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  1. PO
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    <p>I tested the performance difference between <code>x*x*...</code> vs <code>pow(x,i)</code> for small <code>i</code> using this code:</p> <pre><code>#include &lt;cstdlib&gt; #include &lt;cmath&gt; #include &lt;boost/date_time/posix_time/posix_time.hpp&gt; inline boost::posix_time::ptime now() { return boost::posix_time::microsec_clock::local_time(); } #define TEST(num, expression) \ double test##num(double b, long loops) \ { \ double x = 0.0; \ \ boost::posix_time::ptime startTime = now(); \ for (long i=0; i&lt;loops; ++i) \ { \ x += expression; \ x += expression; \ x += expression; \ x += expression; \ x += expression; \ x += expression; \ x += expression; \ x += expression; \ x += expression; \ x += expression; \ } \ boost::posix_time::time_duration elapsed = now() - startTime; \ \ std::cout &lt;&lt; elapsed &lt;&lt; " "; \ \ return x; \ } TEST(1, b) TEST(2, b*b) TEST(3, b*b*b) TEST(4, b*b*b*b) TEST(5, b*b*b*b*b) template &lt;int exponent&gt; double testpow(double base, long loops) { double x = 0.0; boost::posix_time::ptime startTime = now(); for (long i=0; i&lt;loops; ++i) { x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); x += std::pow(base, exponent); } boost::posix_time::time_duration elapsed = now() - startTime; std::cout &lt;&lt; elapsed &lt;&lt; " "; return x; } int main() { using std::cout; long loops = 100000000l; double x = 0.0; cout &lt;&lt; "1 "; x += testpow&lt;1&gt;(rand(), loops); x += test1(rand(), loops); cout &lt;&lt; "\n2 "; x += testpow&lt;2&gt;(rand(), loops); x += test2(rand(), loops); cout &lt;&lt; "\n3 "; x += testpow&lt;3&gt;(rand(), loops); x += test3(rand(), loops); cout &lt;&lt; "\n4 "; x += testpow&lt;4&gt;(rand(), loops); x += test4(rand(), loops); cout &lt;&lt; "\n5 "; x += testpow&lt;5&gt;(rand(), loops); x += test5(rand(), loops); cout &lt;&lt; "\n" &lt;&lt; x &lt;&lt; "\n"; } </code></pre> <p>Results are:</p> <pre><code>1 00:00:01.126008 00:00:01.128338 2 00:00:01.125832 00:00:01.127227 3 00:00:01.125563 00:00:01.126590 4 00:00:01.126289 00:00:01.126086 5 00:00:01.126570 00:00:01.125930 2.45829e+54 </code></pre> <p>Note that I accumulate the result of every pow calculation to make sure the compiler doesn't optimize it away.</p> <p>If I use the <code>std::pow(double, double)</code> version, and <code>loops = 1000000l</code>, I get:</p> <pre><code>1 00:00:00.011339 00:00:00.011262 2 00:00:00.011259 00:00:00.011254 3 00:00:00.975658 00:00:00.011254 4 00:00:00.976427 00:00:00.011254 5 00:00:00.973029 00:00:00.011254 2.45829e+52 </code></pre> <p>This is on an Intel Core Duo running Ubuntu 9.10 64bit. Compiled using gcc 4.4.1 with -o2 optimization.</p> <p>So in C, yes <code>x*x*x</code> will be faster than <code>pow(x, 3)</code>, because there is no <code>pow(double, int)</code> overload. In C++, it will be the roughly same. (Assuming the methodology in my testing is correct.)</p> <hr> <p>This is in response to the comment made by An Markm:</p> <p>Even if a <code>using namespace std</code> directive was issued, if the second parameter to <code>pow</code> is an <code>int</code>, then the <code>std::pow(double, int)</code> overload from <code>&lt;cmath&gt;</code> will be called instead of <code>::pow(double, double)</code> from <code>&lt;math.h&gt;</code>.</p> <p>This test code confirms that behavior:</p> <pre><code>#include &lt;iostream&gt; namespace foo { double bar(double x, int i) { std::cout &lt;&lt; "foo::bar\n"; return x*i; } } double bar(double x, double y) { std::cout &lt;&lt; "::bar\n"; return x*y; } using namespace foo; int main() { double a = bar(1.2, 3); // Prints "foo::bar" std::cout &lt;&lt; a &lt;&lt; "\n"; return 0; } </code></pre>
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    1. POWhat is more efficient? Using pow to square or just multiply it with itself?
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    1. USEmile Cormier
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    1. POWhat is more efficient? Using pow to square or just multiply it with itself?
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    1. COdoes this mean that inserting a "using namespace std" chooses the C option and this will be detrimental to the runtime?
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    2. CO@AnMarkm: See updated answer.
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