程序代码：

void lookup_num()           //按学号查询
{
    char a;
    ifstream ist(filename.c_str());
    if(!ist)
    {
        cout<<"读取信息失败"<<endl;
        exit (0);
    }
    string line;
    string num1;
    string num;
    string oo="学号:";
    while(1)
    {
        cout<<"请输入要查询学生的学号:";
        cin>>num1;
        num=oo+num1;
        while(!ist.eof())
        {
            getline(ist,line);
            if(strstr(line.c_str(),num.c_str())!=NULL)
            {
                cout<<"该学生信息为:"<<line<<endl;
            }
        }
        cout<<"是否继续  1.继续查找    2.返回主菜单"<<endl;
        cin>>a;
        system("cls");
        if(a=='1') continue;
        if(a=='2') break;
    }
    a='0';
    ist.close();
}

程序代码：

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

int main( void )
{
    const string filename = "a.txt";

    ifstream ist(filename.c_str());
    if( !ist )
    {
        cerr << "读取信息失败\n";
        return 1;
    }

    for( string num; cout<<"请输入要查询学生的学号:", cin>>num; )
    {
        ist.clear();
        ist.seekg( 0 );

        for( string line; getline(ist,line); )
        {
            if( line.find("学号:"+num) != string::npos )
                cout << "该学生信息为:" << line << endl;
        }

        cout << "是否继续  1.继续查找    2.返回主菜单" << endl;
        char yn;
        cin >> yn;
        if( yn != '1' )
            break;
    }

    return 0;
}