linux下更改txt文件每行后半部分内容

1.写一个demo.cpp文件

#include<iostream>#include<fstream>#include<string>using namespace std;int main(){    string buff;    string buff0;    int i = 0;    ifstream infile;    ofstream outfile;    infile.open("valid.txt");    if(!infile)      cout<<"error"<<buff<<endl;    outfile.open("result.txt");    while(getline(infile, buff)){      buff0 = buff.substr (0, 16);      i++;      if(i <= 3000){        outfile<<buff0 + '1'<<endl;      }      else      {        outfile<<buff0 + '2'<<endl;      }    }    infile.close();    outfile.close();    return 0;}

2.命令行输入命令编译cpp文件，生成a.out

g++ demo.cpp

3.命令行输入命令执行a.out

 ./a.out