# [R] Calculation of group summaries

From: <Seeliger.Curt_at_epamail.epa.gov>
Date: Wed 13 Jul 2005 - 03:51:03 EST

I know R has a steep learning curve, but from where I stand the slope looks like a sheer cliff. I'm pawing through the available docs and have come across examples which come close to what I want but are proving difficult for me to modify for my use.

Calculating simple group means is fairly straight forward:   data(PlantGrowth)
attach(PlantGrowth)
stack(mean(unstack(PlantGrowth)))

I'd like to do something slightly more complex, using a data frame and groups identified by unique combinations of three id variables. There may be thousands of such combinations in the data. This is easy in SQL:

```         site_id,
visit_no,
mean(undercut) AS meanUndercut,
count(undercut) AS nUndercut,
std(undercut) AS stdUndercut
```

from channelMorphology
group by year, site_id, visit_no

;

Reading a CSV written by SAS and selecting only records expected to have values is also straight forward in R, but getting those summary values for each site visit is currently beyond me:

,na.strings='.'
,sep=','
,strip.white=TRUE

)

undercut<-subset(sub,

```                  ,TRANSDIR %in% c('LF','RT')

,select=c('YEAR','SITE_ID','VISIT_NO','TRANSECT','TRANSDIR'
,'UNDERCUT'
)
,drop=TRUE
)

```

cur

```--
Curt Seeliger, Data Ranger
CSC, EPA/WED contractor
541/754-4638
seeliger.curt@epa.gov

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