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POMahalanobis distance between two vectors
 

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    <p>I tried to apply <a href="http://www.mathworks.com/help/stats/mahal.html" rel="nofollow"><code>mahal</code></a> to calculate the <a href="http://en.wikipedia.org/wiki/Mahalanobis_distance" rel="nofollow">Mahalanobis distance</a> between 2 row-vectors of 27 variables, <em>i.e</em> <code>mahal(X, Y)</code>, where <code>X</code> and <code>Y</code> are the two vectors. However, it comes up with an error:</p> <blockquote> <p><code>The number of rows of X must exceed the number of columns.</code></p> </blockquote> <p>After a few minutes of research I got that I can't use it like this, but I'm still not sure sure why. Can some explain it to me?</p> <p>Also I have below an example of <code>mahal</code> method :</p> <pre><code>&gt;&gt; mahal([1.55 5 32],[5.76 43 34; 6.7 32 5; 3 3 5; 34 12 6;]) ans = 11.1706 </code></pre> <p>Can someone clarify how MATLAB calculate the answer in this case?</p> <p><strong>Edit:</strong><br/> I found this code that calculate the mahalanobis distance:</p> <pre><code>S = cov(X); mu = mean(X); d = (Y-mu)*inv(S)*(Y-mu)' d = ((Y-mu)/S)*(Y-mu)'; % &lt;-- Mathworks prefers this way </code></pre> <p>I tested it on <code>[1.55 5 32]</code>, and <code>[5.76 43 34; 6.7 32 5; 3 3 5; 34 12 6;]</code> and it gave me the same result as if I used the <code>mahal</code> function (11.1706), and I tried to calculate the distance between the 2 vectors of 27 variables and it works. What do you think about it? Can I count on this solution since the <code>mahal</code> function can't do what I need?</p>
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