输出带固定小数位数的浮点类型

#include<bits/stdc++.h>
#define N 105 
#define endl ‘\n‘ 
#define _for(i,a,b) for(int i=a;i<b;i++)
using namespace std;
typedef long long ll;
int main(){
    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    int n,len; int res = 0;
    cin>>n>>len;
    vector<int> a(n); 
    for( int & i : a) cin>>i; 
    sort(a.begin(),a.end()); 
    _for(i,0,n-1){ 
        if( a[i]==a[i+1] ) continue; 
        res = max(res,abs(a[i+1]-a[i]) ) ;
    }
    res = max( res,2*a[0] );
    res = max( res,2*(len-a[n-1]) );
    cout<<fixed<< setprecision(9) << res/2.0 <<endl;
    return 0;
}