如何根据列条件从R中的拆分列表中指定行

Question

I am new to R and to programming in general and am looking for feedback on how to approach what is probably a fairly simple problem in R.

I have the following dataset:

df <- data.frame(county = rep(c("QU","AN","GY"), 3),
                 park = (c("Downtown","Queens", "Oakville","Squirreltown",
                          "Pinhurst", "GarbagePile","LottaTrees","BigHill", 
                          "Jaynestown")),
                 hectares = c(12,42,6,18,92,6,4,52,12))
df<-transform(df, parkrank = ave(hectares, county, 
           FUN = function(x) rank(x, ties.method = "first")))

Which returns a dataframe looking like this:

 county         park hectares parkrank
1     QU     Downtown       12        2
2     AN       Queens       42        1
3     GY     Oakville        6        1
4     QU Squirreltown       18        3
5     AN     Pinhurst       92        3
6     GY  GarbagePile        6        2
7     QU   LottaTrees        4        1
8     AN      BigHill       52        2
9     GY   Jaynestown       12        3

I want to use this to create a two-column data frame that lists each county and the park name corresponding to a specific rank (eg if when I call my function I add "2" as a variable, shows the second biggest park in each county).

I am very new to R and programming and have spent hours looking over the built in R help files and similar questions here on stack overflow but I am clearly missing something. Can anyone give a simple example of where to begin? It seems like I should be using split then lapply or maybe tapply, but everything I try leaves me very confused :(

Thanks.

Answer 1

Try,

df2  <- function(A,x) {
  # A is the name of the data.frame() and x is the rank No
  df  <- A[A[,4]==x,]
  return(df)  
}

> df2(df,2)
  county        park hectares parkrank
1     QU    Downtown       12        2
6     GY GarbagePile        6        2
8     AN     BigHill       52        2