Go back one line on a text file C++

c++ file getline
IzonFreak picture IzonFreak · Dec 6, 2014 · Viewed 14.3k times · Source

my program reads a line from a text file using :

std::ifstream myReadFile("route.txt");
getline(myReadFile, line)

And if it finds something that i'm looking for (tag) it stores that line in a temp String. I wan't to continue this until i find some other tag, if i find an other tag i want to be able to return to the previous line in order for the program to read if again as some that other tag and do something else.

I have been looking at putback() and unget() i'm confuse on how to use them and if they might be the correct answer.

Answer

Christophe picture Christophe · Dec 6, 2014

Best would be to consider a one pass algorithm, that stores in memory what it could need at the first tag without going back.

If this is not possible, you can "bookmark" the stream position and retreive it later with tellg() and seekg(): 

streampos oldpos = myReadFile.tellg();  // stores the position
....
myReadFile.seekg (oldpos);   // get back to the position

If you read recursively embedded tags (html for example), you could even use a stack<streampos> to push and pop the positions while reading. However, be aware that performance is slowed down by such forward/backward accesses.

You mention putback() and unget(), but these are limited to one char, and seem not suited to your getline() approach.

Related questions

c++ Read from .csv file

I have this code which is supposed to cout in console the information from the .csv file; while(file.good()) { getline(file, ID, ','); cout << "ID: " << ID << " " ; getline(file, nome, ',') ; cout &…

Read More
very fast text file processing (C++)

i wrote an application which processes data on the GPU. Code works well, but i have the problem that the reading part of the input file (~3GB, text) is the bottleneck of my application. (The read from the HDD is …

Read More
How can I get the list of files in a directory using C or C++?

How can I determine the list of files in a directory from inside my C or C++ code? I'm not allowed to execute the ls command and parse the results from within my program.

Read More