Pi: Difference between revisions

=={{header|C++}}==

<lang cpp>#include <iostream>

#include <boost/multiprecision/cpp_int.hpp>

using namespace boost::multiprecision;

class Gospers

{

cpp_int q, r, t, i, n;

public:

// use Gibbons spigot algorith based on the Gospers series

Gospers() : q{1}, r{0}, t{1}, i{1}

{

++*this; // move to the first digit

}

// the ++ prefix operator will move to the next digit

Gospers& operator++()

{

n = (q*(27*i-12)+5*r) / (5*t);

while(n != (q*(675*i-216)+125*r)/(125*t))

{

r = 3*(3*i+1)*(3*i+2)*((5*i-2)*q+r);

q = i*(2*i-1)*q;

t = 3*(3*i+1)*(3*i+2)*t;

i++;

n = (q*(27*i-12)+5*r) / (5*t);

}

q = 10*q;

r = 10*r-10*n*t;

return *this;

}

// the dereference operator will give the current digit

int operator*()

{

return (int)n;

}

};

int main()

{

Gospers g;

std::cout << *g << "."; // print the first digit and the decimal point

for(;;) // run forever

{

std::cout << *++g; // increment to the next digit and print

}

}</lang>

{{out}}

<pre>

3.1415926535897932384626433832795028841971693993751058209749445923078164062862 . . .

</pre>