r - Extracting a list from a column, for each group in another column -


i have data frame in long format 2 factor variables columns, first column representing series of unique classes , second column representing grouping variable (in case, city class takes place). 

class <- c("a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k") city <- c("atlanta", "atlanta", "atlanta", "detroit", "detroit", "milwaukee", "milwaukee", "milwaukee", "milwaukee", "portland", "juneau") samp <- data.frame(class, city)  samp # class     city #       atlanta #   b     atlanta #   c     atlanta #   d     detroit #   e     detroit #   f     milwaukee #   g     milwaukee #   h     milwaukee #       milwaukee #   j     portland #   k     juneau

for each city, list of classes take place there. ideally, output this:

class.list <- list(atlanta = c("a", "b", "c"), detroit = c("d", "e"), milwaukee = c("f", "g", "h", "i"), portland = "j", juneau = "k")  class.list # $atlanta # [1] "a" "b" "c" # # $detroit # [1] "d" "e" # # $milwaukee # [1] "f" "g" "h" "i" # # $portland # [1] "j" # # $juneau # [1] "k"

i've tried various solutions , failed. arguably closest came via dcast, wasn't quite output aiming for:

library(reshape2) class.list <-dcast(samp, city ~ class)  class.list # place               b         c        d        e         f    ... # atlanta  atlanta   atlanta   atlanta    <na>     <na>      <na>  ... # detroit    <na>       <na>      <na>   detroit   detroit   <na>  ... # ...

we can use split

split(as.character(samp$class), samp$city) #$atlanta #[1] "a" "b" "c"  #$detroit #[1] "d" "e"  #$juneau #[1] "k"  #$milwaukee #[1] "f" "g" "h" "i"  #$portland #[1] "j"

or option unstack

unstack(samp, class~city)

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