读取文件时两次获得相同的输出

Question

我在使用以下程序时遇到问题，

#include<iostream>
#include<fstream>
#include<windows.h>

using namespace std;

class student
{
    int rollno;
    char name[50];
    int cls;
    int marks;
    char grade;
public:
    void getdata()
    {
        cout<<"Enter Roll No.: ";
        cin>>rollno;
        cout<<"Enter Name: ";
        cin>>name;
        cout<<"Enter Class: ";
        cin>>cls;
        cout<<"Enter Marks: ";
        cin>>marks;
        if(marks>=75)
        {
            grade='A';
        }
        if(marks>=60 && marks<75)
        {
            grade='B';
        }
        if(marks>=45 && marks<60)
        {
            grade='C';
        }
        if(marks>=35 && marks<45)
        {
            grade='D';
        }
        if(marks<35)
        {
            grade='F';
        }
    }

    void display()
    {
        cout<<"\nRoll No.: "<<rollno;
        cout<<"\nName: "<<name;
        cout<<"\nClass: "<<cls;
        cout<<"\nMarks: "<<marks;
        cout<<"\nGrade: "<<grade<<endl;
    }

    int rno()
    {
        return rollno;
    }
} s1, s2, s3;

void append()
{
    fstream fo ("STUDENT.DAT",ios::binary | ios::app | ios::out);
    if(!fo)
    {
        cout<<"CANNOT OPEN FILE!!!!!";
    }

    else
    {
        s1.getdata();
        fo.write((char*)&s1, sizeof(s1));
    }
    fo.close();

}

void sdisplay()
{
    fstream fi ("STUDENT.DAT",ios::binary | ios::in);
    if(!fi)
    {
        cout<<"\nNO RECORDS !!!!";

    }
    else
    {

        while(!fi.eof())
        {
            fi.read((char*)&s2, sizeof(s2));
            if(s2.rno()!='[=10=]')
            {
                s2.display();
            }
        }
    }
    fi.close();
}

void rdisplay()
{
    int n, found;
    fstream fi ("STUDENT.DAT", ios::in | ios::binary);
    if(!fi)
    {
        cout<<"\nNO RECORDS !!!!";
    }

    else
    {
        cout<<"\nEnter Roll No. to be displayed: ";
        cin>>n;
        while(!fi.eof())
        {
            fi.read((char*)&s3, sizeof(s3));
            if(s3.rno()==n)
            {
                s3.display();
                found=1;
            }
        }
        if(found!=1)
        {
            cout<<"\nCannot find roll no!!";
        }
    }
    fi.close();
}

int main()
{
    char ch='y';
    int choice, n;

    while(ch=='y' || ch=='Y')
    {
        cout<<"\n1. Append Data to File";
        cout<<"\n2. Display Entire File";
        cout<<"\n3. Search & Display record based on Roll No.";
        cout<<"\n4. Exit";
        cout<<"\n\n Enter your choice (1-4): ";
        cin>>choice;
        switch(choice)
        {
        case 1:
            append();
            break;

        case 2:
            sdisplay();
            break;

        case 3:
            rdisplay();
            break;
        case 4:
            exit(0);
            break;

        default:
            cout<<"\nWrong Choice !!!!!";
            break;
        }
        cout<<"\n\nDo you want to continue? (Y/N)";
        cin>>ch;
    }
}

因此，附加数据后，当我转到显示数据（选项 2 和 3）时，它会输出相同的条目两次。

Append Data to File

Display Entire File

Search & Display record based on Roll No.

Exit

Enter your choice (1-4): 2

NO RECORDS !!!!

Do you want to continue? (Y/N)y

Append Data to File

Display Entire File

Search & Display record based on Roll No.

Exit

Enter your choice (1-4): 1 Enter Roll No.: 23 Enter Name: James Enter Class: 12 Enter Marks: 85

Do you want to continue? (Y/N)y

Append Data to File

Display Entire File

Search & Display record based on Roll No.

Exit

Enter your choice (1-4): 2

Roll No.: 23 Name: James Class: 12 Marks: 85 Grade: A

Roll No.: 23 Name: James Class: 12 Marks: 85 Grade: A

Do you want to continue? (Y/N)n

Process returned 0 (0x0) execution time : 25.013 s Press any key to continue.

有人可以帮助我吗？谢谢

Answer 1

eof() 直到您读到最后才会设置，直到您第二次阅读才会发生。该读取不会执行任何操作，因此它不会覆盖 s2，它只会失败 - 因此您最终将再次打印该行。

解决方法是不要循环 eof()。只需循环 read():

while(fi.read((char*)&s2, sizeof(s2)))
{
    s2.display();
}

读取文件时两次获得相同的输出

Getting the same output twice when reading a file

c++

fstream

class