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    <p>I want to expand on this a bit:</p> <pre><code>array = [].tap { |ary| ary &lt;&lt; 5 if something } </code></pre> <p>What this does (assuming <code>something</code> is true-ish):</p> <ol> <li><p>assigns <code>array</code> to <code>[]</code>, an empty array. </p> <pre><code>array.object_id = 2152428060 </code></pre></li> <li><p>passes <code>[]</code> to the block as <code>ary</code>. <code>ary</code> and <code>array</code> are pointing to the same array object. </p> <pre><code>array.object_id = 2152428060 ary.object_id = 2152428060 </code></pre></li> <li><p><code>ary &lt;&lt; 5</code> &lt;&lt; is a mutative method, meaning it will modify the receiving object. It is similar to the idiom of appending <code>!</code> to a method call, meaning "modify this in place!", like in <code>.map</code> vs <code>.map!</code> (though the bang does not hold any intrinsic meaning on its own in a method name). <code>ary</code> has 5 inserted, so <code>ary</code> = <code>array</code> = <code>[5]</code></p> <pre><code>array.object_id = 2152428060 ary.object_id = 2152428060 </code></pre></li> </ol> <p>We end with <code>array</code> being equal to <code>[5]</code></p> <p>In the second example:</p> <pre><code>array = [].tap{ |ary| ary += [5] if something } </code></pre> <ol> <li>same</li> <li>same</li> <li><p><code>ary += 5</code> <code>+=</code> is short for <code>ary = ary + 5</code>, so it is first modification (<code>+</code>) and then assignment (<code>=</code>), in that order. It gives the appearance of modifying an object in place, but it actually does not. It creates an entirely new object.</p> <pre><code>array.object_id = 2152428060 ary.object_id = 2152322420 </code></pre></li> </ol> <p>So we end with <code>array</code> as the original object, an empty array with <code>object_id=2152428060</code> , and <code>ary</code>, an array with one item containing 5 with <code>object_id = 2152322420</code>. Nothing happens to <code>ary</code> after this. It is uninvolved with the original assignment of <code>array</code>, that has already happened. Tap executes the block after <code>array</code> has been assigned.</p>
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    1. PORuby 'tap' method - inside assignment
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    1. COthis is a most excellent answer and explanation.
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