r - Finding index of cummax inside a dplyr mutate? -

i have following code:

library(dplyr) set.seed(10) test<-data.frame(x=runif(10,0,1),y=rep(c(1,2),5)) test <- test %>%   group_by(y) %>%   mutate(max_then=cummax(x))  test 

which outputs

source: local data frame [10 x 3] groups: y              x y  max_then 1  0.50747820 1 0.5074782 2  0.30676851 2 0.3067685 3  0.42690767 1 0.5074782 4  0.69310208 2 0.6931021 5  0.08513597 1 0.5074782 6  0.22543662 2 0.6931021 7  0.27453052 1 0.5074782 8  0.27230507 2 0.6931021 9  0.61582931 1 0.6158293 10 0.42967153 2 0.6931021 

i want add mutated column add rownumber/index max_then calculated. imagine following. can't work.

test %>%  group_by(y) %>%      mutate(max_then-cummax(x),             max_index=which(.$x==max_then)) 

expected output is:

            x y  max_then max_index 1  0.50747820 1 0.5074782         1 2  0.30676851 2 0.3067685         2 3  0.42690767 1 0.5074782         1 4  0.69310208 2 0.6931021         4 5  0.08513597 1 0.5074782         1 6  0.22543662 2 0.6931021         4 7  0.27453052 1 0.5074782         1 8  0.27230507 2 0.6931021         4 9  0.61582931 1 0.6158293         9 10 0.42967153 2 0.6931021         4 

any suggestions? i'm curious see if 1 can within mutate() statement. can outside of mutate() statement.

i match unique instances in x

test %>%   mutate(max_index = match(max_then, unique(test$x))) # source: local data frame [10 x 4] # groups: y #  #              x y  max_then max_index #  1  0.50747820 1 0.5074782         1 #  2  0.30676851 2 0.3067685         2 #  3  0.42690767 1 0.5074782         1 #  4  0.69310208 2 0.6931021         4 #  5  0.08513597 1 0.5074782         1 #  6  0.22543662 2 0.6931021         4 #  7  0.27453052 1 0.5074782         1 #  8  0.27230507 2 0.6931021         4 #  9  0.61582931 1 0.6158293         9 #  10 0.42967153 2 0.6931021         4