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POUsing a long double or just a double for calculating pi?
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  1. POUsing a long double or just a double for calculating pi?
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    copied!<p>I'm calculating pi using a long winded formula. I'm trying to get more familiar with floating point numbers etc. I have a working program that uses doubles. The problem with my code is:</p> <ol> <li>If I use a double, pi is only accurate to the 7th decimal place. I can't get it to be any more accurate.</li> <li>If I use a long double, pi is accurate up to the 9th decimal place however the code takes much longer to run. If I check for precision for less than 0.00000001 using a long double, pi returns a value of 9.4246775. I assume that this is due to the long double. </li> </ol> <p>My question is what is the most accurate variable type? How could I change my code to improve the precision of pi?</p> <p>Here is my code:</p> <pre><code>#include &lt;iomanip&gt; #include &lt;cstdlib&gt; #include &lt;iostream&gt; #include &lt;cmath&gt; using namespace std; int main() { double arctan; double pi; double precision; double previous=0; int y=3; int loopcount=0; cout&lt;&lt;"Start\n"; arctan=1-(pow(1,y)/y); do { y=y+2; arctan=arctan+(pow(1,y)/y); y=y+2; arctan=arctan-(pow(1,y)/y); pi=4*(arctan); // cout&lt;&lt;"Pi is: "; // cout&lt;&lt;setprecision(12)&lt;&lt;pi&lt;&lt;endl; precision=(pi*(pow(10,10)/10)); loopcount++; if(precision-previous&lt;0.000000001) break; previous=precision; } while(true); cout&lt;&lt;"Pi is:"&lt;&lt;endl; cout&lt;&lt;setprecision(11)&lt;&lt;pi&lt;&lt;endl; cout&lt;&lt;"Times looped:"&lt;&lt;endl; cout&lt;&lt;loopcount&lt;&lt;endl; return 0; } </code></pre>
 

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