小数点保留若干位小数 %.*f

 T4530 青年π

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

 1 #include <cstdio>
 2 #include <cstdlib>
 3 #include <cmath>
 4 #include <cstring>
 5 #include <time.h>
 6 #include <string>
 7 #include <set>
 8 #include <map>
 9 #include <list>
10 #include <stack>
11 #include <queue>
12 #include <vector>
13 #include <bitset>
14 #include <ext/rope>
15 #include <algorithm>
16 #include <iostream>
17 using namespace std;
18 #define ll long long
19 #define minv 1e-6
20 #define inf 1e9
21 #define pi 3.1415926536
22 #define E  2.7182818284
23 const ll mod=1e9+7;//998244353
24 const int maxn=1e5+10;
25 
26 
27 int main()
28 {
29     int n,m,i;
30     double p=1.0;
31     scanf("%d%d",&n,&m);
32     for (i=1;i<=n;i++)
33         p=p*pi;
34     printf("%.*f",m,p);
35     return 0;
36 }