Add Swap Two Items in a Vector as a clojure til.

README.md

@@ -11,6 +11,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 
 - [Specify the Directory of a Shell Command](clojure/specify-the-directory-of-a-shell-command.md)
 - [Splitting On Whitespace](clojure/splitting-on-whitespace.md)
+- [Swap Two Items in a Vector](clojure/swap-two-items-in-a-vector.md)
 - [Type of Anything](clojure/type-of-anything.md)
 
 ### git

clojure/swap-two-items-in-a-vector.md

@@ -0,0 +1,31 @@
+# Swap Two Items in a Vector
+
+If you want to replace the value at an index in a vector, you can use the
+`assoc` function supplied by `clojure.core` like so:
+
+```clojure
+> (assoc [5 6 7 8] 1 9)
+; [5 9 7 8]
+```
+
+Likewise, if you want to replace two items in a vector, you can extend the
+above like so:
+
+```clojure
+> (assoc [5 6 7 8] 1 2 2 4)
+; [5 2 4 8]
+```
+
+We can take advantage of that second example to construct a function that
+will swap two values:
+
+```clojure
+(defn swap
+  [items i j]
+  (assoc items i (items j) j (items i)))
+```
+
+This function will break on values of `i` and `j` that are out of the bounds
+of `items`, but dealing with that is left as an exercise to the reader.
+
+[source](http://stackoverflow.com/questions/5979538/what-is-the-idiomatic-way-to-swap-two-elements-in-a-vector)