Learning Clojure/Collection functions

Collection functions The clojure namespace contains many functions and macros for handling collections. Rather than repeat reference information from the API documentation, I'll give just a few example operations: (count collection) Returns the number of items in the collection. (count [a b c]) ; return 3 (count nil) ; return 0 (conj collection item) conjoin. Returns a new copy of the collection with the item added. How the item is added depends upon the collection type, e.g. an item conjoined to a list is added to the beginning while an item conjoined to a vector is added to the end. (conj [5 7 3] 9) ; returns [5 7 3 9] (conj (list 5 7 3) 9)) ; returns list (9 5 7 3) (conj nil item) ; returns list (item) (list item*) (vector item*) (hash-map keyval*) where keyval => key value Produce a list, vector, or hashmap, respectively. You can generally always just use literal syntax in place of these functions, but having functions gives us something to pass to other functions expecting function arguments. (nth collection n) Return the nth item from collection, where collection is any collection but not a map. (The collection can be a sequence over a map, but not an actual map.) (nth [31 73 -11] 2) ; returns -11 (nth (list 31 73 -11) 0) ; returns 31 (nth [8 4 4 1] 9) ; exception: out of bounds keywords as functions A keyword can be used as a function to look up a key's value in the various kinds of maps (hashmaps, sets, structmaps, etc.): (:velcro {:velcro 5 :zipper 7}) ; returns 5, the value mapped to :velcro in the hashmap maps as functions The map types themselves can be used as functions to look up their key's values: ({:velcro 5 :zipper 7} :velcro) ; returns 5, the value mapped to :velcro in the hashmap ({3 "cat" :tim "moose"} :tim) ; returns "moose", the value mapped to :tim in the hashmap (["hi" "bye"] 1) ; returns "bye", the value of the second index in the vector Sequence functions Most of the sequence functions can be passed objects which are not sequences but from which a sequence can be derived, e.g. you can pass a list in place of a sequence. Where there are redundancies between sequence operations and more operations specialized for specific collection types, the redundancies are mostly for performance considerations. For instance, to produce a reversed-order sequence of a vector, you can use the sequence operation reverse or the vector specific rseq: while rseq is less general, it is more efficient. (seq collection) Return a sequence representing the collection. seq also works on native Java arrays, Strings, and any object that implements Iterable, Iterator, or Enumeration.