Re: [R] Help needed with aggregate or other solution

Thu, 26 Oct 2017 08:06:48 -0700 

On Thu, 26 Oct 2017, Thomas Adams wrote:


Hi Jeff,

Thank you for the suggestions -- I appreciate your help. Unfortunately, the
result2 has two problems...

(1) there are now 3 date columns (it looks like 2 cols are merged into 1
col)
No, there are two date columns. Result2 includes the grouping value as a row name (pulled from the names of the dta2list items by rbind). 


(2) the output rows should not have any of the basistime dates repeated
(maybe I misstated the problem); I need the max fcst value by basistime, but
also list the date value for that row; for example:


Then try out my code replacing

dta2list <- split( dta2, dta2$date )

with

dta2list <- split( dta2, dta2$basistime )

and

%>% group_by( date )

with

%>% group_by( basistime )
Please study how the code works and ask questions based on gaps in that knowledge rather than how the results are not what you expected. This mailing list is not a do-your-work-for-you coding service. Some help pages you should look at include: 

?rownames
?split
?lapply
?do.call
?rbind
?group_by
?do
You might also find [1] helpful in general, and [2] helpful for understanding dplyr.
[1] H. Wickham, The Split-Apply-Combine Strategy for Data Analysis, Journal of Statistical Software, vol. 40, no. 1, Apr. 2011.
[2] H. Wickham and G. Grolemund, R for Data Science. OReilly UK Ltd, 2017. URL: https://r4ds.had.co.nz. 



             basistime fcst
1   2012-01-25 15:02:00 47.9
2   2012-01-26 15:11:00 50.4
3   2012-01-27 01:41:00 46.0
4   2012-01-27 10:15:00 47.3
5   2012-01-27 15:15:00 47.3
6   2012-01-28 14:22:00 46.2
7   2012-01-29 13:33:00 45.8
8   2012-01-30 14:11:00 44.8
9   2012-01-31 14:24:00 43.9
10  2012-02-01 14:55:00 41.1
11  2012-02-02 14:56:00 38.1
12  2012-02-03 14:40:00 36.2
13  2012-02-04 15:01:00 34.7
14  2012-02-05 15:04:00 33.1
15  2012-02-06 14:37:00 32.2

This is very close to what I need. The basistime dates are all unique, with
the max fcst value for the available basistime dates; but I additionally
need the corresponding 'date' value.

Best,
Tom



On Thu, Oct 26, 2017 at 1:28 AM, Jeff Newmiller <jdnew...@dcn.davis.ca.us>
wrote:
      Thanks for the dput...

