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POAlgorithm to get changes between two arrays
 

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    <p>I needed to create an algorithm which will (efficiently) take an old array and a new array and give me back the changes between the two (which items added, which removed). It happens to need to be in JavaScript (to run in the browser) but the algorithm's more important than the language.</p> <p>This is what I came up with: <a href="http://jsbin.com/osewu3/13" rel="nofollow noreferrer">http://jsbin.com/osewu3/13</a>. Can anyone see any problems with that/suggest any improvements?</p> <p>Thanks!</p> <p>Code Listing:</p> <pre><code>function diff(o, n) { // deal with empty lists if (o == undefined) o = []; if (n == undefined) n = []; // sort both arrays (or this won't work) o.sort(); n.sort(); // don't compare if either list is empty if (o.length == 0 || n.length == 0) return {added: n, removed: o}; // declare temporary variables var op = 0; var np = 0; var a = []; var r = []; // compare arrays and add to add or remove lists while (op &lt; o.length &amp;&amp; np &lt; n.length) { if (o[op] &lt; n[np]) { // push to diff? r.push(o[op]); op++; } else if (o[op] &gt; n[np]) { // push to diff? a.push(n[np]); np++; } else { op++;np++; } } // add remaining items if( np &lt; n.length ) a = a.concat(n.slice(np, n.length)); if( op &lt; o.length ) r = r.concat(o.slice(op, o.length)); return {added: a, removed: r}; } </code></pre> <p>(I have also posted this as a potential solution to another SO question, here: <a href="https://stackoverflow.com/questions/1187518/javascript-array-difference/3476612#3476612">JavaScript array difference</a>)</p>
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    1. COYep, looks like a textbook implementation of the standard set intersection algorithm (which uses a modified version of the "merge" algorithm of mergesort fame). Good work coming up with it yourself.
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    2. CO@David. Thanks... I think :)
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    3. COLooks about right. You don't really need the special check for empty arrays: if there are any, the loop will immediately exit and you'll get the same results.
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