Add Mapping With An Index as a clojure til.

README.md

@@ -33,6 +33,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Evaluate One Liners With lein-exec](clojure/evaluate-one-liners-with-lein-exec.md)
 - [Expanding Macros](clojure/expanding-macros.md)
 - [Get The Value Of An Environment Variable](clojure/get-the-value-of-an-environment-variable.md)
+- [Mapping With An Index](clojure/mapping-with-an-index.md)
 - [Open JavaDocs](clojure/open-javadocs.md)
 - [Pretty Print The Last Thing](clojure/pretty-print-the-last-thing.md)
 - [Quick Clojure Docs](clojure/quick-clojure-docs.md)

clojure/mapping-with-an-index.md

@@ -0,0 +1,11 @@
+# Mapping With An Index
+
+If you're mapping over a collection and you need an index for each item, you
+can reach for `map-indexed`. The `map-indexed` function can be used like so:
+
+```clojure
+> (def foods ["pizza" "hotdog" "chicken alfredo"])
+#'cljs.user/foods
+> (map-indexed (fn [idx item] (str idx " - " item)) foods)
+("0 - pizza" "1 - hotdog" "2 - chicken alfredo")
+```