第3章 使用批量数据

头文件定义了一个类型streamsize，输入/输出库就是用这个来表示长度的。
头文件定义了一个控制器setprecision,这个控制器可以让我们指明输出所包含的有效位数。

streamsize pres=cout.precision();    cout<<setprecision(3)<<3.1415<<setprecision(pres)<<endl;

#include<iostream>#include<algorithm>#include<ios>#include<iomanip>#include<string>#include<vector>using namespace std;int main(){    cout<<"please enter your first name: ";    string name;    cin>>name;    cout<<"Hello, "<<name<<"!"<<endl;    cout<<"please enter your midterm and final exam grades:  ";    double midterm,final;    cin>>midterm>>final;    cout<<"enter all your homework grades,followed by end-of-file: ";    vector<double> homework;    double x;    while(cin>>x)        homework.push_back(x);    typedef vector<double>::size_type  vec_sz;    vec_sz  size=homework.size();    if(size==0){        cout<<endl<<"you must enter your grades,please try again."<<endl;        return 1;    }    sort(homework.begin(),homework.end());    vec_sz mid=size/2;    double median;    median=(size%2==0)?(homework[mid]+homework[mid+1])/2:homework[mid];    streamsize pres=cout.precision();    cout<<"your final grade is "<<setprecision(3)<<0.2*midterm+0.4*median+0.4*final<<setprecision(pres)<<endl;    return 0;}