Dear Contributors
thanks for any help you can provide. I searched the threads
but I could not find any query that satisfied my needs.
This is my database:
 index time         values
13732  27965 DATA.Q211.SUM.Index    04/08/11         1.42
13733  27974 DATA.Q211.SUM.Index    05/10/11         1.45
13734  27984 DATA.Q211.SUM.Index    06/01/11         1.22
13746  28615 DATA.Q211.TDS.Index    04/07/11         1.35
13747  28624 DATA.Q211.TDS.Index    05/20/11         1.40
13754  29262 DATA.Q211.UBS.Index    05/02/11         1.30
13755  29272 DATA.Q211.UBS.Index    05/03/11         1.48
13761  29915 DATA.Q211.UCM.Index    04/28/11         1.43
13768  30565 DATA.Q211.VDE.Index    05/02/11         1.48
13775  31215 DATA.Q211.WF.Index     04/14/11         1.44
13776  31225 DATA.Q211.WF.Index     05/12/11         1.42
13789  31865 DATA.Q211.WPC.Index    04/01/11         1.40
13790  31875 DATA.Q211.WPC.Index    04/08/11         1.42
13791  31883 DATA.Q211.WPC.Index    05/10/11         1.43
13804  32515 DATA.Q211.XTB.Index    04/29/11         1.50
13805  32525 DATA.Q211.XTB.Index    05/30/11         1.40
13806  32532 DATA.Q211.XTB.Index    06/28/11         1.43

I need to select only the rows of this database that correspond to each
of the first occurrences of the string represented in column
index. In the example shown I would like to obtain a new
data.frame which is

index time         values
13732  27965 DATA.Q211.SUM.Index    04/08/11         1.42
13746  28615 DATA.Q211.TDS.Index    04/07/11         1.35
13754  29262 DATA.Q211.UBS.Index    05/02/11         1.30
13761  29915 DATA.Q211.UCM.Index    04/28/11         1.43
13768  30565 DATA.Q211.VDE.Index    05/02/11         1.48
13775  31215 DATA.Q211.WF.Index    04/14/11         1.44
13789  31865 DATA.Q211.WPC.Index    04/01/11         1.40
13804  32515 DATA.Q211.XTB.Index    04/29/11         1.50

As you can see, it is not the whole string to change,
rather a substring that is part of it. I want to select
only the first values related to the row that presents for the first time
the different part of the string(substring).
I know how to select rows according to a substring condition on the
index column, but I cannot use it here because the substring changes
and moreover the number of occurrences per substring is variable.

Thank you for any help you can provide.
Francesca

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