Cache the mean

snippets/standard-deviation.md

@@ -6,12 +6,14 @@ of the values to determine the standard deviation of an array of numbers.
 Since there are two types of standard deviation, population and sample, you can use a flag to switch to population (sample is default).
 
 ```js
-const standardDeviation = (arr, usePopulation) =>
-  Math.sqrt(
-    arr.reduce((acc, val) => acc.concat(Math.pow(val - arr.reduce((acc, val) => acc + val, 0) / arr.length, 2)), [])
+const standardDeviation = (arr, usePopulation) => {
+  const mean = arr.reduce((acc, val) => acc + val, 0);
+  return Math.sqrt(
+    arr.reduce((acc, val) => acc.concat(Math.pow(val - mean / arr.length, 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)
 ```