Build README

README.md

@@ -66,6 +66,7 @@
 * [Similarity between arrays](#similarity-between-arrays)
 * [Sleep](#sleep)
 * [Sort characters in string (alphabetical)](#sort-characters-in-string-alphabetical)
+* [Standard deviation](#standard-deviation)
 * [Sum of array of numbers](#sum-of-array-of-numbers)
 * [Swap values of two variables](#swap-values-of-two-variables)
 * [Tail of list](#tail-of-list)
@@ -696,6 +697,24 @@ const sortCharactersInString = str =>
 // sortCharactersInString('cabbage') -> 'aabbceg'
 ```
 
+### Standard deviation
+
+Use `Array.reduce()` to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then
+determine the standard deviation.
+You can omit the second argument to get the sample standard deviation or set it to `true` to get the population standard deviation.
+
+```js
+const standardDeviation = (arr, usePopulation = false) => {
+  const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
+  return Math.sqrt(
+    arr.reduce((acc, val) => acc.concat(Math.pow(val - mean, 2)), [])
+       .reduce((acc, val) => acc + val, 0) / (arr.length - (usePopulation ? 0 : 1))
+  );
+ }
+// standardDeviation([10,2,38,23,38,23,21]) -> 13.284434142114991 (sample)
+// standardDeviation([10,2,38,23,38,23,21], true) -> 12.29899614287479 (population)
+```
+
 ### Sum of array of numbers
 
 Use `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`.