      #### reproducible example of split-apply-combine ###

      dta <- structure(list(date = structure(c(1L, 2L, 3L, 4L, 5L, 6L,
      7L,
      8L, 9L, 10L, 11L, 12L, 13L, 14L, 15L, 16L, 17L, 18L, 19L, 20L,
      5L, 6L, 7L, 8L, 9L, 10L, 11L, 12L, 13L, 14L, 15L, 16L, 17L, 18L,
      19L, 20L, 21L, 22L, 23L, 24L, 7L, 8L, 9L, 10L, 11L, 12L, 13L,
      14L, 15L, 16L), .Label = c("2012-01-25 18:00:00", "2012-01-26
      00:00:00",
      "2012-01-26 06:00:00", "2012-01-26 12:00:00", "2012-01-26
      18:00:00",
      "2012-01-27 00:00:00", "2012-01-27 06:00:00", "2012-01-27
      12:00:00",
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      06:00:00",
      "2012-01-28 12:00:00", "2012-01-28 18:00:00", "2012-01-29
      00:00:00",
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      18:00:00",
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      1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
      1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
      2L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L,
      3L), .Label = c("2012-01-25 15:02:00", "2012-01-26 15:11:00",
      "2012-01-27 01:41:00", "2012-01-27 10:15:00", "2012-01-27
      15:15:00",
      "2012-01-28 14:22:00", "2012-01-29 13:33:00", "2012-01-30
      14:11:00",
      "2012-01-31 14:24:00", "2012-02-01 14:55:00", "2012-02-02
      14:56:00",
      "2012-02-03 14:40:00", "2012-02-04 15:01:00", "2012-02-05
      15:04:00",
      "2012-02-06 14:37:00", "2012-02-07 14:42:00", "2012-02-08
      14:44:00",
      "2012-02-09 14:27:00", "2012-02-10 14:33:00", "2012-02-11
      15:05:00",
      "2012-02-12 15:09:00", "2012-02-13 15:00:00", "2012-02-14
      15:10:00",
      "2012-02-15 14:46:00", "2012-02-16 14:42:00", "2012-02-17
      14:14:00",
      "2012-02-18 15:27:00", "2012-02-19 13:13:00", "2012-02-20
      13:56:00",
      "2012-02-21 14:39:00", "2012-02-22 15:33:00", "2012-02-23
      15:14:00",
      "2012-02-24 15:14:00", "2012-02-25 14:25:00", "2012-02-26
      14:08:00",
      "2012-02-27 14:55:00", "2012-02-28 14:40:00", "2012-02-29
      14:45:00",
      "2012-03-01 15:18:00", "2012-03-02 15:18:00", "2012-03-03
      15:43:00",
      "2012-03-04 15:16:00", "2012-03-05 15:17:00", "2012-03-06
      15:03:00",
      "2012-03-07 14:44:00", "2012-03-08 14:53:00", "2012-03-09
      15:10:00",
      "2012-03-10 14:43:00", "2012-03-11 13:37:00", "2012-03-12
      14:58:00",
      "2012-03-13 14:36:00", "2012-03-14 13:49:00", "2012-03-15
      14:32:00",
      "2012-03-16 15:01:00", "2012-03-17 13:55:00", "2012-03-18
      13:44:00",
      "2012-03-19 13:55:00", "2012-03-20 13:53:00", "2012-03-21
      14:07:00",
      "2012-03-22 14:03:00", "2012-03-23 14:30:00", "2012-03-24
      14:47:00",
      "2012-03-25 14:32:00", "2012-03-26 14:41:00", "2012-03-27
      13:51:00",
      "2012-03-28 13:32:00", "2012-03-29 13:59:00", "2012-03-30
      14:15:00",
      "2012-03-31 14:12:00", "2012-04-01 14:30:00", "2012-04-02
      13:43:00",
      "2012-04-03 13:54:00", "2012-04-04 13:45:00", "2012-04-05
      13:46:00",
      "2012-04-06 13:41:00", "2012-04-07 13:13:00", "2012-04-08
      12:59:00",
      "2012-04-09 13:53:00", "2012-04-10 14:15:00", "2012-04-11
      14:26:00",
      "2012-04-12 14:05:00", "2012-04-13 13:37:00", "2012-04-14
      13:52:00",
      "2012-04-15 15:00:00", "2012-04-16 14:42:00", "2012-04-17
      14:21:00",
      "2012-04-18 14:40:00", "2012-04-19 14:35:00", "2012-04-20
      15:14:00",
      "2012-04-21 14:46:00", "2012-04-22 14:44:00", "2012-04-23
      14:33:00",
      "2012-04-24 14:41:00", "2012-04-25 14:13:00", "2012-04-26
      14:19:00",
      "2012-04-27 14:35:00", "2012-04-28 13:48:00", "2012-04-29
      14:12:00",
      "2012-04-30 13:53:00", "2012-05-02 14:41:00", "2012-05-03
      15:01:00",
      "2012-05-04 15:21:00", "2012-05-05 14:57:00", "2012-05-06
      14:09:00",
      "2012-05-07 14:30:00", "2012-05-08 14:05:00", "2012-05-08
      21:21:00",
      "2012-05-09 15:02:00", "2012-05-10 14:27:00", "2012-05-11
      14:47:00",
      "2012-05-12 13:24:00", "2012-05-13 14:40:00", "2012-05-13
      19:24:00",
      "2012-05-14 14:33:00", "2012-05-15 14:39:00", "2012-05-16
      14:07:00",
      "2012-05-17 13:42:00", "2012-05-18 14:30:00", "2012-05-19
      13:59:00",
      "2012-05-20 14:14:00", "2012-05-21 14:15:00", "2012-05-22
      14:18:00",
      "2012-05-23 14:14:00", "2012-05-24 13:52:00"), class =
      "factor"),
          fcst = c(38.7, 38.9, 39.2, 39.8, 40.5, 41.5, 42.5, 43.1,
          43.9, 44.5, 44.8, 45, 45, 45.1, 45.4, 45.8, 46.4, 47, 47.5,
          47.9, 39.2, 40, 41.8, 42.2, 42.8, 44.4, 45.7, 46.2, 46.2,
          46.2, 46.5, 47, 47.5, 47.9, 48.4, 49, 49.5, 49.9, 50.2,
      50.4,
          41.3, 42.5, 42.8, 42.8, 42.8, 42.9, 43, 43.3, 43.5, 43.8),
          usgs = c(38.5, 38.6, 38.6, 38.6, 39.1, 39.8, 41.2, 42.9,
          43.4, 43.6, 43.4, 43.1, 43, 43.1, 43.5, 43.8, 44.1, 44.2,
          44.4, 44.5, 39.1, 39.8, 41.2, 42.9, 43.4, 43.6, 43.4, 43.1,
          43, 43.1, 43.5, 43.8, 44.1, 44.2, 44.4, 44.5, 44.5, 44.5,
          44.6, 44.2, 41.2, 42.9, 43.4, 43.6, 43.4, 43.1, 43, 43.1,
          43.5, 43.8)), .Names = c("date", "basistime", "fcst", "usgs"
      ), row.names = c(NA, 50L), class = "data.frame")

Sys.setenv( "Etc/GMT+8" ) # or whatever is appropriate
#> Error in Sys.setenv("Etc/GMT+8"): all arguments must be named

dta2 <- dta
# bad idea to work with dates as factors
dta2$date <- as.POSIXct( as.character( dta2$date ) )
dta2$basistime <- as.POSIXct( as.character( dta2$basistime ) )

