Go to a specific line in file and read it

Question

Problem description

I have a file containing a set of lines. A

File 1:

"Hello How are you"

"The cat ate the mouse"

---

Based on the the beginning and ending of the lines given by the user as input. I want to go to each line in the file and Extract it.

For examples if user types 1 , 17 then I have to go to line 1 that has a size of 17 characters. He can give any line number in the file.

I read the following Answer Read from a specific spot in a file C++. But I didn't really understand. Why do the lines have to be the same size? If i have the information concerning the beginning and ending of every line in the file. Why can't I access it directly?

---

Source code

I tried to use the following code which was inspired by Read Data From Specified Position in File Using Seekg But I couldn't extract the lines why?

    #include <fstream>
    #include <iostream>

    using namespace std::

    void getline(int, int, const ifstream & );
    int main()
    {
      //open file1 containing the sentences
      ifstream file1("file1.txt");

      int beg = 1;
      int end = 17;
      getline(beg,end, file1);

      beg = 2;
      end = 20;
      getline(beg,end, file1);

      return 0;
    }

void getline(int beg, int end, const ifstream & file)
{
   file.seekg(beg, ios::beg); 
   int length = end;

   char * buffer = new char [length];

   file.read (buffer,length);

   buffer [length - 1] = '\0'; 

   cout.write (buffer,length);
   delete[] buffer;
}

Answer 1

This code appears to be using line numbers as byte offsets. If you seek to offset "1" the file seeks forward 1 byte, not 1 line. If you seek to offset 2, the file seeks forward 2 bytes, not 2 lines.

To seek to a specific line you need to read the file and count the number of line breaks until you get to the line you want. There is code that already does this, for example std::getline(). If you don't already know the exact byte offset of the line you want, you can call std::getline() the number of times equal to the line number you want.

Also remember that the first byte of a file is at offset 0 not offset 1, and that different platforms use different bytes to indicate the end of a line (E.g. on Windows it's "\r\n", on Unix it's "\n"). If you're using a library function to read lines, the line ending should be taken care of for you.