数的位数计算

原题链接:http://acm.nefu.edu.cn/JudgeOnline/problemshow.php?problem_id=117

分析：设数为N,则其位数为(int)lg(N)+1;log(10)=ln(10);

素数定理：令Pn表示<n的素数个数，则当n较大时有Pn~nln(n).

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
int main()
{
    int n;
    double e=2.71828;
    while(cin>>n)
    {
        double ans=double(n-log10(n)-log10(log(10)));
        cout<<(int)ans+1<<endl;
    }
    return 0;
}