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copied!<p><strong>Q1:</strong></p> <p>An empty array has 0 elements, so when you try to set its element 0, with negative index <code>-1</code>, it will give an error. Because negative index cycles through the array from the end.</p> <p>So <code>a = []; a[-1] = 3</code> makes it impossible to </p> <p>a) get the element at last position, since its null</p> <p>b) set its value. since it was never captured.</p> <p><code>a[0] = 5</code> will work because you are telling the compiler to</p> <p>a) grab the first element, </p> <p>b) create one if not present, and then assign that to the value you requested.</p> <p>See <a href="http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-5B-5D-3D" rel="nofollow">official api doc</a> specifically mentioning positive index can grow size, negative index past the beginning of the array raises an error.</p> <p><strong>Q2:</strong></p> <p>The above explanation almost answers the second question as well.</p> <p>Given <code>a = [1,2]</code></p> <p><code>a[-3] = 3</code> causes the first point of break. You are trying to access the 3rd element from the end, which does not exist. By design it breaks down.</p> <p>While <code>a[-2..-3]</code> is within the capture range of the defined array. </p> <p>You ask the interpreter to capture the second element from the last (1st from the front in this case), and try to invoke a range which is asking it to increase the array's size, and populate it with whatever you requested.</p> <p>Happily, all is still well and as desired. Good to know.</p>

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