 22 '14 at 04:54

5

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]]

edited Nov 22 '14 at 04:54

Dr. belisarius

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answered Nov 22 '14 at 04:51

Verbeia

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1

Oops! Typing too fast – thanks bel! – Verbeia Nov 22 '14 at 04:55
@ Verbeia thank you for your answer. I did not know it was that simple. – ramesh Nov 22 '14 at 04:58

score 5 · Answer 2 · answered Nov 22 '14 at 04:53

5

 First /@  Split[mydata, #1[[1]] == #2[[1]] &]

answered Nov 22 '14 at 04:53

Dr. belisarius

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@ belisarius, thank you as always. Its that simple. – ramesh Nov 22 '14 at 04:57

How to keep first observation and delete other observations of each group in unbalanced Panel data

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