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Delete file contents

closed account (1vf9z8AR)
I wrote my program but delete is not working.I can't understand what is wrong.(line 85-109)
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#include<iostream>
#include<fstream>
using namespace std;
struct student
{
    char name[80];
    int rno;
};
int main()
{
        student s1;
    int option,n;
    char ch;
    do
    {
    cout<<"Enter 1 to write file\nEnter 2 to read file\n3 to search file\n4 to modify file\n5 to delete";cin>>option;
    if(option==1)
    {
    ofstream f1;
    f1.open("file.dat",ios::binary);
    cout<<"Enter number of customers";cin>>n;
    for(int i=0;i<n;i++)
    {
    cout<<"\nEnter name(dot to terminate)";cin.getline(s1.name,80,'.');
    cout<<"\nEnter roll number";cin>>s1.rno;
        f1.write((char*)&s1,sizeof(student));
    }
      f1.close();
    }

    else if(option==2)
    {
           ifstream r1;
           r1.open("file.dat",ios::binary) ;
                while(   r1.read((char*)&s1,sizeof(student)))
           {
           cout<<"Name"<<s1.name<<endl;
           cout<<"Roll number"<<s1.rno<<endl;
           }
           r1.close();
    }

    else if(option==3)
    {
        int rno;
        cout<<"Enter roll number to search for";cin>>rno;
       bool condition=false;
        ifstream a1;
        a1.open("file.dat",ios::binary);
        while( a1.read((char*)&s1,sizeof(student)))
    {
        if (rno==s1.rno)
                {
                    condition=true;
                    cout<<"Name:"<<s1.name<<endl;
                    cout<<"Roll number"<<s1.rno<<endl;
                };
    }
    if(condition==false)
    cout<<"Not found"<<endl;
    a1.close();
    }

    else if(option==4)
    {
        int r;
        fstream q1;
        q1.open("file.dat",ios::in|ios::out|ios::binary);
        cout<<"Enter roll number to modify";cin>>r;
        bool flag=false;
        while(q1.read((char* )&s1,sizeof(student)))
        {
            if(s1.rno==r)
            {
                flag=true;
                cout<<"\nEnter new name(dot to end input)";cin.getline(s1.name,80,'.');
                cout<<"\nEnter roll number";cin>>s1.rno;
q1.write((char*)&s1,sizeof(student));
            }
        }
        if(!flag)
            cout<<"\nNot found"<<endl;
        q1.close();
    }
    else if(option==5)
    {
        int rno;
              fstream w1;fstream e1;
              e1.open("temp.dat",ios::in|ios::out|ios::binary);
              w1.open("file.dat",ios::in|ios::out|ios::binary);
              cout<<"\nEnter roll number to delete";cin>>rno;
              bool value=false;
              while(w1.read((char*)&s1,sizeof(student)))
              {
                  if(s1.rno==rno)
                      value=true;
                  else
                         e1.write((char*)&s1,sizeof(student));
              }
                             if(value==false)
                cout<<"\nNot found"<<endl;
                else
                {
                     remove("file.dat");
              rename("temp.dat","file.dat");
                }
              e1.close();
              w1.close();
    }

    cout<<"Do you want to continue?(y/n)";cin>>ch;
    }while((ch=='y')||(ch=='Y'));
               return 0;
}


EDIT:
The delete coding magnified and changed.But whole file gets deleted now.
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    else if(option==5)
    {
        int rno;
              fstream w1;fstream e1;
              e1.open("temp.dat",ios::in|ios::out|ios::binary);
              w1.open("file.dat",ios::in|ios::out|ios::binary);
              cout<<"\nEnter roll number to delete";cin>>rno;
              bool value=false;
              while(w1.read((char*)&s1,sizeof(student)))
              {
                  if(s1.rno==rno)
                      value=true;
                  else
                         e1.write((char*)&s1,sizeof(student));
              }
                                      e1.close();
              w1.close();
                             if(value==false)
                             {
                cout<<"\nNot found"<<endl;
                             }
                else
                {
                     remove("file.dat");
              rename("temp.dat","file.dat");
                }
    }
Last edited on
coder777 (8444)
Close the files before remove(...)/rename(...). stream operations are buffered and might not be physically written.
closed account (1vf9z8AR)
Now the whole file gets deleted

