C ++中的隨機訪問文件

Question

嗨，我無法從我的二進制文件中具體記錄。 這是列出所有記錄的方法。

int student :: showall(fstream &fp)
{
    student rec;
    fp.seekg(0,ios::beg);
    int i=0;
    cout<<"Position\tRoll No\t\tName\tBalance"<<endl;
    while(fp.read((char*)&rec,sizeof rec))
    {   
        cout<<i*sizeof rec<<"\t";
        rec.show();
        i++;
    }
}

運行此程序時，我得到以下輸出

  Position Roll No Name Balance 0 1 Heartly 10 28 2 Heartly 20 56 3 Heartly 30 84 4 Heartly 40 112 5 Heartly 50 140 6 Heartly 60 168 7 Heartly 70 196 8 Heartly 80 224 9 Heartly 90 252 10 Heartly 100 

現在，我想搜索特定記錄。 我給了RecNo 5搜索。 這是我的View方法

int student :: view(fstream &fp,int RecNum)
{
    student rec;
    std::ios::pos_type SearchPosition = RecNum*sizeof rec;
    cout<<endl<<"Position="<<SearchPosition<<endl;
    if( fp.seekg(SearchPosition) == 0 )
    {
        cout<<endl<<"Position="<<SearchPosition<<endl;
            if(fp.read((char*)&rec,sizeof rec))
                    rec.show();
        else
            cout<<endl<<"Read Error .. ! "<<endl;
        }
    else
    {
        cout<<endl<<"Record Number Not Found"<<endl;
    }   
}

運行此方法后，我得到了以下結果。

記錄數（-1取消）：5

位置= 140

找不到記錄號

為什么沒有找到特定的記錄？

printf("\nMenu : AddDummy, Add, View, Edit, List or Quit ( U, A, V, E, L or Q) : ");  switch(toupper(getchar()))  {     
     case 'U' :
        file.open("abc.cli",ios::in|ios::out|ios::binary|ios::app);
            cout<<"Creating dummy file of 1000 entries"<<endl;
            record.adddummy(file);
        file.clear();
            break;
    case 'A' :
        file.open("abc.cli",ios::in|ios::out|ios::binary|ios::app);
            record.get();
        record.add(record,file);
            cout<<"Record Added"<<endl;         
            file.clear();
            break;              
    case 'E' :      
            file.open("abc.cli",ios::in|ios::out|ios::binary|ios::ate);
            cout<<endl<<"Record number (-1 Cancels): "<<endl;
            cin>>Rec;
            if (Rec > -1){
                record.get();
         if(!record.update(record,file,Rec))
            cout<<"Record Edited"<<endl;            
             else
            cout<<"Record Edited Failed"<<endl;     
        file.clear();}
             break;
    case 'V' :      
             file.open("abc.cli",ios::in|ios::out|ios::binary|ios::ate);
             cout<<endl<<"Record number (-1 Cancels): "<<endl;
             cin>>Rec;
             if (Rec > -1)
                record.view(file,Rec);
         file.clear();
             break;
    case 'L' :
         file.open("abc.cli",ios::in|ios::out|ios::binary|ios::ate);
             record.showall(file); 
         file.clear();
             break; 
    case 'Q' :
            file.close();
        return 0;   
    default :
        cout<<" \nNot Matching\n ";     
}while(getchar() != '\n');

為什么不讀取特定的記錄？在此處輸入代碼

Answer 1

fstream::seekg返回一個fstream引用，永遠不會等於0。因此，您要檢查的條件將永遠不會運行。 fstream :: seekg