C++ program to modify the content of a Binary File - GeeksforGeeks

01 Nov, 2019
This article explains how to modify the content of a Binary File.
Given a binary file that contains the records of students, the task is to modify or alter the record of the specified student. If the record of no such student exists, then print “record not found”.
Examples:
Input:
old roll no: 1
new roll no: 11
new name: "Geek"
Output:
roll no: 11
name: "Geek"
record successfully modified

Input:
old roll no: 234
new roll no: 14
new name: "Geek2"
Output:
record not found

In this example, the existing roll number of the student whose record is to be modified is taken from the user and a newly updated record is created that replaces the previous record.
Approach:
Step 1: Searching for the roll number in the binary file.
Step 2: While searching in the file, the variable “pos” stores the position of file pointer record then traverse(continue) reading of the record.
Step 3: If the roll number to be searched exists then place the write pointer (to ending of the previous record) i.e. at pos.
Step 4: Call getdata function to take the new record.
Step 5: Write the new object at the position “pos” and hence the record is updated and print “record successfully updated”.
Step 6: If the roll number does not exists then print “record not found”.
Standard Library Functions used:
// tells the position of read pointer 
file.tellg();

// places the writing pointer at 
// position "pos" in the file
file.seekp(pos);

Below is the implementation of the above approach:
// C++ program to modify the content// of a binary file  #include <bits/stdc++.h>using namespace std;  class abc {    int roll;    char name[20];  public:    void getdata(int, char[]);    void update(int, int, char[]);    void testcase1();    void testcase2();    void putdata();};  // Code to display the data of the// data of the objectvoid abc::putdata(){    cout << "roll no: ";    cout << roll;    cout << "\nname: ";    cout << name;}  // Code to set the value to the objectvoid abc::getdata(int a, char str[]){    // setting the new roll no    roll = a;      // setting new name    strcpy(name, str);}  void abc::update(int rno, int r, char str[]){    // code to update and modify    // the content of the binary file    int pos, flag = 0;      // rno=9    fstream fs;    fs.open("he.dat",            ios::in | ios::binary | ios::out);      while (!fs.eof()) {        // storing the position of        // current file pointeri.e. at        // the end of previously read record        pos = fs.tellg();          fs.read((char*)this, sizeof(abc));        if (fs) {              // comparing the roll no with that            // of the entered roll number            if (rno == roll) {                flag = 1;                  // setting the new (modified )                // data of the object or new record                getdata(r, str);                  // placing the put(writing) pointer                // at the starting of the  record                fs.seekp(pos);                  // writing the object to the file                fs.write((char*)this, sizeof(abc));                  // display the data                putdata();                break;            }        }    }    fs.close();      if (flag == 1)        cout << "\nrecord successfully modified \n";    else        cout << "\nrecord not found \n";}  // Sample input 1void abc::testcase1(){    int rno, r;    char name[20];      // roll no to be searched    rno = 1;      // new roll no    r = 11;      // new name    strcpy(name, "Geek");      // call update function with new values    update(rno, r, name);}  // Sample input 2void abc::testcase2(){    int rno, r;    char name[20];      // roll no to be searched    rno = 4;      // new roll no    r = 14;      // new name    strcpy(name, "Geek2");      // call update function with the new values    update(rno, r, name);}  // Driver codeint main(){    abc s;      // sample case 1    s.testcase1();      // sample case 2    s.testcase2();      return 0;}
Output:
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