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    "question_id" : 11227809,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/11227809/why-is-processing-a-sorted-array-faster-than-processing-an-unsorted-array",
    "title" : "Why is processing a sorted array faster than processing an unsorted array?",
    "body" : "<p>In this C++ code, sorting the data (<em>before</em> the timed region) makes the primary loop ~6x faster:</p>\n<pre class=\"lang-cpp prettyprint-override\"><code>#include &lt;algorithm&gt;\n#include &lt;ctime&gt;\n#include &lt;iostream&gt;\n\nint main()\n{\n    // Generate data\n    const unsigned arraySize = 32768;\n    int data[arraySize];\n\n    for (unsigned c = 0; c &lt; arraySize; ++c)\n        data[c] = std::rand() % 256;\n\n    // !!! With this, the next loop runs faster.\n    std::sort(data, data + arraySize);\n\n    // Test\n    clock_t start = clock();\n    long long sum = 0;\n    for (unsigned i = 0; i &lt; 100000; ++i)\n    {\n        for (unsigned c = 0; c &lt; arraySize; ++c)\n        {   // Primary loop.\n            if (data[c] &gt;= 128)\n                sum += data[c];\n        }\n    }\n\n    double elapsedTime = static_cast&lt;double&gt;(clock()-start) / CLOCKS_PER_SEC;\n\n    std::cout &lt;&lt; elapsedTime &lt;&lt; '\\n';\n    std::cout &lt;&lt; &quot;sum = &quot; &lt;&lt; sum &lt;&lt; '\\n';\n}\n</code></pre>\n<ul>\n<li>Without <code>std::sort(data, data + arraySize);</code>, the code runs in 11.54 seconds.</li>\n<li>With the sorted data, the code runs in 1.93 seconds.</li>\n</ul>\n<p>(Sorting itself takes more time than this one pass over the array, so it's not actually worth doing if we needed to calculate this for an unknown array.)</p>\n<hr />\n<p>Initially, I thought this might be just a language or compiler anomaly, so I tried Java:</p>\n<pre class=\"lang-java prettyprint-override\"><code>import java.util.Arrays;\nimport java.util.Random;\n\npublic class Main\n{\n    public static void main(String[] args)\n    {\n        // Generate data\n        int arraySize = 32768;\n        int data[] = new int[arraySize];\n\n        Random rnd = new Random(0);\n        for (int c = 0; c &lt; arraySize; ++c)\n            data[c] = rnd.nextInt() % 256;\n\n        // !!! With this, the next loop runs faster\n        Arrays.sort(data);\n\n        // Test\n        long start = System.nanoTime();\n        long sum = 0;\n        for (int i = 0; i &lt; 100000; ++i)\n        {\n            for (int c = 0; c &lt; arraySize; ++c)\n            {   // Primary loop.\n                if (data[c] &gt;= 128)\n                    sum += data[c];\n            }\n        }\n\n        System.out.println((System.nanoTime() - start) / 1000000000.0);\n        System.out.println(&quot;sum = &quot; + sum);\n    }\n}\n</code></pre>\n<p>With a similar but less extreme result.</p>\n<hr />\n<p>My first thought was that sorting brings the data into the <a href=\"https://en.wikipedia.org/wiki/CPU_cache\" rel=\"noreferrer\">cache</a>, but that's silly because the array was just generated.</p>\n<ul>\n<li>What is going on?</li>\n<li>Why is processing a sorted array faster than processing an unsorted array?</li>\n</ul>\n<p>The code is summing up some independent terms, so the order should not matter.</p>\n<hr />\n<h2>Related / follow-up Q&amp;As with more modern C++ compilers</h2>\n<ul>\n<li><a href=\"https://stackoverflow.com/q/66521344\">Why is processing an unsorted array the same speed as processing a sorted array with modern x86-64 clang?</a> - <strong>modern C++ compilers auto-vectorize the loop</strong>, especially when SSE4.1 or AVX2 is available.  This avoids any data-dependent branching so performance isn't data-dependent.</li>\n<li><a href=\"https://stackoverflow.com/q/28875325\">gcc optimization flag -O3 makes code slower than -O2</a> - branchless scalar with <code>cmov</code> can result in a longer dependency chain (especially when GCC chooses poorly), creating a latency bottleneck that makes it slower than branchy asm for the sorted case.</li>\n</ul>\n"
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    "tags" : [ "java", "methods", "parameter-passing", "pass-by-reference", "pass-by-value" ],
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      "display_name" : "user4315",
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    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value",
    "title" : "Is Java &quot;pass-by-reference&quot; or &quot;pass-by-value&quot;?",
    "body" : "<p>I always thought Java uses <strong>pass-by-reference</strong>. However, I read <a href=\"http://javadude.com/articles/passbyvalue.htm\" rel=\"noreferrer\">a blog post</a> which claims that Java uses <strong>pass-by-value</strong>. I don't think I understand the distinction the author is making.</p>\n<p>What is the explanation?</p>\n"
