R help - Mar 2014 - dataframe calculations based on certain values of a column

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

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