C++如何确保输出的小数位数——以计算加权平均值为例

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <iomanip>
using namespace std;
const int maxn = 55;
int x[maxn];
int w[maxn];

int main() {
/* Enter your code here. Read input from STDIN. Print output to STDOUT */
int n;
cin >> n;
for(int i = 0; i < n; i++) {
cin >> x[i];
}
for(int i = 0; i < n; i++) {
cin >> w[i];
}
double res = 0.0;
double wei = 0.0;
for(int i = 0; i < n; i++) {
res += x[i] * w[i];
wei += w[i];
}
cout << fixed << setprecision(1) << res * 1.0 / wei << endl;
return 0;
}