Added standard deviation and variance

README.md

@@ -18,6 +18,8 @@ var moreNumbers = [1, 2, 1, 3];
 var mean = stats.mean(numbers); // returns 2
 var median = stats.median(numbers); // returns 2
 var modes = stats.mode(moreNumbers); // returns [1]
+var standardDeviation = stats.standardDeviation(numbers); // returns 0.816496580927726
+var variance = stats.variance(numbers); // returns 0.6666666666666666
 ```
 
 ### Tests
@@ -26,10 +28,7 @@ var modes = stats.mode(moreNumbers); // returns [1]
 `npm test`
 
 ### To do
-- [ ] Standard Deviation
-
-
-  ![alt text](https://www.mathsisfun.com/data/images/standard-deviation-formula.gif "Standard Deviation")
+- [ ] :thinking:
 
 ### Like it? :see_no_evil:
 

index.js

@@ -49,8 +49,33 @@ var mode = numbers => {
     return modes;
 }
 
+var standardDeviation = numbers => {
+    if (!Array.isArray(numbers)) {
+        return null;
+    }
+
+    return Math.sqrt(variance(numbers));
+}
+
+var variance = numbers => {
+    if (!Array.isArray(numbers)) {
+        return null;
+    }
+    var initialMean = mean(numbers);
+    var varianceNumbersArray = [], i;
+    for(i = 0; i < numbers.length; i += 1) {
+        var varianceNumber = numbers[i] - initialMean;
+        varianceNumbersArray.push(varianceNumber * varianceNumber);
+    }
+    var variance = mean(varianceNumbersArray);
+
+    return variance;
+}
+
 module.exports = {
     mean: mean,
     median: median,
-    mode: mode
+    mode: mode,
+    standardDeviation: standardDeviation,
+    variance: variance
 }

package.json

@@ -1,6 +1,6 @@
 {
   "name": "math-stats",
-  "version": "0.1.4",
+  "version": "0.2.0",
   "description": "Mean, Median, Mode",
   "main": "index.js",
   "scripts": {

test/test.js

@@ -20,5 +20,13 @@ describe('#stats', function() {
                 var result = stats.mean([1, 2, 3]);
                 expect(result).to.equal(2);
         });
+        it('should return 0.816', function() {
+                var result = stats.standardDeviation([1, 2, 3]);
+                expect(result).to.equal(0.816496580927726);
+        })
+        it('should return 0.666', function() {
+                var result = stats.variance([1, 2, 3]);
+                expect(result).to.equal(0.6666666666666666);
+        })
 });