update records in file (c++) - c++

Question

I wanted to ask how can I append strings to the end of fixed number of lines (fixed position). I am trying and searching books and websites for my answer but I couldn't find what I am doing wrong.
My structure :
const int numberofdays=150 ;
const int numberofstudents=2;
struct students
{
char attendance[numberofdays]; int rollno;
char fullname[50],fathersname[50];
}
Creating a text file
ofstream datafile("data.txt", ios::out );
Then I take input from the user and save it to the file.
How I save my data to text files :
datafile <<attendance <<endl<< rollno <<endl<<
fullname <<endl<< fathersname <<endl ;
How it looks like in text files :
p // p for present - 1st line
1 // roll number
Monte Cristo // full name
Black Michael // Fathers name
a // a for absent - 5th line
2 // roll number
Johnson // full name
Nikolas // Fathers name
How I try to update the file. (updating attendance for everyday)
datafile.open("data.txt", ios::ate | ios::out | ios::in);
if (!datafile)
{
cerr <<"File couldn't be opened";
exit (1);
}
for (int i=1 ; i<=numberofstudents ; i++)
{
long int offset = ( (i-1) * sizeof(students) );
system("cls");
cout <<"\t\tPresent : p \n\t\t Absent : a"<<endl;
cout <<"\nRoll #"<<i<<" : ";
cin >> ch1;
if (ch1 != 'p')
ch1 = 'a';
datafile.seekp(offset);
datafile <<ch1;
datafile.seekg(0);
}
I just want to add (append) characters 'p' or 'a' to the first or fifth line, I tried every possible way but I am unable to do it.

Answer 1

What you are doing is fairly common, but as you say it is inefficient if the size of data grows. Two solutions are to have fixed size records and index files.
For fixed-size records, in the file write the exact bytes of your data structure rather than a variable length text. This would mean you don't have a text file any more, but a binary file. You can then calculate the position to seek to easily.
To create an index file, write two files at once, one your variable size record file, and the other write a binary value with either the offset of the data from the start of the file. Since the index is a fixed size, you can seek to the index, read it, then seek to the position in the data file. If the new record will fit, you can update it in place, otherwise fill in with blanks and put the updated record at the end of the data file, then update the index file to point to the new location. This is basically how early PC databases worked.
Fixed size records are rather inflexible, and by the time you've implemented the index file system and tested it, now-a-days you probably would use a in-process database instead.

Answer 2

I came up with my own (easy & inefficient) logic to copy every line (except the line I want to update) to the another file.
I made my text file to be created like this :
=p // p for present - 1st line
1 // roll number
Monte Cristo // full name
Black Michael // Fathers name
=a // a for absent - 5th line
2 // roll number
Johnson // full name
Nikolas // Fathers name
Then I made the following code to update 1st and 5th line :
ifstream datafile("data.txt", ios ::in);
ofstream tempfile("temp.txt" , ios ::out);
string data, ch1;
while (getline(datafile,data))
{
if (data[0]=='=')
{
system("cls");
cout <<"\t\tPresent : p\n\t\tAbsent : a"<<endl;
cout <<"\nRoll #"<<i<<" : ";
cin >> ch1;
++i;
if (ch1 != "p")
ch1 = "a";
data=data+ch1; // Appending (updating) lines.
}
tempfile <<data <<endl; // If it was 1st or 5th line, it got updated
}
datafile.close(); tempfile.close();
remove("data.txt"); rename("temp.txt" , "data.txt");
But as you can see, this logic is inefficient. I will still wait for someone to inform me if I could somehow move my file pointer to exact location (1st and 5th line) and update them.
Cheers!