# base R solution

dates <- unique( dta2$date )
dta2list <- split( dta2, dta2$date )
grplist <- lapply( dta2list
                 , function( DF ) {
                     DF[ which.max( DF$fcst ), ]
                   }
                 )
result2 <- do.call( rbind, grplist )
result2
#>                                    date           basistime fcst
usgs
#> 2012-01-25 18:00:00 2012-01-25 18:00:00 2012-01-25 15:02:00 38.7
38.5
#> 2012-01-26 00:00:00 2012-01-26 00:00:00 2012-01-25 15:02:00 38.9
38.6
#> 2012-01-26 06:00:00 2012-01-26 06:00:00 2012-01-25 15:02:00 39.2
38.6
#> 2012-01-26 12:00:00 2012-01-26 12:00:00 2012-01-25 15:02:00 39.8
38.6
#> 2012-01-26 18:00:00 2012-01-26 18:00:00 2012-01-25 15:02:00 40.5
39.1
#> 2012-01-27 00:00:00 2012-01-27 00:00:00 2012-01-25 15:02:00 41.5
39.8
#> 2012-01-27 06:00:00 2012-01-27 06:00:00 2012-01-25 15:02:00 42.5
41.2
#> 2012-01-27 12:00:00 2012-01-27 12:00:00 2012-01-25 15:02:00 43.1
42.9
#> 2012-01-27 18:00:00 2012-01-27 18:00:00 2012-01-25 15:02:00 43.9
43.4
#> 2012-01-28 00:00:00 2012-01-28 00:00:00 2012-01-25 15:02:00 44.5
43.6
#> 2012-01-28 06:00:00 2012-01-28 06:00:00 2012-01-26 15:11:00 45.7
43.4
#> 2012-01-28 12:00:00 2012-01-28 12:00:00 2012-01-26 15:11:00 46.2
43.1
#> 2012-01-28 18:00:00 2012-01-28 18:00:00 2012-01-26 15:11:00 46.2
43.0
#> 2012-01-29 00:00:00 2012-01-29 00:00:00 2012-01-26 15:11:00 46.2
43.1
#> 2012-01-29 06:00:00 2012-01-29 06:00:00 2012-01-26 15:11:00 46.5
43.5
#> 2012-01-29 12:00:00 2012-01-29 12:00:00 2012-01-26 15:11:00 47.0
43.8
#> 2012-01-29 18:00:00 2012-01-29 18:00:00 2012-01-26 15:11:00 47.5
44.1
#> 2012-01-30 00:00:00 2012-01-30 00:00:00 2012-01-26 15:11:00 47.9
44.2
#> 2012-01-30 06:00:00 2012-01-30 06:00:00 2012-01-26 15:11:00 48.4
44.4
#> 2012-01-30 12:00:00 2012-01-30 12:00:00 2012-01-26 15:11:00 49.0
44.5
#> 2012-01-30 18:00:00 2012-01-30 18:00:00 2012-01-26 15:11:00 49.5
44.5
#> 2012-01-31 00:00:00 2012-01-31 00:00:00 2012-01-26 15:11:00 49.9
44.5
#> 2012-01-31 06:00:00 2012-01-31 06:00:00 2012-01-26 15:11:00 50.2
44.6
#> 2012-01-31 12:00:00 2012-01-31 12:00:00 2012-01-26 15:11:00 50.4
44.2

# alternately, use tidyverse

library(dplyr)
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#>     filter, lag
#> The following objects are masked from 'package:base':
#>
#>     intersect, setdiff, setequal, union
result3 <- (   dta2
           %>% group_by( date )
           %>% do({
                 DF <- .
                 DF[ which.max( DF$fcst ), ]
               })
           %>% ungroup
           %>% as.data.frame
           )
result3
#>                   date           basistime fcst usgs
#> 1  2012-01-25 18:00:00 2012-01-25 15:02:00 38.7 38.5
#> 2  2012-01-26 00:00:00 2012-01-25 15:02:00 38.9 38.6
#> 3  2012-01-26 06:00:00 2012-01-25 15:02:00 39.2 38.6
#> 4  2012-01-26 12:00:00 2012-01-25 15:02:00 39.8 38.6
#> 5  2012-01-26 18:00:00 2012-01-25 15:02:00 40.5 39.1
#> 6  2012-01-27 00:00:00 2012-01-25 15:02:00 41.5 39.8
#> 7  2012-01-27 06:00:00 2012-01-25 15:02:00 42.5 41.2
#> 8  2012-01-27 12:00:00 2012-01-25 15:02:00 43.1 42.9
#> 9  2012-01-27 18:00:00 2012-01-25 15:02:00 43.9 43.4
#> 10 2012-01-28 00:00:00 2012-01-25 15:02:00 44.5 43.6
#> 11 2012-01-28 06:00:00 2012-01-26 15:11:00 45.7 43.4
#> 12 2012-01-28 12:00:00 2012-01-26 15:11:00 46.2 43.1
#> 13 2012-01-28 18:00:00 2012-01-26 15:11:00 46.2 43.0
#> 14 2012-01-29 00:00:00 2012-01-26 15:11:00 46.2 43.1
#> 15 2012-01-29 06:00:00 2012-01-26 15:11:00 46.5 43.5
#> 16 2012-01-29 12:00:00 2012-01-26 15:11:00 47.0 43.8
#> 17 2012-01-29 18:00:00 2012-01-26 15:11:00 47.5 44.1
#> 18 2012-01-30 00:00:00 2012-01-26 15:11:00 47.9 44.2
#> 19 2012-01-30 06:00:00 2012-01-26 15:11:00 48.4 44.4
#> 20 2012-01-30 12:00:00 2012-01-26 15:11:00 49.0 44.5
#> 21 2012-01-30 18:00:00 2012-01-26 15:11:00 49.5 44.5
#> 22 2012-01-31 00:00:00 2012-01-26 15:11:00 49.9 44.5
#> 23 2012-01-31 06:00:00 2012-01-26 15:11:00 50.2 44.6
#> 24 2012-01-31 12:00:00 2012-01-26 15:11:00 50.4 44.2

####################################################


On Thu, 26 Oct 2017, Thomas Adams wrote:

      Hello all!

      I've been struggling with is for many hours today; I'm
      close to getting
      what I want, but not close enough...

      I have a dataframe consisting of two date-time columns
      followed by two
      numeric columns. what I need is the max value (in the
      first numeric column)
      based on the 2nd date-time column, which is essentially a
      factor. But, I
      want the result to provide both date-time values
      corresponding to the max
      value.

