Try this:

A.split[[1]]["time"]
A.split[[1]][["concentration"]]

test <- NULL
for(i in 2:4){
    test[i] <- (A.split[[1]][["time"]][i] - A.split[[1]][["time"]][i-1]) *
   (A.split[[1]][["concentration"]][i] - A.split[[1]][["concentration"]][i-1])
* 0.5
}


On 28/11/2007, leeznar <leeznar@yahoo.com.tw> wrote:
>
>
>
> Dear all:
>
>
> I am a new R-user and I have 2 questions
> about it.
>
>
> 1) I need to find specific sub-dataframe,
> and then use specific column to calculate.
> For example, after splitting dataframe, I find specific the
> sub-dataframe, such as "A.split [1]".
> But, I don't know how to find "time" and "concentration" columns of
> "A.split
> [1]".
>
>
> 2) The equation used to sub-dataframe is
> (time[i]-time[i-1])*(concentration[i]- concentration[i-1])*1/2.  I don't
> know how to calculate it.  How can I find the specific column and use it to
> calculate ?
>
>
>
>
>
>
> x<-rep(c(1.2,6.8),4)
>
>
> y<-rep(c(1, 2),4)
>
>
> z<-rep(c(1,2),4)
>
>
> t<-rep(c(0,1,2,3),length.out=8)
>
>
> c<-rep(c(0,0.5,1,2),length.out=8)
>
>
> df<-data.frame(pH=x, formulation=y,
> subject=z, time=t, concentration=c)
>
>
> A.split<-split(df, list(df$pH
> ,df$formulation, df$subject) )
>
>
> A.split [1]
>
>
>
>
>
>
>
> Best regards,
>
>
> Hsin-Ya Lee
>
>
>
>
>
>
>
> ______________________________________________________________________________________
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>



-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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