update and delete data from file in c++ - c++

Question

How can I update data from file in c++ without display the old data? I want to delete specific data and update. For example I want to update the name only and another time update gpa only with delete the old gpa?
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
int k=0;
string line;
string find;
char name[25];
int id=0;
float gpa=0;
ofstream myfile;
myfile.open("data.txt");
while(k!=3){
cout<<"press 1 for adding data"<<endl;
cout<<"press 2 for update "<<endl;
cin>>k;
if(k==1)
{
cout<<"enter ID "<<endl;
cin>>id;
cout<<"enter Name"<<endl;
cin>>name;
cout<<"enter GPA "<<endl;
cin>>gpa;
myfile<<name<<endl;
myfile<<id<<endl;
myfile<<gpa<<endl<<endl<<endl;
}
if(k==2)
{
cout<<"name u want to update "<<endl;
cin>>find;
ifstream file;
file.open("data.txt");
while (!file.eof() && line!=find)
{
getline(file,line);
}
cout<<"enter ID "<<endl;
cin>>id;
cout<<"enter Name"<<endl;
cin>>name;
cout<<"enter GPA "<<endl;
cin>>gpa;
myfile<<name<<endl;
myfile<<id<<endl;
myfile<<gpa<<endl<<endl<<endl;
}
if(k==3){
myfile.close();
}
}
return 0;
}

Answer 1

First you have to copy all the other records to a temporary file eg. temp.txt and then delete the old file data.txt and rename temp.txt to data.txt. Now write the new record to file data.txt.
code:
if(k==2)
{
cout<<"name u want to update "<<endl;
cin>>find;
ifstream myfile2; //for reading records
myfile2.open("data.txt");
ofstream temp;
temp.open("temp.txt");
while (getline(myfile2, line))
{
if (line != find)
temp << line << endl;
}
myfile2.close();
temp.close();
remove("data.txt");
rename("temp.txt", "data.txt");
//Now add new Record to file
cout<<"enter ID "<<endl;
cin>>id;
cout<<"enter Name"<<endl;
cin>>name;
cout<<"enter GPA "<<endl;
cin>>gpa;
ofstream myfile;
myfile.open("data.txt", ios::app | ios::out);
myfile<<name<<endl;
myfile<<id<<endl;
myfile<<gpa<<endl<<endl<<endl;
}

Answer 2

Line by line read old file and copy lines to a new file. When you find the line which you want to update, change it with your line and copy it to the new file. When you reach EOF, delete old file and rename new file with the name of old file.
It will be something like that:
int replace_line(const char *fname, const char *oldline, const char* newline)
{
int done = 0;
/* Open templorary file */
FILE *newfp = fopen("file.tmp", "a");
if (newfp != NULL)
{
/* Open exiting file */
FILE *oldfp = fopen(fname, "r");
if(oldfp != NULL)
{
ssize_t read;
size_t len = 0;
char * line = NULL;
/* Line-by-line read from exiting file */
while ((read = getline(&line, &len, oldfp)) != -1)
{
if(strstr(line, oldline) == NULL) fprintf(newfp, "%s", line);
else fprintf(newfp, "%s", newline);
}
/* Clean up memory */
fclose(oldfp);
if (line) free(line);
done = 1;
}
fclose(newfp);
remove(fname);
rename("file.tmp", fname);
return done;
}
return 0;
}