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  1. POWhy is it faster to process a sorted array than an unsorted array?
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    <p>Here is a piece of C++ code that seems very peculiar. For some strange reason, sorting the data miraculously makes the code almost six times faster.</p> <pre class="lang-cpp prettyprint-override"><code>#include &lt;algorithm&gt; #include &lt;ctime&gt; #include &lt;iostream&gt; int main() { // Generate data const unsigned arraySize = 32768; int data[arraySize]; for (unsigned c = 0; c &lt; arraySize; ++c) data[c] = std::rand() % 256; // !!! With this, the next loop runs faster std::sort(data, data + arraySize); // Test clock_t start = clock(); long long sum = 0; for (unsigned i = 0; i &lt; 100000; ++i) { // Primary loop for (unsigned c = 0; c &lt; arraySize; ++c) { if (data[c] &gt;= 128) sum += data[c]; } } double elapsedTime = static_cast&lt;double&gt;(clock() - start) / CLOCKS_PER_SEC; std::cout &lt;&lt; elapsedTime &lt;&lt; std::endl; std::cout &lt;&lt; "sum = " &lt;&lt; sum &lt;&lt; std::endl; } </code></pre> <ul> <li>Without <code>std::sort(data, data + arraySize);</code>, the code runs in 11.54 seconds.</li> <li>With the sorted data, the code runs in 1.93 seconds.</li> </ul> <p>Initially, I thought this might be just a language or compiler anomaly. So I tried it in Java.</p> <pre class="lang-java prettyprint-override"><code>import java.util.Arrays; import java.util.Random; public class Main { public static void main(String[] args) { // Generate data int arraySize = 32768; int data[] = new int[arraySize]; Random rnd = new Random(0); for (int c = 0; c &lt; arraySize; ++c) data[c] = rnd.nextInt() % 256; // !!! With this, the next loop runs faster Arrays.sort(data); // Test long start = System.nanoTime(); long sum = 0; for (int i = 0; i &lt; 100000; ++i) { // Primary loop for (int c = 0; c &lt; arraySize; ++c) { if (data[c] &gt;= 128) sum += data[c]; } } System.out.println((System.nanoTime() - start) / 1000000000.0); System.out.println("sum = " + sum); } } </code></pre> <p>With a somewhat similar but less extreme result.</p> <hr> <p>My first thought was that sorting brings the data into the cache, but then I thought how silly that is because the array was just generated.</p> <ul> <li>What is going on?</li> <li>Why is it faster to process a sorted array than an unsorted array?</li> <li>The code is summing up some independent terms, and the order should not matter.</li> </ul>
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  2. POThe Definitive C++ Book Guide and List
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    <p>This question attempts to collect the few pearls among the dozens of bad C++ books that are published every year.</p> <p>Unlike many other programming languages, which are often picked up on the go from tutorials found on the Internet, few are able to quickly pick up C++ without studying a well-written C++ book. It is way too big and complex for doing this. In fact, it is so big and complex, that <strong><em>there are very many very bad C++ books</em></strong> out there. And we are not talking about bad style, but things like sporting <em>glaringly obvious factual errors</em> and <em>promoting abysmally bad programming styles</em>.</p> <p>Please edit the accepted answer to provide <strong>quality books</strong> and an approximate skill level — <em>preferably</em> <strong>after</strong> <em>discussing your addition in <a href="http://chat.stackoverflow.com/rooms/10/loungec">the C++ chat room</a></em>. (The regulars might mercilessly undo your work if they disagree with a recommendation.) Add a short blurb/description about each book that you have personally read/benefited from. Feel free to debate quality, headings, etc. Books that meet the criteria will be added to the list. Books that have reviews by the Association of C and C++ Users (ACCU) have links to the review. </p> <p><sub>*Note: FAQs and other resources can be found in the <a href="https://stackoverflow.com/tags/c%2b%2b/info">C++ tag info</a> and under <a href="/questions/tagged/c%2b%2b-faq" class="post-tag" title="show questions tagged &#39;c++-faq&#39;" rel="tag">c++-faq</a>. </sub></p>
