Thread: How to Reset a text file to bof??

Hi,

I'm wondering how to reset a text file (ifstream infile("blah.txt")) to the beginning of the file using C++ (no C) after you've been working through it with some getline's.

From what I've read, it seems to be a bit easier to work with files in C using the FILE type and all the functions that go w/it but I really want to know how to do it in C++ with no old headers.

Thanks for your help.

Swaine777

File streams, at least ifstreams, have a seekg method that is use to repoint the get pointer for that stream. I.e. to read through a file twice you would do something along the lines of:

Code:

#include <fstream>
using namespace std;

int main()
{
    ifstream infile("Blah.Txt");

    // Loop through file once getting data, doing stuff, etc...

    infile.seekg(0,ios::beg);  // Seek to 0 bytes from the beginning of the file, i.e. the beginning

    // Loop through the file a second time doing more stuff...

    return 0;
}

Code not tested but I think that is the general idea.

Once you get to a point where infile.eof() is true, you need to reset some flags within the infile stream by calling infile.clear(). This must be done prior to the infile.seekg(0,ios::beg) call.

You can probably use the other version of the getline function, for example:

Code:

getline(infile,dummy);

As long as your lines in the text file have newline characters at the end of them this will do fine.

The infile.close() is not really needed but it's ok to have it there. Once the function is over, the destructor will automatically be called which will close the stream by itself.

An easier way to do this might be to consider an STL container class to store the data, perhaps a map<int,vector<string> > container, i.e:

Code:

#include <vector>
#include <map>
#include <string>
#include <fstream>
#include <iostream>
#include <iterator>    // For ostream_iterator
#include <algorithm>   // For copy
using namespace std;

void SearchForVerse()
{
    map<int,vector<string> > Verses;
    map<int,vector<string> >::iterator itVerses;
    vector<string> aVerse;
    ifstream infile("verses.txt");
    string temp;
    unsigned int verseChoice, i = 0;

    // Read input file, store all verses into map.

    getline(infile,temp);
    while( !infile.eof() )
    {
        if( temp.length() != 0 )
        {
            if( temp[0] == '|'  )
            {
                if( i > 0 )
                {
                    Verses[i] = aVerse;
                    aVerse.clear();
                }
                aVerse.push_back(temp);
                ++i;
            }
            else aVerse.push_back(temp);
        }
        getline(infile,temp);
    }
    Verses[i] = aVerse; // [edit] added this[/edit]

    // Show all entries and get users choice.

    cout << "Here are the references for the verses stored in verses.txt...\n\n";
    for( itVerses = Verses.begin(); itVerses != Verses.end(); ++itVerses )
        cout << itVerses->first << ". " << itVerses->second[0] << endl;
    cout << "Choose a verse to display: ";
    cin >> verseChoice;

    // Output selected verse to screen.

    if( verseChoice <= Verses.size() && verseChoice != 0 )
    {
        ostream_iterator<string> osit(cout,"\n");
        copy(Verses[verseChoice].begin(),Verses[verseChoice].end(),osit);
    }

}

Thank you for the advice about vectors & maps. Maps are really powerful if you know how to use them. I didn't really understand this chunk of code that you used to display the contents of the map/vector:

Code:

if( verseChoice <= Verses.size() && verseChoice != 0 )
    {
        ostream_iterator<string> osit(cout,"\n");
        copy(Verses[verseChoice].begin(),Verses[verseChoice].end(),osit);
    }

My C++ books are too elementary to have more advance STL stuff like this. I can only guess about the declaration of the ostream_iterator and the copy line. Declare an ostream_iterator that iterates through strings called "osit" that displays the contents of the output stream as it iterates and uses the newline char as the delimiter? Copy from the beginning of the chosen vector within the map to the end of the vector . . . to osit, an ostream_iterator which is set up to display it's contents line by line. How'm I doin'?

I was wondering, how to delete a vector from the map and rewrite the text file with the new map? I've been trying to figure it out by myself but I keep scratching my head trying to figure out what next.

Oop, 1 other thing...I'm trying to figure out how to get rid of the pipe after the search when I display the verse that was found.

Thanks for all your help.

Swaine777

Originally Posted by Swaine777

My C++ books are too elementary to have more advance STL stuff like this. I can only guess about the declaration of the ostream_iterator and the copy line. Declare an ostream_iterator that iterates through strings called "osit" that displays the contents of the output stream as it iterates and uses the newline char as the delimiter? Copy from the beginning of the chosen vector within the map to the end of the vector . . . to osit, an ostream_iterator which is set up to display it's contents line by line. How'm I doin'?

That's basically what it's doing... yes.

I was wondering, how to delete a vector from the map and rewrite the text file with the new map? I've been trying to figure it out by myself but I keep scratching my head trying to figure out what next.