  }, {
    "tags" : [ "java", "date", "timezone" ],
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      "display_name" : "Freewind",
      "link" : "https://stackoverflow.com/users/342235/freewind"
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    "question_id" : 6841333,
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    "link" : "https://stackoverflow.com/questions/6841333/why-is-subtracting-these-two-epoch-milli-times-in-year-1927-giving-a-strange-r",
    "title" : "Why is subtracting these two epoch-milli Times (in year 1927) giving a strange result?",
    "body" : "<p>If I run the following program, which parses two date strings referencing times 1 second apart and compares them:</p>\n<pre><code>public static void main(String[] args) throws ParseException {\n    SimpleDateFormat sf = new SimpleDateFormat(&quot;yyyy-MM-dd HH:mm:ss&quot;);  \n    String str3 = &quot;1927-12-31 23:54:07&quot;;  \n    String str4 = &quot;1927-12-31 23:54:08&quot;;  \n    Date sDt3 = sf.parse(str3);  \n    Date sDt4 = sf.parse(str4);  \n    long ld3 = sDt3.getTime() /1000;  \n    long ld4 = sDt4.getTime() /1000;\n    System.out.println(ld4-ld3);\n}\n</code></pre>\n<p><strong>The output is:</strong></p>\n<pre><code>353\n</code></pre>\n<p>Why is <code>ld4-ld3</code>, not <code>1</code> (as I would expect from the one-second difference in the times), but <code>353</code>?</p>\n<p>If I change the dates to times 1 second later:</p>\n<pre><code>String str3 = &quot;1927-12-31 23:54:08&quot;;  \nString str4 = &quot;1927-12-31 23:54:09&quot;;  \n</code></pre>\n<p>Then <code>ld4-ld3</code> will be <code>1</code>.</p>\n<hr />\n<p><strong>Java version:</strong></p>\n<pre class=\"lang-none prettyprint-override\"><code>java version &quot;1.6.0_22&quot;\nJava(TM) SE Runtime Environment (build 1.6.0_22-b04)\nDynamic Code Evolution Client VM (build 0.2-b02-internal, 19.0-b04-internal, mixed mode)\n</code></pre>\n<pre class=\"lang-java prettyprint-override\"><code>Timezone(`TimeZone.getDefault()`):\n\nsun.util.calendar.ZoneInfo[id=&quot;Asia/Shanghai&quot;,\noffset=28800000,dstSavings=0,\nuseDaylight=false,\ntransitions=19,\nlastRule=null]\n\nLocale(Locale.getDefault()): zh_CN\n</code></pre>\n"
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      "display_name" : "Johnny Maelstrom",
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    "question_id" : 309424,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/309424/how-do-i-read-convert-an-inputstream-into-a-string-in-java",
    "title" : "How do I read / convert an InputStream into a String in Java?",
    "body" : "<p>If you have a <code>java.io.InputStream</code> object, how should you process that object and produce a <code>String</code>?</p>\n<hr />\n<p>Suppose I have an <code>InputStream</code> that contains text data, and I want to convert it to a <code>String</code>, so for example I can write that to a log file.</p>\n<p>What is the easiest way to take the <code>InputStream</code> and convert it to a <code>String</code>?</p>\n<pre><code>public String convertStreamToString(InputStream is) {\n// ???\n}\n</code></pre>\n"
  }, {
    "tags" : [ "java", "nullpointerexception", "null" ],
    "owner" : {
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      "display_name" : "Goran Martinic",
      "link" : "https://stackoverflow.com/users/34856/goran-martinic"
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    "is_answered" : true,
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    "protected_date" : 1368308875,
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    "community_owned_date" : 1319187757,
    "score" : 4439,
    "last_activity_date" : 1748456350,
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    "question_id" : 271526,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/271526/how-do-i-avoid-checking-for-nulls-in-java",
    "title" : "How do I avoid checking for nulls in Java?",
    "body" : "<p>I use <code>x != null</code> to avoid <a href=\"https://docs.oracle.com/javase/9/docs/api/java/lang/NullPointerException.html\" rel=\"noreferrer\"><code>NullPointerException</code></a>. Is there an alternative?</p>\n<pre class=\"lang-java prettyprint-override\"><code>if (x != null) {\n    // ...\n}\n</code></pre>\n"
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    "tags" : [ "java", "collections", "hashmap", "hashtable" ],
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      "display_name" : "dmanxiii",
      "link" : "https://stackoverflow.com/users/4316/dmanxiii"
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    "last_edit_date" : 1596280763,
