我们数学上经常遇到的一个问题，椭圆周长的计算，今天我们用程序来实现一下看看吧。
// use 2nd one
// calculate circumference of an ellipse
#include "iostream"
using namespace std;
#define _USE_MATH_DEFINES
#include "cmath"
const int LIMIT = 10; // items number
double circumference(double semimajor, double semiminor);
int main()
{
double res;
res = circumference(1, 2);
cout << "The circumference is: " << res << endl;
}
double circumference(double semimajor, double semiminor)
{
double item[LIMIT];
double e;
if(semimajor >= semiminor)
{
e = sqrt(1 - pow(semiminor / semimajor, 2));
}
else
{
e = sqrt(1 - pow(semimajor / semiminor, 2));
}
double l_deno, l_numa, pe, r_deno, mo, ne;
int i;
for(int j = 1; j < LIMIT; ++j)
{
for(i = 1, l_deno = l_numa = mo = 1, ne = 2; i < j + 1;
++i, mo += 2, ne += 2)
{
l_numa *= mo;
l_deno *= ne;
}
pe = pow(e, 2 * j);
r_deno = 2 * j - 1;
item[j - 1] = l_numa * l_numa * pe /(l_deno * l_deno * r_deno);
}
double tmp = 0.0;
for(int k = 0; k < LIMIT - 1; ++k)
{
tmp += item[k];
}
double l = 2 * M_PI * semimajor * (1 - tmp);