R help - Jan 2011 - how to subset unique factor combinations from a data frame.

Hi All
 
I have these questions and request members expert view on this. 
 
a) I have a dataframe (df) with five factors (identity variables) and value
(measured value). The id variables are Year, Country, Commodity, Attribute,
Unit. Value is a value for each combination of this.
 
I would like to get just the unique combination of Commodity, Attribute and
Unit. I just need the unique factor combination into a dataframe or a table.
I know aggregate and subset but dont how to use them in this context. 
 
b) Is it possible to inclue non- aggregate columns with aggregate function
 
say in the above case > aggregate(Value ~ Commodity + Attribute, data = df,
FUN = count). The use of count(Value) is just a round about to return the
combinations of Commodity & Attribute, and I would like to include
'Unit'
column in the returned data frame?
 
c) Is it possible to subset based on unique combination, some thing like
this.
 
> subset(df, unique(Commodity), select = c(Commodity, Attribute, Unit)). I
know this is not correct as it returns an error 'subset needs a logical
evaluation'. Trying various ways to accomplish the task. 
 
will be grateful for any ideas and help 
 
Regards,
 
SNVK 

	[[alternative HTML version deleted]]

Hi

r-help-bounces at r-project.org napsal dne 04.01.2011 05:21:25:
> Hi All
> 
> I have these questions and request members expert view on this. 
> 
> a) I have a dataframe (df) with five factors (identity variables) and 
value
> (measured value). The id variables are Year, Country, Commodity, 
Attribute,
> Unit. Value is a value for each combination of this.
> 
> I would like to get just the unique combination of Commodity, Attribute 
and
> Unit. I just need the unique factor combination into a dataframe or a 
table.
> I know aggregate and subset but dont how to use them in this context. 
aggregate(Value, list(Comoditiy, Atribute, Unit), function)
> 
> b) Is it possible to inclue non- aggregate columns with aggregate 
function
> 
> say in the above case > aggregate(Value ~ Commodity + Attribute, data = 
df,
> FUN = count). The use of count(Value) is just a round about to return 
the
> combinations of Commodity & Attribute, and I would like to include 
'Unit'
> column in the returned data frame?
Hm. Maybe xtabs? But without any example it is only a guess.
> 
> c) Is it possible to subset based on unique combination, some thing like
> this.
> 
> > subset(df, unique(Commodity), select = c(Commodity, Attribute, Unit)).
I
> know this is not correct as it returns an error 'subset needs a logical
> evaluation'. Trying various ways to accomplish the task. 
> 
Probably sqldf package has tools for doing it but I do not use it so you 
have to try yourself.

df[Comodity==something, c("Commodity", "Attribute",
"Unit")]

can be other way.

Anyway your explanation is ambiguous. Let say you have three rows with the 
same Commodity. Which row do you want to select?

Regards
Petr

> will be grateful for any ideas and help 
> 
> Regards,
> 
> SNVK 
> 
>    [[alternative HTML version deleted]]
> 
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