[R] dataframe calculations based on certain values of a column

[Bert Gunter]

I believe this will generalize. But check carefully!

Using your example (Excellent!), use ave():

with(df,ave(seq_along(var1),var2,FUN=function(i)
var3[i]/var3[i][var1[i]=="c"]))

[1] 0.5000000 1.0000000 1.0000000 0.8333333 0.3333333 1.0000000 1.7500000
[8] 1.0000000 1.0000000

This is kind of a low level brute force approach. Others may have more
elegant approaches.

-- Bert


Bert Gunter
Genentech Nonclinical Biostatistics
(650) 467-7374

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
H. Gilbert Welch




On Wed, Mar 26, 2014 at 9:09 AM, Johannes Radinger
<johannesradinger at gmail.com> wrote:
> Hi,
>
> I have data in a dataframe in following structure
> var1 <- c("a","b","c","a","b","c","a","b","c")
> var2 <- c("X","X","X","Y","Y","Y","Z","Z","Z")
> var3 <- c(1,2,2,5,2,6,7,4,4)
> df <- data.frame(var1,var2,var3)
>
> Now I'd like to calculate relative values of var3. This values
> should be relative to the base value (where var1=c) which is
> indicated for each group (var2).
>
> To illustrate how my result column should look like I divide
> the column var3 by a vector c(2,2,2,6,6,6,4,4,4) (= for each group
> of var2 the value c)
>
> Of course this can also be done like this:
> df$div <- rep(df$var3[df$var1=="c"],each=length(unique(df$var1)))
> df$result_calc <- df$var3/df$div
>
>
> However what when the dataframe is not as simple and not that well ordered
> as
> in the example here. So for example there is always a value c for each group
> but all the "c"s are clumped in the last rows of the dataframe or scatterd
> in a random
> mannar. Is there a simple way to still calculate such relative values.
> Probably with an approach using apply, but maybe someone can give me a hint.
> Or do I need to sort my dataframe in order to do such calculations?
>
> best,
>
> /Johannes
>
>         [[alternative HTML version deleted]]
>
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