From 69703d882d96a53168e7091b9bbe86c6f67056cd Mon Sep 17 00:00:00 2001
From: Rakshit Menpara <deltasquare4@gmail.com>
Date: Fri, 1 Aug 2014 14:58:50 +0530
Subject: [PATCH] Fixed correlation coefficient calculation

* Correlation coefficient r() was returning NaN
  if the standard deviation of y was 0 (because
  all y values were equal)
* Added test for calculating linear regression
  with points containing same y coordinate values
---
 lib/stats.js |  6 +++++-
 test/test.js | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/stats.js b/lib/stats.js
index c875a34..5b54d12 100644
--- a/lib/stats.js
+++ b/lib/stats.js
@@ -316,7 +316,11 @@
 				return (num - meanX) / stdDevX;
 			});
 			yCoords.map(function(num) {
-				return (num - meanY) / stdDevY;
+				if (stdDevY === 0) {
+					return 0;
+				} else {
+					return (num - meanY) / stdDevY;
+				}
 			});
 
 			// Multiply each element in the x by the corresponding value in
diff --git a/test/test.js b/test/test.js
index 03baea9..12b64fd 100644
--- a/test/test.js
+++ b/test/test.js
@@ -128,6 +128,20 @@ describe('#linReg()', function() {
 		expect(reg.yIntercept).to.be.within(.00001).of(3.61352);
 		expect(reg.r).to.be.within(.000001).of(-.693356);
 	});
+
+	it('should not return NaN if the y coordinate values are same', function() {
+		var reg1 = stats([
+			{ x: 1,  y: 4 },
+			{ x: 3,  y: 4 },
+			{ x: 6,  y: 4 },
+			{ x: 4,  y: 4 },
+			{ x: 9,  y: 4 },
+		]).linReg();
+
+		expect(reg1.slope).to.eql(0);
+		expect(reg1.yIntercept).to.eql(4);
+		expect(reg1.r).to.eql(0);
+	});
 });
 
 describe('#powReg()', function() {