r - How to replicate a ddply behavior that uses a custom function with dplyr? -

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i'm trying replace plyr calls dplyr. there still few snags , 1 of them group_by function. imagine acts same way second ddply argument , split, apply , combine based on grouping variables list. doesn't appear case. here rather trivial example.

let's define silly function

mm <- function(x) return(x[1:5, ])

now can split species in irisdataset , apply function each piece.

ddply(iris, .(species), mm)

this works intended. however, when try same dplyr, doesn't work expected. 

iris %>% group_by(species) %>% mm

what doing wrong?

as shown in ?do, can refer group . in expression. following replicate ddply output:

iris %>% group_by(species) %>% do(.[1:5, ])  # source: local data frame [15 x 5] # groups: species # #    sepal.length sepal.width petal.length petal.width    species # 1           5.1         3.5          1.4         0.2     setosa # 2           4.9         3.0          1.4         0.2     setosa # 3           4.7         3.2          1.3         0.2     setosa # 4           4.6         3.1          1.5         0.2     setosa # 5           5.0         3.6          1.4         0.2     setosa # 6           7.0         3.2          4.7         1.4 versicolor # 7           6.4         3.2          4.5         1.5 versicolor # 8           6.9         3.1          4.9         1.5 versicolor # 9           5.5         2.3          4.0         1.3 versicolor # 10          6.5         2.8          4.6         1.5 versicolor # 11          6.3         3.3          6.0         2.5  virginica # 12          5.8         2.7          5.1         1.9  virginica # 13          7.1         3.0          5.9         2.1  virginica # 14          6.3         2.9          5.6         1.8  virginica # 15          6.5         3.0          5.8         2.2  virginica

more generally, apply custom function groups dplyr, can following (thanks @docendodiscimus):

iris %>% group_by(species) %>% do(mm(.))

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