How to select the rows with maximum values in each group with dplyr?

Question

I would like to select a row with maximum value in each group with dplyr.

Firstly I generate some random data to show my question

set.seed(1)
df <- expand.grid(list(A = 1:5, B = 1:5, C = 1:5))
df$value <- runif(nrow(df))

In plyr, I could use a custom function to select this row.

library(plyr)
ddply(df, .(A, B), function(x) x[which.max(x$value),])

In dplyr, I am using this code to get the maximum value, but not the rows with maximum value (Column C in this case).

library(dplyr)
df %>% group_by(A, B) %>%
    summarise(max = max(value))

How could I achieve this? Thanks for any suggestion.

sessionInfo()
R version 3.1.0 (2014-04-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252   
[3] LC_MONETARY=English_Australia.1252 LC_NUMERIC=C                      
[5] LC_TIME=English_Australia.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] dplyr_0.2  plyr_1.8.1

loaded via a namespace (and not attached):
[1] assertthat_0.1.0.99 parallel_3.1.0      Rcpp_0.11.1        
[4] tools_3.1.0        

Answer 1

Try this:

result <- df %>% 
             group_by(A, B) %>%
             filter(value == max(value)) %>%
             arrange(A,B,C)

Seems to work:

identical(
  as.data.frame(result),
  ddply(df, .(A, B), function(x) x[which.max(x$value),])
)
#[1] TRUE

As pointed out in the comments, slice may be preferred here as per @RoyalITS' answer below if you strictly only want 1 row per group. This answer will return multiple rows if there are multiple with an identical maximum value.