读取运算十几万行几列的数据文件运算并输出，但是速度非常慢，得好几分钟vector有缺陷，但是是不是预分配空间就可以，但是不知道vector < vector > vec;怎么分配，好像没有效果！求助！

 cpp


 #include <fstream>
#include <iostream>
#include <string>
#include <sstream>
#include <vector>
using namespace std;
#define pi 3.141592653
int main()
{
    ifstream readfile;
    ofstream outfile;
    string file_name;
    string file_name2;
    string sline;

    float data;
    int n;
    int m;
    int i;
    int j;

    vector <float> v;

    vector < vector <float> > vec;

    double a;
    double b;
    a = 0.005*pi/180;
    b = 0.005;
    cout << "输入文件名：";
    cin >> file_name;
    vec.clear();
    readfile.open((char *)file_name.c_str(), ios::in);
    i = 0;

    while(!readfile.eof())
    {
        getline(readfile, sline);
        stringstream buf(sline);
        v.clear();
        while(!buf.eof())
        {
            //v.reserve(850000);
            buf >> data;
            v.push_back(data);
        }
        //vec.reserve(850000);
        vec.push_back(v);
        i++;
    }
    readfile.close();
    cout << "输入文件名：";
    cin >> file_name2;
    outfile.open((char *)file_name2.c_str(), ios::out);
    n = vec.size();
    for(i=0;i<n;i++)
    {
        m = vec[i].size();
        for(j=0;j<m;j++)
        {
            outfile<<vec[i][j] * (j==0?1:(j<4?a:b));
            cout<<endl;
            if(j<m-1)
                outfile << char(0x20);
        }
        if(i<n-1)
            outfile << endl;
    }
    outfile.close();
    return 0;
}