      My data:

      structure(list(date = structure(c(1L, 2L, 3L, 4L, 5L, 6L,
      7L,
      8L, 9L, 10L, 11L, 12L, 13L, 14L, 15L, 16L, 17L, 18L, 19L,
      20L,
      5L, 6L, 7L, 8L, 9L, 10L, 11L, 12L, 13L, 14L, 15L, 16L,
      17L, 18L,
      19L, 20L, 21L, 22L, 23L, 24L, 7L, 8L, 9L, 10L, 11L, 12L,
      13L,
      14L, 15L, 16L), .Label = c("2012-01-25 18:00:00",
      "2012-01-26 00:00:00",
      "2012-01-26 06:00:00", "2012-01-26 12:00:00", "2012-01-26
      18:00:00",
      "2012-01-27 00:00:00", "2012-01-27 06:00:00", "2012-01-27
      12:00:00",
      "2012-01-27 18:00:00", "2012-01-28 00:00:00", "2012-01-28
      06:00:00",
      "2012-01-28 12:00:00", "2012-01-28 18:00:00", "2012-01-29
      00:00:00",
      "2012-01-29 06:00:00", "2012-01-29 12:00:00", "2012-01-29
      18:00:00",
      "2012-01-30 00:00:00", "2012-01-30 06:00:00", "2012-01-30
      12:00:00",
      "2012-01-30 18:00:00", "2012-01-31 00:00:00", "2012-01-31
      06:00:00",
      "2012-01-31 12:00:00", "2012-01-31 13:00:00", "2012-01-31
      18:00:00",
      "2012-02-01 00:00:00", "2012-02-01 06:00:00", "2012-02-01
      12:00:00",
      "2012-02-01 18:00:00", "2012-02-02 00:00:00", "2012-02-02
      06:00:00",
      "2012-02-02 12:00:00", "2012-02-02 18:00:00", "2012-02-03
      00:00:00",
      "2012-02-03 06:00:00", "2012-02-03 12:00:00", "2012-02-03
      18:00:00",
      "2012-02-04 00:00:00", "2012-02-04 06:00:00", "2012-02-04
      12:00:00",
      "2012-02-04 18:00:00", "2012-02-05 00:00:00", "2012-02-05
      06:00:00",
      "2012-02-05 12:00:00", "2012-02-05 18:00:00", "2012-02-06
      00:00:00",
      "2012-02-06 06:00:00", "2012-02-06 12:00:00", "2012-02-06
      18:00:00",
      "2012-02-07 00:00:00", "2012-02-07 06:00:00", "2012-02-07
      12:00:00",
      "2012-02-07 18:00:00", "2012-02-08 00:00:00", "2012-02-08
      06:00:00",
      "2012-02-08 12:00:00", "2012-02-08 18:00:00", "2012-02-09
      00:00:00",
      "2012-02-09 06:00:00", "2012-02-09 12:00:00", "2012-02-09
      18:00:00",
      "2012-02-10 00:00:00", "2012-02-10 06:00:00", "2012-02-10
      12:00:00",
      "2012-02-10 18:00:00", "2012-02-11 00:00:00", "2012-02-11
      06:00:00",
      "2012-02-11 12:00:00", "2012-02-11 18:00:00", "2012-02-12
      00:00:00",
      "2012-02-12 06:00:00", "2012-02-12 12:00:00", "2012-02-12
      18:00:00",
      "2012-02-13 00:00:00", "2012-02-13 06:00:00", "2012-02-13
      12:00:00",
      "2012-02-13 18:00:00", "2012-02-14 00:00:00", "2012-02-14
      06:00:00",
      "2012-02-14 12:00:00", "2012-02-14 18:00:00", "2012-02-15
      00:00:00",
      "2012-02-15 06:00:00", "2012-02-15 12:00:00", "2012-02-15
      18:00:00",
      "2012-02-16 00:00:00", "2012-02-16 06:00:00", "2012-02-16
      12:00:00",
      "2012-02-16 18:00:00", "2012-02-17 00:00:00", "2012-02-17
      06:00:00",
      "2012-02-17 12:00:00", "2012-02-17 18:00:00", "2012-02-18
      00:00:00",
      "2012-02-18 06:00:00", "2012-02-18 12:00:00", "2012-02-18
      18:00:00",
      "2012-02-19 00:00:00", "2012-02-19 06:00:00", "2012-02-19
      12:00:00",
      "2012-02-19 18:00:00", "2012-02-20 00:00:00", "2012-02-20
      06:00:00",
      "2012-02-20 12:00:00", "2012-02-20 18:00:00", "2012-02-21
      00:00:00",
      "2012-02-21 06:00:00", "2012-02-21 12:00:00", "2012-02-21
      18:00:00",
      "2012-02-22 00:00:00", "2012-02-22 06:00:00", "2012-02-22
      12:00:00",
      "2012-02-22 18:00:00", "2012-02-23 00:00:00", "2012-02-23
      06:00:00",
      "2012-02-23 12:00:00", "2012-02-23 18:00:00", "2012-02-24
      00:00:00",
      "2012-02-24 06:00:00", "2012-02-24 12:00:00", "2012-02-24
      18:00:00",
      "2012-02-25 00:00:00", "2012-02-25 06:00:00", "2012-02-25
      12:00:00",
      "2012-02-25 18:00:00", "2012-02-26 00:00:00", "2012-02-26
      06:00:00",
      "2012-02-26 12:00:00", "2012-02-26 18:00:00", "2012-02-27
      00:00:00",
      "2012-02-27 06:00:00", "2012-02-27 12:00:00", "2012-02-27
      18:00:00",
      "2012-02-28 00:00:00", "2012-02-28 06:00:00", "2012-02-28
      12:00:00",
      "2012-02-28 18:00:00", "2012-02-29 00:00:00", "2012-02-29
      06:00:00",
      "2012-02-29 12:00:00", "2012-02-29 18:00:00", "2012-03-01
      00:00:00",
      "2012-03-01 06:00:00", "2012-03-01 12:00:00", "2012-03-01
      18:00:00",
      "2012-03-02 00:00:00", "2012-03-02 06:00:00", "2012-03-02
      12:00:00",
      "2012-03-02 18:00:00", "2012-03-03 00:00:00", "2012-03-03
      06:00:00",
      "2012-03-03 12:00:00", "2012-03-03 18:00:00", "2012-03-04
      00:00:00",
      "2012-03-04 06:00:00", "2012-03-04 12:00:00", "2012-03-04
      18:00:00",
