Add Combinations Of Items From A Sequence as a clojure til

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_391 TILs and counting..._
+_392 TILs and counting..._
 
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@@ -43,6 +43,7 @@ _391 TILs and counting..._
 
 - [Aggregation Using merge-with](clojure/aggregation-using-merge-with.md)
 - [Argument Requirements For A Function](clojure/argument-requirements-for-a-function.md)
+- [Combinations Of Items From A Sequence](clojure/combinations-of-items-from-a-sequence.md)
 - [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)

clojure/combinations-of-items-from-a-sequence.md

@@ -0,0 +1,24 @@
+# Combinations Of Items From A Sequence
+
+Sometimes we want all combinations of items from a list. For instance, we
+may have 5 people and we want to know all the ways that we can unique pair
+up 2 people from that group of 5. What we want is the number of
+combinations of 2 people from the 5.
+
+The
+[clojure/math.combinatorics](https://github.com/clojure/math.combinatorics)
+library provides a `combinations` function that gives us exactly that
+functionality.
+
+```clojure
+(use '[clojure.math.combinatorics :as combo])
+
+(combo/combinations ["Liz", "Tracy", "Kenneth", "Jack", "Jenna"] 2)
+; (("Liz" "Tracy") ("Liz" "Kenneth") ("Liz" "Jack")
+;  ("Liz" "Jenna") ("Tracy" "Kenneth") ("Tracy" "Jack")
+;  ("Tracy" "Jenna") ("Kenneth" "Jack") ("Kenneth" "Jenna")
+;  ("Jack" "Jenna"))
+```
+
+The `combinations` function takes a list of items and then a number for the
+size of the grouping combinations that it is supposed to produce.