clojure-dev

1. def groupBy[K](f : (A) => K) : Map[K, This]
Examples:

1. scala> val groups = (1 to 20).toList groupBy {
2. | case i if(i<5> "g1"
3. | case i if(i<10> "g2"
4. | case i if(i<15> "g3"
5. | case _ => "g4"
6. | }
7. res4: scala.collection.Map[java.lang.String,List[Int]] = Map(g1 -> List(1, 2, 3, 4), g2 -> List(5, 6, 7, 8, 9), g3 -> List(10, 11, 12, 13, 14), g4 -> List(15, 16, 17, 18, 19, 20))
8.
9. scala> groups.keySet
10. res6: scala.collection.Set[java.lang.String] = Set(g1, g2, g3, g4)
11.
12. scala> groups("g1")
13. res7: List[Int] = List(1, 2, 3, 4)
14.
15. scala> val mods = (1 to 20).toList groupBy ( _ % 4 )
16. mods: scala.collection.Map[Int,List[Int]] = Map(1 -> List(1, 5, 9, 13, 17), 2 -> List(2, 6, 10, 14, 18), 3 -> List(3, 7,
17. 11, 15, 19), 0 -> List(4, 8, 12, 16, 20))
18.
19. scala> mods.keySet
20. res9: scala.collection.Set[Int] = Set(1, 2, 3, 0)
21.
22. scala> mods(1)
23. res11: List[Int] = List(1, 5, 9, 13, 17)

(defmacro -$>
"Threads the expr through the forms. Inserts x into
the first form at the position marked by the $ symbol.
If the second form is not a list then it behaves as ->.
If there are more forms, inserts the first form into the second
form at the position marked by the $ symbol, etc."
([x form] (if (seq? form)
(let [split (split-with (partial (complement =) '$)
form)]
`(~@(first split) ~x ~@(rest (second split))))
(list form x)))
([x form & more] `(-$> (-$> ~x ~form) ~@more)))

(defn qsort [xs]
(loop [[part & parts :as work] (list xs), out []]
(if-not work
out
(cond
(coll? part) (let [[pivot & xs] part, smaller #(< % pivot)]
(recur (conj parts
(seq (remove smaller xs))
pivot
(seq (filter smaller xs)))
out))
(nil? part) (recur parts out)
part (recur parts (conj out part))))))