• 0

[C++] How to jump a specific line when reading a file?


Question

11 answers to this question

Recommended Posts

  • 0

There's no way to go to a specific line without reading up to that point, since each line in the file you are reading probably has a different length. You could try read the entire file while looking for newlines and for each newline you find, add the position to an array. Then you can seek to that position by using the line number as the array's index and the value as the position in the file.

  • 0
  Banjo said:
There's no way to go to a specific line without reading up to that point, since each line in the file you are reading probably has a different length. You could try read the entire file while looking for newlines and for each newline you find, add the position to an array. Then you can seek to that position by using the line number as the array's index and the value as the position in the file.

That sounds incredibly taxing, though :/ Especially if it's a really big file that you need to go through multiple times. Or am I reading it wrong?

  • 0

You would only need to go through it once to store the line indexes in the array. After that, everything is fast since you just look up two values in the array - the start of the line you want and the start of the next line (so you know how much to read)

If you are in control of what the file contains, an even easier method is to just pad each line to the same length and use multiplication to find out where the line starts

  • 0
  The Tjalian said:
That sounds incredibly taxing, though :/ Especially if it's a really big file that you need to go through multiple times. Or am I reading it wrong?

Well how else would you do it? Unless the file has lines of a fixed length, or has its own index, somebody needs to scan through it to find the line breaks. Whether it's a library / API or your own code, it needs to be done.

If you just need to scan to a certain line number once and make a single pass through the file, you don't need to save off the indices into an array as suggested. That's an optimization that you can do if you're going to need to seek to different lines within the file several times (and possibly out-of-order). Likely you'll want to make this part of your "loading" process for the file as users are accustomed to this behavior.

  • 0

by using the seekg method I posted earlier, the user can set the seekg to the 0th position from ios::end, access tellg to get the length of the file, then set seekg to the position he wanted knowing his upper and lower limits... very simple an efficient.

  • 0

how about using getline() ?

Here you can find this code:

ifstream fin("data.txt");
  string s;
  while( getline(fin,s) ) {
	cout << "Read from file: " << s << endl;
  }

which is reading string line by line. Just store the s.size(); (string's length) to a variable, and sum it up for each loop. You may set the loop limit as the argument pass, and return the sum for seekg();.

If I got a chance, I'll put the code.

Or maybe just use tellg(); after each loop for loop limit (line number). return the last tellg(); value for seekg();

This may not work. I just type it out here:

long goto_file_line(int iLine_Number) {
	ifstream fin("data.txt");
	string s;
	long length;

	fin.seekg (0, ios::beg); // go to the first line

	for (int i=0; i<=iLine_Number; i++) // loop 'till the desired line
		getline(fin, s);

	length = fin.tellg(); // tell the first position at the line, maybe +1 or -1. try if it's not working
	return length;
}

or add seekg(); for void, so you can continue your i/o after the function.

void goto_file_line(int iLine_Number) {
	ifstream fin("data.txt");
	string s;
	long length;

	fin.seekg (0, ios::beg); // go to the first line

	for (int i=0; i<=iLine_Number; i++) // loop 'till the desired line
		getline(fin, s);

	length = fin.tellg(); // tell the first position at the line, maybe +1 or -1. try if it's not working
	fin.seekg(length);
}

Edited by chu121su12
This topic is now closed to further replies.
  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Posts

    • dBpoweramp Music Converter 2025-06-05 by Razvan Serea Audio conversion perfected, effortlessly convert between formats. dBpoweramp contains a multitude of audio tools in one: CD Ripper, Music Converter, Batch Converter, ID Tag Editor and Windows audio shell enhancements. Preloaded with essential codecs (mp3, wave, FLAC, m4a, Apple Lossless, AIFF), additional codecs can be installed from [Codec Central], as well as Utility Codecs which perform actions on audio files. After 21 days the trial will end, reverting to dBpoweramp Free edition (learn the difference between Reference and dBpoweramp Free, here). dBpoweramp is compatible with Windows 10, 8, 7, Vista, both 32 and 64 bit. dBpoweramp Music Converter features: Convert audio files with elegant simplicity. mp3, mp4, m4a (iTunes / iPod), Windows Media Audio (WMA), Ogg Vorbis, AAC, Monkeys Audio, FLAC, Apple Lossless (ALAC) to name a few! Multi CPU Encoding Support Rip digitally record audio CDs (with CD Ripper) Batch Convert large numbers of files with 1 click Windows Integration popup info tips, audio properties, columns, edit ID-Tags DSP Effects such as Volume Normalize, or Graphic EQ [Power Pack Option] Command Line Encoding: invoke the encoder from the command line DSP Effects - process the audio with Volume Normalize, or Sample / Bit Rate Conversion, with over 30 effects dBpoweramp is a fully featured mp3 Converter dBpoweramp integrates into Windows Explorer, an mp3 converter that is as simple as right clicking on the source file >> Convert To. Popup info tips, Edit ID-Tags are all provided. dBpoweramp Music Converter 2025.06.05 changelog: Darkmode added Core Converter Debug log dumps ID Tags written VST Effect Folders dialog fixed missing InitCommonControls would not show correctly FLAC/Ogg/Opus/etc - allows editing of CDTOC ID Tag CD Ripper secure ripping log where shows TOC was not showing CD Extra correctly CD Ripper was incorrectly setting data track length on main display (for certain drives) CD Ripper internally better handling of corrupt TOCs CD TOC to Tag was incorrectly adding 150 to CD Extra disc CD Ripper shows "AccurateRip Unconfigured" in rip status rather than "not in accuraterip" if unconfigured CD Ripper art paste accepts https CueSheet added as standard - log file written to same folder as cue and folder.jpg AIFF internal code merge (macos >> windows) Download: dBpoweramp Music Converter R2025.06.05 | 82.2 MB (Shareware) View: dBpowerAMP Music Converter Website | Screenshot Get alerted to all of our Software updates on Twitter at @NeowinSoftware
    • Staged. It's a requirement that vehicles are strapped down to the bed. Usually wheel and/or chassis tie downs are used. That appears to just be on the winch.
    • I feel Apple's big problem is the lack of big data to train any AI LLM model. They have statistics on usage, but they don't have the written social media, messaging (they were early adopters of end-to-end encryption), they didn't scrape the Internet before the book companies and new sources were wise. So they have no choice but to use a third party LLM provider. Which ties them in knots with their own stance on security and privacy. In short, they are royally stuffed when it comes to developing an in-house AI.
    • Nothing is black and white. Democracy can suck, just as communism can. The risk is people who blindly think one is vastly superior over the other. Democracy needs a lot to make it work well, and there are many examples around the world of it. Good education, mandatory voting, accessible voting, and removing money from politics are just a few elements that need to be sorted for a functional democracy. The USA is the playbook on what not to do with democracy.
  • Recent Achievements

    • Week One Done
      abortretryfail earned a badge
      Week One Done
    • First Post
      Mr bot earned a badge
      First Post
    • First Post
      Bkl211 earned a badge
      First Post
    • One Year In
      Mido gaber earned a badge
      One Year In
    • One Year In
      Vladimir Migunov earned a badge
      One Year In
  • Popular Contributors

    1. 1
      +primortal
      495
    2. 2
      snowy owl
      255
    3. 3
      +FloatingFatMan
      252
    4. 4
      ATLien_0
      226
    5. 5
      +Edouard
      189
  • Tell a friend

    Love Neowin? Tell a friend!