小数点保留若干位小数 %.*f

T4530 青年π

https://www.luogu.org/problemnew/show/T4530

 #include <cstdio>
 #include <cstdlib>
 #include <cmath>
 #include <cstring>
 #include <time.h>
 #include <string>
 #include <set>
 #include <map>
 #include <list>
 #include <stack>
 #include <queue>
 #include <vector>
 #include <bitset>
 #include <ext/rope>
 #include <algorithm>
 #include <iostream>
 using namespace std;
 #define ll long long
 #define minv 1e-6
 #define inf 1e9
 #define pi 3.1415926536
 #define E  2.7182818284
 const ll mod=1e9+;//
 const int maxn=1e5+;

 int main()
 {
     int n,m,i;
     double p=1.0;
     scanf("%d%d",&n,&m);
     for (i=;i<=n;i++)
         p=p*pi;
     printf("%.*f",m,p);
     return ;
 }