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#include<iostream>
#include<fstream>
using namespace std;
struct student
{
    char name[80];
    int rno;
};
int main()
{
        student s1;
    int option,n;
    char ch;
    do
    {
    cout<<"Enter 1 to write file\nEnter 2 to read file\n3 to search file\n4 to modify file\n5 to delete";cin>>option;
    if(option==1)
    {
    ofstream f1;
    f1.open("file.dat",ios::binary);
    cout<<"Enter number of customers";cin>>n;
    for(int i=0;i<n;i++)
    {
    cout<<"\nEnter name(dot to terminate)";cin.getline(s1.name,80,'.');
    cout<<"\nEnter roll number";cin>>s1.rno;
        f1.write((char*)&s1,sizeof(student));
    }
      f1.close();
    }

    else if(option==2)
    {
           ifstream r1;
           r1.open("file.dat",ios::binary) ;
                while(   r1.read((char*)&s1,sizeof(student)))
           {
           cout<<"Name"<<s1.name<<endl;
           cout<<"Roll number"<<s1.rno<<endl;
           }
           r1.close();
    }

    else if(option==3)
    {
        int rno;
        cout<<"Enter roll number to search for";cin>>rno;
       bool condition=false;
        ifstream a1;
        a1.open("file.dat",ios::binary);
        while( a1.read((char*)&s1,sizeof(student)))
    {
        if (rno==s1.rno)
                {
                    condition=true;
                    cout<<"Name:"<<s1.name<<endl;
                    cout<<"Roll number"<<s1.rno<<endl;
                };
    }
    if(condition==false)
    cout<<"Not found"<<endl;
    a1.close();
    }

    else if(option==4)
    {
        int r;
        fstream q1;
        q1.open("file.dat",ios::in|ios::out|ios::binary);
        cout<<"Enter roll number to modify";cin>>r;
        bool flag=false;
        while(q1.read((char* )&s1,sizeof(student)))
        {
            if(s1.rno==r)
            {
                flag=true;
                cout<<"\nEnter new name(dot to end input)";cin.getline(s1.name,80,'.');
                cout<<"\nEnter roll number";cin>>s1.rno;
q1.write((char*)&s1,sizeof(student));
            }
        }
        if(!flag)
            cout<<"\nNot found"<<endl;
        q1.close();
    }
    else if(option==5)
    {
        int rno;
              fstream w1;fstream e1;
              e1.open("temp.dat",ios::in|ios::out|ios::binary);
              w1.open("file.dat",ios::in|ios::out|ios::binary);
              cout<<"\nEnter roll number to delete";cin>>rno;
              bool value=false;
              while(w1.read((char*)&s1,sizeof(student)))
              {
                  if(s1.rno==rno)
                      value=true;
                  else
                         e1.write((char*)&s1,sizeof(student));
              }
                                      e1.close();
              w1.close();
                             if(value==false)
                             {
                cout<<"\nNot found"<<endl;
                             }
                else
                {
                     remove("file.dat");
              rename("temp.dat","file.dat");
                }
    }

    cout<<"Do you want to continue?(y/n)";cin>>ch;
    }while((ch=='y')||(ch=='Y'));
               return 0;
}
Last edited on
Enoizat (1343)
Hints:
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#include <cstdio>
#include <fstream>
#include <iostream>
#include <limits>
#include <stdexcept>
#include <string>

struct Student {
    std::string name;
    int rno {};
};

void displayMenu();
int askForInteger(int min, int max);
void writeToFile();
void readFromFile();
void searchRollNumberInFile();
void modifyFile();
void deleteFromFile();

int main()
{
    std::string line;
    do {
        displayMenu();
        int option = askForInteger(1, 5);
        switch(option) {
        case 1:
            writeToFile();
            break;
        case 2:
            readFromFile();
            break;
        case 3:
            searchRollNumberInFile();
            break;
        case 4:
            modifyFile();
            break;
        case 5:
            deleteFromFile();
            break;
        }
        std::cout << "Do you want to continue (y/n)? ";
        std::getline(std::cin, line);
    } while(line.at(0) == 'y' || line.at(0) == 'Y');
    std::cout << "\nPress ENTER to continue...\n";
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    return 0;
}

void displayMenu()
{
    std::cout << "1 - write to file\n2 - read from file\n3 - search in file\n"
                 "4 - modify file\n5 - delete from file\n";
}

