[datatable-help] datatable and function "which.max"

[Dominic Roye]

Hello,

I hope you can help me with the datatable. My aim is to know the
position of the daily maximum temperature (Ta).

str(y)
Classes ‘data.table’ and 'data.frame':  17135 obs. of  6 variables:
 $ X  : chr  "01/01/2010" "01/01/2010" "01/01/2010" "01/01/2010" ...
 $ X.1: chr  "0:10:00" "0:20:00" "0:30:00" "0:40:00" ...
 $ Ta : num  5.28 5.45 5.54 5.54 5.51 5.51 5.5 5.56 5.58 5.63 ...
 $ HR : int  100 100 100 100 100 100 100 100 100 100 ...
 $ RS : int  0 0 0 0 0 0 0 0 0 0 ...
 $ v  : num  2.3 2.5 1.1 2.3 2.2 2.1 2.2 2.9 2.4 2.6 ...
 - attr(*, ".internal.selfref")=<externalptr>

For this i made the datatable and than:

> z <- y[,which.max(Ta),by=X]
> z
              X  V1
  1: 01/01/2010  84
  2: 02/01/2010 132
  3: 03/01/2010  84
  4: 04/01/2010  92
  5: 05/01/2010   1
 ---
115: 25/04/2010  92
116: 26/04/2010  96
117: 27/04/2010  86
118: 28/04/2010  90
119: 29/04/2010  88


it works well, but the index don´t correspond with the index of the
original table. Have you any idea?

When i try it with z <- y[,max(Ta),by=X] it works perfect, but so I
don`t know the time,

Thanks for your help.

Best regards,

Dominic