Accessing to information in a ".txt" file and go to a determinated row

When accessing a text file, I want to read from a specific line. Let's suppose that my file has 1000 rows and I want to read row 330. Each row has a different number of characters and could possibly be quite long (let's say around 100,000,000 characters per row). I'm thinking fseek() can't be used effectively here.

I was thinking about a loop to track linebreaks, but I don't know how exactly how to implement it, and I don't know开发者_开发百科 if that would be the best solution.

Can you offer any help?

Unless you have some kind of index saying "line M begins at position N" in the file, you have to read characters from the file and count newlines until you find the desired line.

You can easily read lines using std::getline if you want to save the contents of each line, or std::istream::ignore if you want to discard the contents of the lines you read until you find the desired line.

There is no way to know where row 330 starts in an arbitrary text file without scanning the whole file, finding the line breaks, and then counting.

If you only need to do this once, then scan. If you need to do it many times, then you can scan once, and build up a data structure listing where all of the lines start. Now you can figure out where to seek to to read just that line. If you're still just thinking about how to organize data, I would suggest using some other type of data structure for random access. I can't recommend which one without knowing the actual problem that you are trying to solve.

Create an index on the file. You can do this "lazily" but as you read a buffer full you may as well scan it for each character.

If it is a text file on Windows that uses a 2-byte '\n' then the number of characters you read to the point where the newline occurs will not be the offset. So what you should do is a "seek" after each call to getline().

something like:

std::vector< off_t > lineNumbers;
std::string line;
lineNumbers.push_back(0); // first line begins at 0
while( std::getline( ifs, line ) )
{
   lineNumbers.push_back(ifs.tellg());
}

last value will tell you where EOF is.

I think you need to scan the file and count the \n occurrences since you find the desired line. If this is a frequent operation, and you are the only one you write the file, you can possibly mantain an index file containing such information side by side with the one containing the data, a sort of "poor-man-index", but can save a lot of time.

Try running fgets in a loop

/* fgets example */
#include <stdio.h>

int main()
{
   FILE * pFile;
   char mystring [100];

   pFile = fopen ("myfile.txt" , "r");
   if (pFile == NULL) perror ("Error opening file");
   else {
     fgets (mystring , 100 , pFile);
     puts (mystring);
     fclose (pFile);
   }
   return 0;
}