      "2012-03-05 00:00:00", "2012-03-05 06:00:00", "2012-03-05
      12:00:00",
      "2012-03-05 18:00:00", "2012-03-06 00:00:00", "2012-03-06
      06:00:00",
      "2012-03-06 12:00:00", "2012-03-06 18:00:00", "2012-03-07
      00:00:00",
      "2012-03-07 06:00:00", "2012-03-07 12:00:00", "2012-03-07
      18:00:00",
      "2012-03-08 00:00:00", "2012-03-08 06:00:00", "2012-03-08
      12:00:00",
      "2012-03-08 18:00:00", "2012-03-09 00:00:00", "2012-03-09
      06:00:00",
      "2012-03-09 12:00:00", "2012-03-09 18:00:00", "2012-03-10
      00:00:00",
      "2012-03-10 06:00:00", "2012-03-10 12:00:00", "2012-03-10
      18:00:00",
      "2012-03-11 00:00:00", "2012-03-11 06:00:00", "2012-03-11
      12:00:00",
      "2012-03-11 18:00:00", "2012-03-12 00:00:00", "2012-03-12
      06:00:00",
      "2012-03-12 12:00:00", "2012-03-12 18:00:00", "2012-03-13
      00:00:00",
      "2012-03-13 06:00:00", "2012-03-13 12:00:00", "2012-03-13
      18:00:00",
      "2012-03-14 00:00:00", "2012-03-14 06:00:00", "2012-03-14
      12:00:00",
      "2012-03-14 18:00:00", "2012-03-15 00:00:00", "2012-03-15
      06:00:00",
      "2012-03-15 12:00:00", "2012-03-15 18:00:00", "2012-03-16
      00:00:00",
      "2012-03-16 06:00:00", "2012-03-16 12:00:00", "2012-03-16
      18:00:00",
      "2012-03-17 00:00:00", "2012-03-17 06:00:00", "2012-03-17
      12:00:00",
      "2012-03-17 18:00:00", "2012-03-18 00:00:00", "2012-03-18
      06:00:00",
      "2012-03-18 12:00:00", "2012-03-18 18:00:00", "2012-03-19
      00:00:00",
      "2012-03-19 06:00:00", "2012-03-19 12:00:00", "2012-03-19
      18:00:00",
      "2012-03-20 00:00:00", "2012-03-20 06:00:00", "2012-03-20
      12:00:00",
      "2012-03-20 18:00:00", "2012-03-21 00:00:00", "2012-03-21
      06:00:00",
      "2012-03-21 12:00:00", "2012-03-21 18:00:00", "2012-03-22
      00:00:00",
      "2012-03-22 06:00:00", "2012-03-22 12:00:00", "2012-03-22
      18:00:00",
      "2012-03-23 00:00:00", "2012-03-23 06:00:00", "2012-03-23
      12:00:00",
      "2012-03-23 18:00:00", "2012-03-24 00:00:00", "2012-03-24
      06:00:00",
      "2012-03-24 12:00:00", "2012-03-24 18:00:00", "2012-03-25
      00:00:00",
      "2012-03-25 06:00:00", "2012-03-25 12:00:00", "2012-03-25
      18:00:00",
      "2012-03-26 00:00:00", "2012-03-26 06:00:00", "2012-03-26
      12:00:00",
      "2012-03-26 18:00:00", "2012-03-27 00:00:00", "2012-03-27
      06:00:00",
      "2012-03-27 12:00:00", "2012-03-27 18:00:00", "2012-03-28
      00:00:00",
      "2012-03-28 06:00:00", "2012-03-28 12:00:00", "2012-03-28
      18:00:00",
      "2012-03-29 00:00:00", "2012-03-29 06:00:00", "2012-03-29
      12:00:00",
      "2012-03-29 18:00:00", "2012-03-30 00:00:00", "2012-03-30
      06:00:00",
      "2012-03-30 12:00:00", "2012-03-30 18:00:00", "2012-03-31
      00:00:00",
      "2012-03-31 06:00:00", "2012-03-31 12:00:00", "2012-03-31
      18:00:00",
      "2012-04-01 00:00:00", "2012-04-01 06:00:00", "2012-04-01
      12:00:00",
      "2012-04-01 18:00:00", "2012-04-02 00:00:00", "2012-04-02
      06:00:00",
      "2012-04-02 12:00:00", "2012-04-02 18:00:00", "2012-04-03
      00:00:00",
      "2012-04-03 06:00:00", "2012-04-03 12:00:00", "2012-04-03
      18:00:00",
      "2012-04-04 00:00:00", "2012-04-04 06:00:00", "2012-04-04
      12:00:00",
      "2012-04-04 18:00:00", "2012-04-05 00:00:00", "2012-04-05
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      "2012-04-05 12:00:00", "2012-04-05 18:00:00", "2012-04-06
      00:00:00",
      "2012-04-06 06:00:00", "2012-04-06 12:00:00", "2012-04-06
      18:00:00",
      "2012-04-07 00:00:00", "2012-04-07 06:00:00", "2012-04-07
      12:00:00",
      "2012-04-07 18:00:00", "2012-04-08 00:00:00", "2012-04-08
      06:00:00",
      "2012-04-08 12:00:00", "2012-04-08 18:00:00", "2012-04-09
      00:00:00",
      "2012-04-09 06:00:00", "2012-04-09 12:00:00", "2012-04-09
      18:00:00",
      "2012-04-10 00:00:00", "2012-04-10 06:00:00", "2012-04-10
      12:00:00",
      "2012-04-10 18:00:00", "2012-04-11 00:00:00", "2012-04-11
      06:00:00",
      "2012-04-11 12:00:00", "2012-04-11 18:00:00", "2012-04-12
      00:00:00",
      "2012-04-12 06:00:00", "2012-04-12 12:00:00", "2012-04-12
      18:00:00",
      "2012-04-13 00:00:00", "2012-04-13 06:00:00", "2012-04-13
      12:00:00",
      "2012-04-13 18:00:00", "2012-04-14 00:00:00", "2012-04-14
      06:00:00",
      "2012-04-14 12:00:00", "2012-04-14 18:00:00", "2012-04-15
      00:00:00",
      "2012-04-15 06:00:00", "2012-04-15 12:00:00", "2012-04-15
      18:00:00",
      "2012-04-16 00:00:00", "2012-04-16 06:00:00", "2012-04-16