int askForInteger(int min, int max)
{
    std::cout << "Please insert an integer (" << min << '-' << max << "): ";
    int answ {};
    for(std::string line; std::getline(std::cin, line); /**/) {
        try {
            answ = std::stoi(line); // may throw std::invalid_argument or
                                    // std::out_of_range
            if(answ < min || max < answ) { 
                throw std::out_of_range("User input out of range");
            }
            break; // no errors: exit for-loop
        // } catch(std::invalid_argument& e) {
            // //
        // } catch(std::out_of_range& e) {
            // //
        } catch(std::logic_error& e) {
            std::cout << "Please insert an integer (" << min << '-' << max 
                      << "): ";
        }
    }
    return answ;
}

void writeToFile()
{
    std::cout << "How many customers do you want to enter?\n";
    int n = askForInteger(1, std::numeric_limits<int>::max());
    std::ofstream f1("file.dat");
    for(int i=0; i<n; i++) {
        std::cout << "\nCustomer n. " << i+1 << ":\nname (max 80 characters)? ";
        std::string line;
        std::getline(std::cin, line);
        if(line.length() != 80) { line.resize(80, ' '); }
        std::cout << "roll number?\n";
        int rno = askForInteger(1, std::numeric_limits<int>::max());
        f1 << line << ' ' << rno << '\n';
    }
    f1.close();
}

void readFromFile()
{
    std::ifstream r1("file.dat");
    for(std::string line; std::getline(r1, line); /**/) {
        // The following lines are pointless: just to show how to create
        // a Student from the data inside the file:
        Student s;
        s.rno = std::stoi(line.substr(line.rfind(' ')));
        s.name = line.substr(0, line.find("  "));
        std::cout << "Name: " << s.name << "; roll number: " << s.rno << '\n';
    }
    r1.close();
}

void searchRollNumberInFile()
{
    std::cout << "Enter roll number to search for.\n";
    int rno = askForInteger(1, std::numeric_limits<int>::max());
    std::ifstream a1("file.dat");
    for(std::string line; std::getline(a1, line); /**/) {
        int rno_int = std::stoi(line.substr(line.rfind(' ')));
        if(rno == rno_int) {
            Student s;
            s.rno = rno_int;
            s.name = line.substr(0, line.find("  "));
            std::cout << "Name: " << s.name << "; roll number: " << s.rno << '\n';
            return;
        }
    }
    std::cout << "Not found.\n";
    a1.close();
}

void modifyFile()
{
    std::cout << "Enter roll number to modify.\n";
    int r = askForInteger(1, std::numeric_limits<int>::max());
    std::ifstream q1("file.dat");
    bool found { false };
    for(std::string line; std::getline(q1, line); /**/) {
        if(r == std::stoi(line.substr(line.rfind(' ')))) {
            found = true;
            break;
        }
    }
    q1.close();
    if(found) { // now we know for sure the searched rno exists.
        std::ifstream q1("file.dat");
        std::ofstream q2("tmp_file.dat");
        for(std::string line; std::getline(q1, line); /**/) {
            if(r == std::stoi(line.substr(line.rfind(' ')))) { // get new data
                std::cout << "Enter new name (max 80 characters): ";
                std::getline(std::cin, line);
                if(line.length() != 80) { line.resize(80, ' '); }
                std::cout <<"\nEnter new roll number.\n";
                int rno = askForInteger(1, std::numeric_limits<int>::max());
                line += ' ' + std::to_string(rno);
            }
            q2 << line << '\n';
        }
        q1.close();
        q2.close();
        std::remove("file.dat");
        std::rename("tmp_file.dat", "file.dat");
    } else {
        std::cout << " Not found.\n";
    }
}

void deleteFromFile()
{
    std::cout << "Enter roll number to modify.\n";
    int r = askForInteger(1, std::numeric_limits<int>::max());
    std::ifstream q1("file.dat");
    bool found { false };
    for(std::string line; std::getline(q1, line); /**/) {
        if(r == std::stoi(line.substr(line.rfind(' ')))) {
            found = true;
            break;
        }
    }
    q1.close();
    if(found) { // now we know for sure the searched rno exists.
        std::ifstream q1("file.dat");
        std::ofstream q2("tmp_file.dat");
        for(std::string line; std::getline(q1, line); /**/) {
            if(r != std::stoi(line.substr(line.rfind(' ')))) {
                q2 << line << '\n';
            }
        }
        q1.close();
        q2.close();
        std::remove("file.dat");
        std::rename("tmp_file.dat", "file.dat");
    } else {
        std::cout << " Not found.\n";
    }
}

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