Build README

README.md

@@ -41,6 +41,7 @@
 * [Measure time taken by function](#measure-time-taken-by-function)
 * [Median of array of numbers](#median-of-array-of-numbers)
 * [Object from key value pairs](#object-from-key-value-pairs)
+* [Percentile](#percentile)
 * [Pipe](#pipe)
 * [Powerset](#powerset)
 * [Random integer in range](#random-integer-in-range)
@@ -411,6 +412,17 @@ const objectFromPairs = arr => arr.reduce((a,v) => (a[v[0]] = v[1], a), {});
 // objectFromPairs([['a',1],['b',2]]) -> {a: 1, b: 2}
 ```
 
+### Percentile
+
+Use `Array.reduce()` to calculate how many numbers are below the value and how many are the same value and
+apply the percentile formula.
+
+```js
+const percentile = (arr, val) => 
+  100 * arr.reduce((acc,v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0) / arr.length;
+// percentile([1,2,3,4,5,6,7,8,9,10], 6) -> 55
+ ```
+
 ### Pipe
 
 Use `Array.reduce()` to pass value through functions.