      12:00:00",
      "2012-04-16 18:00:00", "2012-04-17 00:00:00", "2012-04-17
      06:00:00",
      "2012-04-17 12:00:00", "2012-04-17 18:00:00", "2012-04-18
      00:00:00",
      "2012-04-18 06:00:00", "2012-04-18 12:00:00", "2012-04-18
      18:00:00",
      "2012-04-19 00:00:00", "2012-04-19 06:00:00", "2012-04-19
      12:00:00",
      "2012-04-19 18:00:00", "2012-04-20 00:00:00", "2012-04-20
      06:00:00",
      "2012-04-20 12:00:00", "2012-04-20 18:00:00", "2012-04-21
      00:00:00",
      "2012-04-21 06:00:00", "2012-04-21 12:00:00", "2012-04-21
      18:00:00",
      "2012-04-22 00:00:00", "2012-04-22 06:00:00", "2012-04-22
      12:00:00",
      "2012-04-22 18:00:00", "2012-04-23 00:00:00", "2012-04-23
      06:00:00",
      "2012-04-23 12:00:00", "2012-04-23 18:00:00", "2012-04-24
      00:00:00",
      "2012-04-24 06:00:00", "2012-04-24 12:00:00", "2012-04-24
      18:00:00",
      "2012-04-25 00:00:00", "2012-04-25 06:00:00", "2012-04-25
      12:00:00",
      "2012-04-25 18:00:00", "2012-04-26 00:00:00", "2012-04-26
      06:00:00",
      "2012-04-26 12:00:00", "2012-04-26 18:00:00", "2012-04-27
      00:00:00",
      "2012-04-27 06:00:00", "2012-04-27 12:00:00", "2012-04-27
      18:00:00",
      "2012-04-28 00:00:00", "2012-04-28 06:00:00", "2012-04-28
      12:00:00",
      "2012-04-28 18:00:00", "2012-04-29 00:00:00", "2012-04-29
      06:00:00",
      "2012-04-29 12:00:00", "2012-04-29 18:00:00", "2012-04-30
      00:00:00",
      "2012-04-30 06:00:00", "2012-04-30 12:00:00", "2012-04-30
      18:00:00",
      "2012-05-01 00:00:00", "2012-05-01 06:00:00", "2012-05-01
      12:00:00",
      "2012-05-01 18:00:00", "2012-05-02 00:00:00", "2012-05-02
      06:00:00",
      "2012-05-02 12:00:00", "2012-05-02 18:00:00", "2012-05-03
      00:00:00",
      "2012-05-03 06:00:00", "2012-05-03 12:00:00", "2012-05-03
      18:00:00",
      "2012-05-04 00:00:00", "2012-05-04 06:00:00", "2012-05-04
      12:00:00",
      "2012-05-04 18:00:00", "2012-05-05 00:00:00", "2012-05-05
      06:00:00",
      "2012-05-05 12:00:00", "2012-05-05 18:00:00", "2012-05-06
      00:00:00",
      "2012-05-06 06:00:00", "2012-05-06 12:00:00", "2012-05-06
      18:00:00",
      "2012-05-07 00:00:00", "2012-05-07 06:00:00", "2012-05-07
      12:00:00",
      "2012-05-07 18:00:00", "2012-05-08 00:00:00", "2012-05-08
      06:00:00",
      "2012-05-08 12:00:00", "2012-05-08 18:00:00", "2012-05-09
      00:00:00",
      "2012-05-09 06:00:00", "2012-05-09 12:00:00", "2012-05-09
      18:00:00",
      "2012-05-10 00:00:00", "2012-05-10 06:00:00", "2012-05-10
      12:00:00",
      "2012-05-10 18:00:00", "2012-05-11 00:00:00", "2012-05-11
      06:00:00",
      "2012-05-11 12:00:00", "2012-05-11 18:00:00", "2012-05-12
      00:00:00",
      "2012-05-12 06:00:00", "2012-05-12 12:00:00", "2012-05-12
      18:00:00",
      "2012-05-13 00:00:00", "2012-05-13 06:00:00", "2012-05-13
      12:00:00",
      "2012-05-13 18:00:00", "2012-05-14 00:00:00", "2012-05-14
      06:00:00",
      "2012-05-14 12:00:00", "2012-05-14 18:00:00", "2012-05-15
      00:00:00",
      "2012-05-15 06:00:00", "2012-05-15 12:00:00", "2012-05-15
      18:00:00",
      "2012-05-16 00:00:00", "2012-05-16 06:00:00", "2012-05-16
      12:00:00",
      "2012-05-16 18:00:00", "2012-05-17 00:00:00", "2012-05-17
      06:00:00",
      "2012-05-17 12:00:00", "2012-05-17 18:00:00", "2012-05-18
      00:00:00",
      "2012-05-18 06:00:00", "2012-05-18 12:00:00", "2012-05-18
      18:00:00",
      "2012-05-19 00:00:00", "2012-05-19 06:00:00", "2012-05-19
      12:00:00",
      "2012-05-19 18:00:00", "2012-05-20 00:00:00", "2012-05-20
      06:00:00",
      "2012-05-20 12:00:00", "2012-05-20 18:00:00", "2012-05-21
      00:00:00",
      "2012-05-21 06:00:00", "2012-05-21 12:00:00", "2012-05-21
      18:00:00",
      "2012-05-22 00:00:00", "2012-05-22 06:00:00", "2012-05-22
      12:00:00",
      "2012-05-22 18:00:00", "2012-05-23 00:00:00", "2012-05-23
      06:00:00",
      "2012-05-23 12:00:00", "2012-05-23 18:00:00", "2012-05-24
      00:00:00",
      "2012-05-24 06:00:00", "2012-05-24 12:00:00", "2012-05-24
      18:00:00",
      "2012-05-25 00:00:00", "2012-05-25 06:00:00", "2012-05-25
      12:00:00",
      "2012-05-25 18:00:00", "2012-05-26 00:00:00", "2012-05-26
      06:00:00",
      "2012-05-26 12:00:00", "2012-05-26 18:00:00", "2012-05-27
      00:00:00",
      "2012-05-27 06:00:00", "2012-05-27 12:00:00", "2012-05-27
      18:00:00",
      "2012-05-28 00:00:00", "2012-05-28 06:00:00", "2012-05-28
      12:00:00",
      "2012-05-28 18:00:00", "2012-05-29 00:00:00", "2012-05-29