The map container comes equipped with an erase member function. Though there are different versions of this function, one that should interest you is the one that takes as a parameter the key value that you wish to find and erase from the map. It returns the number of elements removed, which in the case of a map should always be either 0 or 1. For the example we are working with, if we wanted to erase the second verse from the map we could say simply:

Code:

Verses.erase(2);

However, if we did that and then tried to redisplay the list of choices immediately after the call to erase, we would then get:

Code:

Here are the references for the verses stored in verses.txt...

1. Romans 3:23
3. Romans 1:21

Note that our numbering is now off a little. I would recommend you write the new map contents to the file and reload it before displaying and asking for the users choice. Also, a better way than what I had before to validate the users choice for verse might be to do this:

Code:

if( Verses.find(verseChoice) != Verses.end() )
{
    ostream_iterator<string> osit(cout,"\n");
    copy(Verses[verseChoice].begin(),Verses[verseChoice].end(),osit);
}

Writing the new data in the map container to your file is a simple matter of using an iterator to loop over the whole map. Within that loop, you then need another iterator in a loop to output all the strings for a particular verse (remembering to also output a pipe symbol for the first line fo each new verse).

Originally Posted by Swaine777

Oop, 1 other thing...I'm trying to figure out how to get rid of the pipe after the search when I display the verse that was found.

Make following change in red

Code:

if( temp[0] == '|'  )
{
    if( i > 0 )
    {
        Verses[i] = aVerse;
        aVerse.clear();
    }
    aVerse.push_back(temp.substr(1));
    ++i;
}

Originally Posted by Swaine777

I appreciate it so much! I was just reading a map tutorial today. It was good but it didn't have an example of find() and erase().

How'd U learn all that stuff about maps anyway? Can you remember which book or class you took?

Gratefully,

Swaine777

A good book about the whole STL area of programming is Nicolai Josuttis' The C++ Standard Library - A Tutorial and Reference. I carry that book around with me everywhere. It's not a beginners book but it is a must have.

Originally Posted by Swaine777

Hi again,

I've been working on implementing the suggestions you made for my program concerning maps and vectors and I realized that I don't know how to access the elements of the vectors within the map...

Code:

cout << "Rewriting verse file...\n\n";

int count = 0;

for(itVerses = Verses.begin(), itVerses != Verses.end(), itVerses++)
{
for(vit = Verses[count].begin(), vit != Verses[count].end(), vit++)
{
if(Verses[count] //scratch head here


I made an iterator for the vector as you said and hopefully I'm using it correctly in the for loop. I know you can get a vector's element by going "aVerse[i]" but how do you do that for a vector within a map? Verses[count].aVerse[count]?????

Code to iterate through the map and vector elements should look something like this:

Code:

map<int,vector<string> >::iterator itVerses;
vector<string>::iterator vit;

// Iterate through the map values...

for( itVerses = Verses.begin(); itVerses != Verses.end(); ++itVerses )
{
    // Now iterate throught the particular string vector containing the single verse.
    // itVerses as a map iterator has some important members, first and second,
    // itVerses->first is the key and itVerses->second is the value which
    // in this case is the vector<string> container that we stored in the map
    // Therefore, itVerses->second.begin() returns a vector<string>
    // iterator to the beginning of the particular vector element, i.e. string
    // that we are dealing with

    for( vit = itVerses->second.begin(); vit != itVerses->second.end(); ++vit )
    {
        // Do something with the individual string we are looking at
        // Such as write it to the screen...

        cout << *vit << endl;
    }

}

The above code will output the entire contents of the map, all of the verses to the screen. If you are rewritting the map to a file in order to store a modified set of verses, then you will need to remember to add that pipe symbol to the output for the first string element of the vector. In the above code, this could be accomplished by inserting the necessary code before the inner for loop.

Originally Posted by Swaine777

Also,

I'm assuming that if I have a function that loads the map, that when I leave the scope of the function, the map is gone too. Is that right? Or maybe since this is a global function it's still there? What's the best way to make a LoadMap() function that is accessable by many other functions? Should I have the LoadMap() function return a reference to the map?

If I want to have a function that loads the map and then displays the contents of the map that all other
functions can access (i.e. LoadVerses() and DeleteVerse()) does that mean that I have to make a class? I've tried to learn classes a number of times and I it's just not getting through my head. I need to practice working with classes, accessing member variables and member functions of classes, inheritance, polymorphism, all that stuff.

You would do well to make individual functions to load, display, write, and delete elements from the map. You should pass the map as a reference parameter to your functions so that they can access and modify it. Then you can have your map declaration in main for instance and pass it to all of your functions which will perform all your necessary operations:

Code:

LoadVersesFromFile(map<int,vector<string> >& Verses);
DeleteSelectedVerse(map<int,vector<string> >& Verses);
WriteVersesToFile(map<int,vector<string> >& Verses);
DisplayUsersVerse(map<int,vector<string> >& Verses);

int main()
{
    map<int,vector<string> > Verses;

    LoadVersesFromFile( Verses );

    DisplayUsersVerse( Verses );

    DeleteSelectedVerse( Verses );

    WriteVersesToFile( Verses );

    return 0;
}