How to read text file into deque

Question

I am trying to construct a deque of strings (in C++) from a txt file by putting a new entry in the deque for each line in the file. Below is my attempt at the function - I know that the while loop is being executed the proper number of times, however after calling this function the queue is always empty. I'm sure I'm missing something small (very new to C++ syntax and workings...), and any help is greatly appreciated.

void read_file(string file_name, deque<string> str_queue) {
    ifstream filestr;
    filestr.open(file_name);
    if (!filestr.is_open()){
        perror ("Error opening file.");
    }
    else {
        while (filestr) {
            string s;
            getline(filestr,s);
            str_queue.push_back(s);
        }
    }
}

you could create a stream extraction operator overload instead ... — AJG85, Jul 08 '11 at 21:34

score 9 · Answer 1 · answered Jul 08 '11 at 21:30

9

You are passing the queue by value, not by reference. Try this:

void read_file(const string &file_name, deque<string> &str_queue) {

answered Jul 08 '11 at 21:30

Nicol Bolas

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@rowerguy229 Because Nicol's answer helped with your problem, it's polite to mark it as accepted by clicking the green checkmark. Accepting answers also rewards you with Stack Overflow reputation points. – Greg Bacon Jul 09 '11 at 17:47

score 3 · Answer 2 · answered Jul 08 '11 at 21:31

3

Pass the deque<string> either by reference or pointer. You are creating a local deque which goes out of scope at end of call.

answered Jul 08 '11 at 21:31

user803563

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11

score 1 · Answer 3 · edited May 23 '17 at 12:04

I suggest using STL to work for you (see working demo on codepad[1]); this program will replicate itself on stdout:

#include <iostream>
#include <fstream>
#include <iterator>
#include <deque>
#include <vector>

using namespace std;

struct newlinesep: ctype<char>
{
    newlinesep(): ctype<char>(get_table()) {}

    static ctype_base::mask const* get_table()
    {
        static vector<ctype_base::mask> rc(ctype<char>::table_size,ctype_base::mask());
        rc['\r'] = rc['\n'] = ctype_base::space;
        return &rc[0];
    }
};

int main()
{
    deque<string> str_queue;

    ifstream ifs("t.cpp");
    ifs.imbue(locale(locale(), new newlinesep));
    copy(istream_iterator<string>(ifs), istream_iterator<string>(), back_inserter(str_queue));
    copy(str_queue.begin(), str_queue.end(), ostream_iterator<string>(cout, "\n"));
    return 0;
}

The idea of imbuing the custom locale (with newlinesep) was borrowed from this answer: Reading formatted data with C++'s stream operator >> when data has spaces

_{[1] interestingly this tells us quite a bit about the implementation details of codepad.org; not only have I guess the source filename used (t.cpp) but also can we see that the source code gets modified slightly (what is prelude.h? - Perhaps it is a giant pre-compiled header to reduce the server load)}

score 1 · Answer 4 · edited May 23 '17 at 11:56

1

I would start with one of the answers to a previous question. In my answer there, I gave one example using a std::set and one using a std::vector, but the code should continue to work fine if you use a std::deque instead:

std::deque<std::string> lines;

std::copy(std::istream_iterator<line>(std::cin), 
          std::istream_iterator<line>(),
          std::back_inserter(lines));

edited May 23 '17 at 11:56

Community

1
1

answered Jul 08 '11 at 22:33

Jerry Coffin

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How to read text file into deque

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