      06:00:00",
      "2012-05-29 12:00:00"), class = "factor"), basistime =
      structure(c(1L,
      1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
      1L, 1L,
      1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
      2L, 2L,
      2L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L,
      3L, 3L,
      3L), .Label = c("2012-01-25 15:02:00", "2012-01-26
      15:11:00",
      "2012-01-27 01:41:00", "2012-01-27 10:15:00", "2012-01-27
      15:15:00",
      "2012-01-28 14:22:00", "2012-01-29 13:33:00", "2012-01-30
      14:11:00",
      "2012-01-31 14:24:00", "2012-02-01 14:55:00", "2012-02-02
      14:56:00",
      "2012-02-03 14:40:00", "2012-02-04 15:01:00", "2012-02-05
      15:04:00",
      "2012-02-06 14:37:00", "2012-02-07 14:42:00", "2012-02-08
      14:44:00",
      "2012-02-09 14:27:00", "2012-02-10 14:33:00", "2012-02-11
      15:05:00",
      "2012-02-12 15:09:00", "2012-02-13 15:00:00", "2012-02-14
      15:10:00",
      "2012-02-15 14:46:00", "2012-02-16 14:42:00", "2012-02-17
      14:14:00",
      "2012-02-18 15:27:00", "2012-02-19 13:13:00", "2012-02-20
      13:56:00",
      "2012-02-21 14:39:00", "2012-02-22 15:33:00", "2012-02-23
      15:14:00",
      "2012-02-24 15:14:00", "2012-02-25 14:25:00", "2012-02-26
      14:08:00",
      "2012-02-27 14:55:00", "2012-02-28 14:40:00", "2012-02-29
      14:45:00",
      "2012-03-01 15:18:00", "2012-03-02 15:18:00", "2012-03-03
      15:43:00",
      "2012-03-04 15:16:00", "2012-03-05 15:17:00", "2012-03-06
      15:03:00",
      "2012-03-07 14:44:00", "2012-03-08 14:53:00", "2012-03-09
      15:10:00",
      "2012-03-10 14:43:00", "2012-03-11 13:37:00", "2012-03-12
      14:58:00",
      "2012-03-13 14:36:00", "2012-03-14 13:49:00", "2012-03-15
      14:32:00",
      "2012-03-16 15:01:00", "2012-03-17 13:55:00", "2012-03-18
      13:44:00",
      "2012-03-19 13:55:00", "2012-03-20 13:53:00", "2012-03-21
      14:07:00",
      "2012-03-22 14:03:00", "2012-03-23 14:30:00", "2012-03-24
      14:47:00",
      "2012-03-25 14:32:00", "2012-03-26 14:41:00", "2012-03-27
      13:51:00",
      "2012-03-28 13:32:00", "2012-03-29 13:59:00", "2012-03-30
      14:15:00",
      "2012-03-31 14:12:00", "2012-04-01 14:30:00", "2012-04-02
      13:43:00",
      "2012-04-03 13:54:00", "2012-04-04 13:45:00", "2012-04-05
      13:46:00",
      "2012-04-06 13:41:00", "2012-04-07 13:13:00", "2012-04-08
      12:59:00",
      "2012-04-09 13:53:00", "2012-04-10 14:15:00", "2012-04-11
      14:26:00",
      "2012-04-12 14:05:00", "2012-04-13 13:37:00", "2012-04-14
      13:52:00",
      "2012-04-15 15:00:00", "2012-04-16 14:42:00", "2012-04-17
      14:21:00",
      "2012-04-18 14:40:00", "2012-04-19 14:35:00", "2012-04-20
      15:14:00",
      "2012-04-21 14:46:00", "2012-04-22 14:44:00", "2012-04-23
      14:33:00",
      "2012-04-24 14:41:00", "2012-04-25 14:13:00", "2012-04-26
      14:19:00",
      "2012-04-27 14:35:00", "2012-04-28 13:48:00", "2012-04-29
      14:12:00",
      "2012-04-30 13:53:00", "2012-05-02 14:41:00", "2012-05-03
      15:01:00",
      "2012-05-04 15:21:00", "2012-05-05 14:57:00", "2012-05-06
      14:09:00",
      "2012-05-07 14:30:00", "2012-05-08 14:05:00", "2012-05-08
      21:21:00",
      "2012-05-09 15:02:00", "2012-05-10 14:27:00", "2012-05-11
      14:47:00",
      "2012-05-12 13:24:00", "2012-05-13 14:40:00", "2012-05-13
      19:24:00",
      "2012-05-14 14:33:00", "2012-05-15 14:39:00", "2012-05-16
      14:07:00",
      "2012-05-17 13:42:00", "2012-05-18 14:30:00", "2012-05-19
      13:59:00",
      "2012-05-20 14:14:00", "2012-05-21 14:15:00", "2012-05-22
      14:18:00",
      "2012-05-23 14:14:00", "2012-05-24 13:52:00"), class =
      "factor"),
         fcst = c(38.7, 38.9, 39.2, 39.8, 40.5, 41.5, 42.5,
      43.1,
         43.9, 44.5, 44.8, 45, 45, 45.1, 45.4, 45.8, 46.4, 47,
      47.5,
         47.9, 39.2, 40, 41.8, 42.2, 42.8, 44.4, 45.7, 46.2,
      46.2,
         46.2, 46.5, 47, 47.5, 47.9, 48.4, 49, 49.5, 49.9, 50.2,
      50.4,
         41.3, 42.5, 42.8, 42.8, 42.8, 42.9, 43, 43.3, 43.5,
      43.8),
         usgs = c(38.5, 38.6, 38.6, 38.6, 39.1, 39.8, 41.2,
      42.9,
         43.4, 43.6, 43.4, 43.1, 43, 43.1, 43.5, 43.8, 44.1,
      44.2,
         44.4, 44.5, 39.1, 39.8, 41.2, 42.9, 43.4, 43.6, 43.4,
      43.1,
         43, 43.1, 43.5, 43.8, 44.1, 44.2, 44.4, 44.5, 44.5,
      44.5,
         44.6, 44.2, 41.2, 42.9, 43.4, 43.6, 43.4, 43.1, 43,
      43.1,
         43.5, 43.8)), .Names = c("date", "basistime", "fcst",
      "usgs"
      ), row.names = c(NA, 50L), class = "data.frame")