    "question_id" : 40471,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/40471/what-are-the-differences-between-a-hashmap-and-a-hashtable-in-java",
    "title" : "What are the differences between a HashMap and a Hashtable in Java?",
    "body" : "<p>What are the differences between a <a href=\"https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/HashMap.html\" rel=\"noreferrer\"><code>HashMap</code></a> and a <a href=\"https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Hashtable.html\" rel=\"noreferrer\"><code>Hashtable</code></a> in Java?</p>\n\n<p>Which is more efficient for non-threaded applications?</p>\n"
  }, {
    "tags" : [ "java", "random", "integer" ],
    "owner" : {
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      "link" : "https://stackoverflow.com/users/42155/user42155"
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    "question_id" : 363681,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/363681/how-do-i-generate-random-integers-within-a-specific-range-in-java",
    "title" : "How do I generate random integers within a specific range in Java?",
    "body" : "<p>How do I generate a random <code>int</code> value in a specific range?</p>\n<p>The following methods have bugs related to integer overflow:</p>\n<pre class=\"lang-java prettyprint-override\"><code>randomNum = minimum + (int)(Math.random() * maximum);\n// Bug: `randomNum` can be bigger than `maximum`.\n</code></pre>\n<pre class=\"lang-java prettyprint-override\"><code>Random rn = new Random();\nint n = maximum - minimum + 1;\nint i = rn.nextInt() % n;\nrandomNum =  minimum + i;\n// Bug: `randomNum` can be smaller than `minimum`.\n</code></pre>\n"
  }, {
    "tags" : [ "java", "arrays", "arraylist", "type-conversion" ],
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      "display_name" : "Ron Tuffin",
      "link" : "https://stackoverflow.com/users/939/ron-tuffin"
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    "question_id" : 157944,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/157944/create-arraylist-from-array",
    "title" : "Create ArrayList from array",
    "body" : "<pre class=\"lang-java prettyprint-override\"><code>Element[] array = {new Element(1), new Element(2), new Element(3)};\n</code></pre>\n<p>How do I convert the above variable of type <code>Element[]</code> into a variable of type <a href=\"https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/ArrayList.html\" rel=\"noreferrer\"><code>ArrayList&lt;Element&gt;</code></a>?</p>\n<pre class=\"lang-java prettyprint-override\"><code>ArrayList&lt;Element&gt; arrayList = ...;\n</code></pre>\n"
  }, {
    "tags" : [ "java", "android", "usermanager" ],
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      "display_name" : "Ovidiu Latcu",
      "link" : "https://stackoverflow.com/users/542091/ovidiu-latcu"
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    "question_id" : 13375357,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat",
    "title" : "Proper use cases for Android UserManager.isUserAGoat()?",
    "body" : "<p>I was looking at the new APIs introduced in <a href=\"http://en.wikipedia.org/wiki/Android_version_history#Android_4.1.2F4.2_Jelly_Bean\" rel=\"noreferrer\">Android 4.2</a>.\nWhile looking at the <a href=\"http://developer.android.com/reference/android/os/UserManager.html\" rel=\"noreferrer\"><code>UserManager</code></a> class I came across the following method:</p>\n\n<blockquote>\n<pre><code>public boolean isUserAGoat()\n</code></pre>\n  \n  <p>Used to determine whether the user making this call is subject to teleportations.</p>\n  \n  <p>Returns whether the user making this call is a goat.</p>\n</blockquote>\n\n<p>How and when should this be used?</p>\n"
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    "tags" : [ "java", "dictionary", "collections", "iteration" ],
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      "display_name" : "iMack",
      "link" : "https://stackoverflow.com/users/4807/imack"
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    "question_id" : 46898,
    "content_license" : "CC BY-SA 4.0",
    "link" : "https://stackoverflow.com/questions/46898/how-do-i-efficiently-iterate-over-each-entry-in-a-java-map",
    "title" : "How do I efficiently iterate over each entry in a Java Map?",
    "body" : "<p>If I have an object implementing the <code>Map</code> interface in Java and I wish to iterate over every pair contained within it,  what is the most efficient way of going through the map?</p>\n\n<p>Will the ordering of elements depend on the specific map implementation that I have for the interface?</p>\n"
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