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    1. USAndy
    1. USgrepsedawk
  3. POWhat is the single most influential book every programmer should read?
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    1. USRobert Harvey
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  4. POThe definitive guide to form-based website authentication
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    <h2>Form-based authentication for websites</h2> <p>We believe that Stack&nbsp;Overflow should not just be a resource for very specific technical questions, but also for general guidelines on how to solve variations on common problems. "Form based authentication for websites" should be a fine topic for such an experiment.</p> <h3>It should include topics such as:</h3> <ul> <li>How to log in</li> <li>How to log out</li> <li>How to remain logged in</li> <li>Managing cookies (including recommended settings)</li> <li>SSL/HTTPS encryption</li> <li>How to store passwords</li> <li>Using secret questions</li> <li>Forgotten username/password functionality</li> <li>Use of <a href="https://en.wikipedia.org/wiki/Cryptographic_nonce" rel="noreferrer">nonces</a> to prevent <a href="https://en.wikipedia.org/wiki/Cross-site_request_forgery" rel="noreferrer">cross-site request forgeries (CSRF)</a></li> <li><a href="http://openid.net/" rel="noreferrer">OpenID</a></li> <li>"Remember me" checkbox</li> <li>Browser autocompletion of usernames and passwords</li> <li>Secret URLs (public <a href="https://en.wikipedia.org/wiki/Uniform_Resource_Locator" rel="noreferrer">URL</a> protected by digest)</li> <li>Checking password strength</li> <li>E-mail validation</li> <li><em>and much more about</em> <a href="http://en.wikipedia.org/wiki/Form-based_authentication" rel="noreferrer">form based authentication</a>...</li> </ul> <h3>It should not include things like:</h3> <ul> <li>Roles and authorization</li> <li>HTTP basic authentication</li> </ul> <h3>Please help us by:</h3> <ol> <li>Suggesting subtopics</li> <li>Submitting good articles about this subject</li> <li>Editing the official answer</li> </ol>
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    1. USMichiel de Mare
  5. PO"Thinking in AngularJS" if I have a jQuery background?
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  6. POHow to undo the most recent commits in Git?
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    1. USHamza Yerlikaya
  7. PORegEx match open tags except XHTML self-contained tags
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  8. POWhat is the best comment in source code you have ever encountered?
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  9. POHidden features of Python
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    <p>What are the lesser-known but useful features of the Python programming language?</p> <ul> <li>Try to limit answers to Python core.</li> <li>One feature per answer.</li> <li>Give an example and short description of the feature, not just a link to documentation.</li> <li>Label the feature using a title as the first line.</li> </ul> <h2>Quick links to answers:</h2> <ul> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#111176">Argument Unpacking</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#112303">Braces</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101945">Chaining Comparison Operators</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101447">Decorators</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#113198">Default Argument Gotchas / Dangers of Mutable Default arguments</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#102062">Descriptors</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#111970">Dictionary default <code>.get</code> value</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#102065">Docstring Tests</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python/112316#112316">Ellipsis Slicing Syntax</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#117116">Enumeration</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#114420">For/else</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#102202">Function as iter() argument</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101310">Generator expressions</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101276"><code>import this</code></a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#102037">In Place Value Swapping</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101840">List stepping</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#112286"><code>__missing__</code> items</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101537">Multi-line Regex</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#113164">Named string formatting</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101549">Nested list/generator comprehensions</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#108297">New types at runtime</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#113833"><code>.pth</code> files</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#1024693">ROT13 Encoding</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#143636">Regex Debugging</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#101739">Sending to Generators</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#168270">Tab Completion in Interactive Interpreter</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#116480">Ternary Expression</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#114157"><code>try/except/else</code></a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#3267903">Unpacking+<code>print()</code> function</a></li> <li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python#109182"><code>with</code> statement</a></li> </ul>
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    1. USCommunity
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