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    <p>In your example</p> <pre><code>a.sort </code></pre> <p>is equivalent to</p> <pre><code>a.sort { |x, y| x &lt;=&gt; y } </code></pre> <p>As you know, to sort an array, you need to be able to compare its elements (if you doubt that, just try to implement any sort algorithm without using any comparison, no <code>&lt;</code>, <code>&gt;</code>, <code>&lt;=</code> or <code>&gt;=</code>).</p> <p>The block you provide is really a function which will be called by the <code>sort</code> algorithm to compare two items. <strong>That is <code>x</code> and <code>y</code> will always be some elements of the input array chosen by the <code>sort</code> algorithm during its execution.</strong></p> <p>The <code>sort</code> algorithm will assume that this comparison function/block will meet the requirements for method <code>&lt;=&gt;</code>:</p> <ul> <li>return -1 if x &lt; y</li> <li>return 0 if x = y</li> <li>return 1 if x > y</li> </ul> <p>Failure to provide an adequate comparison function/block will result in array whose order is undefined.</p> <p>You should now understand why</p> <pre><code>a.sort { |x, y| x &lt;=&gt; y } </code></pre> <p>and</p> <pre><code>a.sort { |x, y| y &lt;=&gt; x } </code></pre> <p>return the same array in opposite orders.</p> <hr/> <p>To elaborate on what Tate Johnson added, if you implement the comparison function <code>&lt;=&gt;</code> on any of your classes, you gain the following</p> <ol> <li>You may include the module <code>Comparable</code> in your class which will automatically define for you the following methods: <code>between?</code>, <code>==</code>, <code>&gt;=</code>, <code>&lt;</code>, <code>&lt;=</code> and <code>&gt;</code>.</li> <li>Instances of your class can now be sorted using the default (ie without argument) invocation to <code>sort</code>.</li> </ol> <p>Note that the <code>&lt;=&gt;</code> method is already provided wherever it makes sense in ruby's standard library (<code>Bignum</code>, <code>Array</code>, <code>File::Stat</code>, <code>Fixnum</code>, <code>String</code>, <code>Time</code>, etc...).</p>
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    1. POHow does Array#sort work when a block is passed?
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    1. POHow does Array#sort work when a block is passed?
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    1. COI think it's worth adding that `<=>` is a method defined in `Comparable` and mixed in with `String`. `Fixnum` and `Bignum` also define `<=>`. You can implement `<=>` in any class where sorting or comparrisons are necessary.
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    2. COI highlighted the important sentence. x and y will be 2 elements of your array, chosen by the sort algorithm itself.
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    3. COThanks. I get it now!
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