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    <p>A Hamming distance should be done between two strings of equal length and with the order taken into account.</p> <p>As your documents are certainly of different length and if the words places do not count, cosine similarity is better (please note that depending your needs, better solutions exist). :)</p> <p>Here is a cosine similarity function of 2 arrays of words:</p> <pre><code>function cosineSimilarity($tokensA, $tokensB) { $a = $b = $c = 0; $uniqueTokensA = $uniqueTokensB = array(); $uniqueMergedTokens = array_unique(array_merge($tokensA, $tokensB)); foreach ($tokensA as $token) $uniqueTokensA[$token] = 0; foreach ($tokensB as $token) $uniqueTokensB[$token] = 0; foreach ($uniqueMergedTokens as $token) { $x = isset($uniqueTokensA[$token]) ? 1 : 0; $y = isset($uniqueTokensB[$token]) ? 1 : 0; $a += $x * $y; $b += $x; $c += $y; } return $b * $c != 0 ? $a / sqrt($b * $c) : 0; } </code></pre> <p>It is fast (<code>isset()</code> instead of <code>in_array()</code> is a killer on large arrays).</p> <p>As you can see, the results does not take into account the "magnitude" of each the word.</p> <p>I use it to detect multi-posted messages of "almost" copy-pasted texts. It works well. :)</p> <p><strong>The best link about string similarity metrics</strong>: <a href="http://www.dcs.shef.ac.uk/~sam/stringmetrics.html" rel="noreferrer">http://www.dcs.shef.ac.uk/~sam/stringmetrics.html</a></p> <p>For further interesting readings:</p> <p><a href="http://www.miislita.com/information-retrieval-tutorial/cosine-similarity-tutorial.html" rel="noreferrer">http://www.miislita.com/information-retrieval-tutorial/cosine-similarity-tutorial.html</a> <a href="http://bioinformatics.oxfordjournals.org/cgi/content/full/22/18/2298" rel="noreferrer">http://bioinformatics.oxfordjournals.org/cgi/content/full/22/18/2298</a></p>
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    1. POCosine similarity vs Hamming distance
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    1. COThank you very much. :) But isn't Mike's solution (selected answer) better? The code is shorter and seems to be as fast as yours. What are the differences?
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    2. COMike's function is not really exact. Try `echo check(array('a', 'b', 'c'), array('a', 'b', 'c'));` It should return 1 (cos(0)) but his function returns 0.33. :(
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    3. COIs your function really correct? It gives 0.71 for [1, 1, 1] and [1, 1, 0]. But http://www.miislita.com/searchito/binary-similarity-calculator.html gives 0.82?! Is it still necessary do divide the similarity value by the document length?
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