      aggregate(fcst ~ basistime,data=legacy, FUN= max)

      A snippet...

                   basistime fcst
      1   2012-01-25 15:02:00 47.9
      2   2012-01-26 15:11:00 50.4
      3   2012-01-27 01:41:00 46.0
      4   2012-01-27 10:15:00 47.3
      5   2012-01-27 15:15:00 47.3
      6   2012-01-28 14:22:00 46.2
      7   2012-01-29 13:33:00 45.8
      8   2012-01-30 14:11:00 44.8
      9   2012-01-31 14:24:00 43.9
      10  2012-02-01 14:55:00 41.1
      11  2012-02-02 14:56:00 38.1
      12  2012-02-03 14:40:00 36.2
      13  2012-02-04 15:01:00 34.7
      14  2012-02-05 15:04:00 33.1
      15  2012-02-06 14:37:00 32.2

      This is what I want, except I need the other corresponding
      date-time column
      as well. I've tried this with the date-times as factors
      and as POSIXct
      values and using many of the combinations suggested in the
      documentation
      and other examples. The closest I can get returns the
      second date as a
      numeric value:

      aggregate(cbind(fcst,date) ~ basistime,data=legacy, FUN=
      max)

      Produces:

                   basistime fcst       date
      1   2012-01-25 15:02:00 47.9 1327942800
      2   2012-01-26 15:11:00 50.4 1328029200
      3   2012-01-27 01:41:00 46.0 1328072400
      4   2012-01-27 10:15:00 47.3 1328032800
      5   2012-01-27 15:15:00 47.3 1328115600
      6   2012-01-28 14:22:00 46.2 1328202000
      7   2012-01-29 13:33:00 45.8 1328288400
      8   2012-01-30 14:11:00 44.8 1328374800
      9   2012-01-31 14:24:00 43.9 1328461200
      10  2012-02-01 14:55:00 41.1 1328547600
      11  2012-02-02 14:56:00 38.1 1328634000
      12  2012-02-03 14:40:00 36.2 1328720400
      13  2012-02-04 15:01:00 34.7 1328806800
      14  2012-02-05 15:04:00 33.1 1328893200
      15  2012-02-06 14:37:00 32.2 1328979600
      16  2012-02-07 14:42:00 31.2 1329066000
      17  2012-02-08 14:44:00 30.4 1329152400
      18  2012-02-09 14:27:00 30.0 1329238800


      Help is greatly appreciated!

      Regards,
      Tom

      --

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--
Thomas E Adams, III1724 Sage Lane
Blacksburg, VA 24060
tea...@gmail.com (personal)
t...@terrapredictions.org (work)

1 (513) 739-9512 (cell)





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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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