how to extract file names with .txt extension from a text file?

Question

I have a text file named myfile.txt which lists the contents of drive D:\. In my program, I have a functon which will read myfile.txt. It will extract filenames from the .txt extension. I don't know much C++, so can you make it please "simple"? I am confused about the starting position of the substring as how would I know from where it will start.

#include<iostream>
#include<string>
#include<fstream>

using namespace std;

int main(void)
{
    system("cls");
    string temp;
    fstream file;
    file.open("D:\\myfile.txt", ios::in);

    while( file.good() )
    {     
        file>>temp;
        string str2, str3;
        size_t pos;

        str2 = temp.substr (4,4); // confused with this

        pos = temp.find(".txt");    // position of ".txt" in temp
        str3 = temp.substr (pos);  

        cout << str2 << ' ' << str3 << endl;
    }

    system("pause");
    return 0;
}

Another point - if you're on a unix system, you could do the filtering before you even get to your program. i.e. `find / -type f -iname "*.txt" > textFiles.txt` — I82Much, Apr 07 '11 at 18:40
Or, if you already have the list, `grep 'txt$' < files.txt > textfiles.txt`. — Robᵩ, Apr 07 '11 at 18:59

score 4 · Answer 1 · answered Apr 07 '11 at 17:08

4

Read the text file that contains the file name.
If the file name ends with .txt, insert it into the array, otherwise discard it.
Continue it until you reach at the end of the file.

For reference: ifstream, string.

answered Apr 07 '11 at 17:08

Donotalo

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how will i make sure that it ends with .txt? – Mahee Apr 07 '11 at 17:16
take a substring of the filename and compare if the substring is .txt or not. Look into the string link i've provided to know how to take substring. – Donotalo Apr 07 '11 at 18:00
thanks!!. What if i dont know the starting position of the substring as required in string substr ( size_t pos = 0, size_t n = npos ) const; how will i determine the starting position. – Mahee Apr 08 '11 at 21:33
i got this after compilation when i implemented the program as you suggested....Unhandled exception at 0x7c81eb33 in file.exe: Microsoft C++ exception: std::out_of_range at memory location 0x0012fc38.. – Mahee Apr 08 '11 at 21:38
you can leave the parameters as default. – Donotalo Apr 08 '11 at 22:32

score 4 · Answer 2 · edited May 23 '17 at 11:55

4

Load file, extract all lines from it (store in mutable list) ( http://www.linuxquestions.org/questions/programming-9/c-text-file-line-by-line-each-line-to-string-array-126337/ )
Loop through list and delete all the strings that do not end with .txt ( Find if string ends with another string in C++ )

edited May 23 '17 at 11:55

Community

1
1

answered Apr 07 '11 at 17:09

I82Much

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1

That seems like a waste of memory if only .001% of the file names end in ".TXT". How about `std::istream_iterator` instead of creating the first list? – Robᵩ Apr 07 '11 at 17:52
I agree that this is not the most efficient approach. However, it is probably the simplest conceptually, and I'd rather the OP get a working implementation and worry about improving it later rather than worrying prematurely optimizing it. – I82Much Apr 07 '11 at 18:39

score 0 · Answer 3 · answered Apr 07 '11 at 18:10

#include <fstream>
#include <vector>
#include <string>

int main (int argc, char* argv[])
{
  if (argc != 2)
    return 1;
  std::ifstream file(argv[1]);
  if (!file.is_open())
    return 2;

  std::vector<std::string> files;
  std::string line;
  while (std::getline(file, line)) {
    if(line.length() < 4)
      continue;
    if(line.substr(line.length() - 4, std::string::npos) == ".txt")
      files.push_back(line);
  }

  /* all files ending with .txt are in "files" */

  return 0;
}

score 0 · Answer 4 · answered Apr 08 '11 at 21:26

#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>

// inspried by http://www.cplusplus.com/reference/std/iterator/istream_iterator/
struct getline :
  public std::iterator<std::input_iterator_tag, std::string>
{
    std::istream* in;
    std::string line;
    getline(std::istream& in) : in(&in) {
        ++*this;
    }
    getline() : in(0) {
    }
    getline& operator++() {
        if(in && !std::getline(*in, line)) in = 0;
    }
    std::string operator*() const {
        return line;
    }
    bool operator!=(const getline& rhs) const {
        return !in != !rhs.in;
    }
};

bool doesnt_end_in_txt(const std::string& s) {
    if (s.size() < 4)
        return true;
    if (s.compare(s.size()-4, 4, ".txt") != 0)
        return true;
}

int main() {
    std::vector<std::string> v;
    std::remove_copy_if(getline(std::cin), getline(),
        std::back_inserter(v),
        doesnt_end_in_txt);
}

how to extract file names with .txt extension from a text file?

4 Answers4