[R] Calculate the difference using ave

[Dimitris Rizopoulos]

Maybe one approach could be:

set.seed(123)
df1 <- data.frame(measure = rep(c("A1", "A2", "A3"), each=3),
                   water = sample(c(100:200), 9),
           tide  = sample(c(-10:+10), 9))


100 * tail(df1$tide, -3) / head(df1$water, -3)


I hope it helps.

Best,
Dimitris


On 10/26/2011 12:02 PM, Patrick Hausmann wrote:
> Dear R users,
>
> It may be very simple but it is being difficult for me.
> I'd like to calculate the difference in percent between to measures.
> My data looks like this:
>
> set.seed(123)
> df1 <- data.frame(measure = rep(c("A1", "A2", "A3"), each=3),
> water = sample(c(100:200), 9),
> tide = sample(c(-10:+10), 9))
> df1
>
> # What I want to calculate is:
> # tide_[A2] / water_[A1],
> # tide_[A3] / water_[A2]
>
> # This 'works' for the example, but I am
> # looking for a more general solution.
>
> df1$tide_diff <- ave(df1$tide, FUN=function(L) L /
> c(NA, NA, NA, df1$water)) * 100
> df1
>
> Thanks for any help!
> Patrick
>
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-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center

Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
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