Convert text document to a char array c++

Question

I am attempting to convert text from a text document to a char array. To begin I tried to implement a dynamic array. However, for some reason when I attempt to save each char from the text into the new array it returns a bunch of equals signs. Down below is what I have:

Say for example the text said something like "Make this a char array".

#include <iostream>
#include <fstream>
using namespace std;
int main()
{
    char x;
    int y = 0;
    ifstream file;
    file.open("text.txt");
    while (file >> x)
        y++;

    char *phrase = NULL;
    phrase = new char[y];

    for (int i = 0; file >> x; i++)
    {
        phrase[i] = x;
    }
    for (int i = 0; i < y; i++)
    {
        cout << phrase[i];
    }
}

It will end up outputting: "=================="

I did research on this issue but I couldn't find anything that clears it up.

You need to go back to the beginning of the file. You can't just keep reading the file after you have read everything to the end. — Galik, Jun 11 '18 at 23:07
It would be easier to use a `std::vector` which will automatically resize its internal array as needed. — NO_NAME, Jun 11 '18 at 23:12
This doesn't address the question, but don't do pointless initialization followed by modifying objects. Instead of `ifstream file; file.open("text.txt");` use `ifstream file("text.txt");`. Similarly, instead of `char *phrase = NULL; phrase = new char[y];` use `char *phrase = new char[y];`. Your code block doesn't go this far, but I'll bet there's `file.close()` further down; that's not needed, either: the destructor will close the file. — Pete Becker, Jun 11 '18 at 23:36
@einpoklum - I also helped mess it up, if that `iostream` was a cut-and-paste from mine. In mine it's a spell-check-induced error. I took the liberty of fixing that in your answer. — Pete Becker, Jun 11 '18 at 23:42
(checking...) Oh yeah. Right. Wll, all's well that ends well. — einpoklum, Jun 11 '18 at 23:43
I think douplicate of https://stackoverflow.com/questions/116038/what-is-the-best-way-to-read-an-entire-file-into-a-stdstring-in-c — M.M, Jun 12 '18 at 00:14

einpoklum · Answer 1 · 2018-06-12T07:33:14.817

You seem to be "measuring" the length of the file by repeatedly reading individual characters from it. You don't need to do that - just determine the size before opening the file:

#include <filesystem>
#include <fstream>  

int main() {
    auto file_name { "text.txt" };
    auto size = std::filesystem::file_size(file_name);
    std::ifstream file(file_name);
    // etc. etc.

See the documentation for the file_size() function. It's in C++17; if you're using an earlier version of the language, try >experimental/filesystem> with C++14, or otherwise the boost::filesystem library with any version of C++.

... but actually, you don't need to do that at all!

You can just read the entire file using plain vanilla C++ standard library facilities:

#include <sstream>
#include <iostream>
#include <fstream>

int main() {
    std::ifstream file("text.txt");
    if (not file) { /* handle error */ }
    std::stringstream sstr;
    sstr << file.rdbuf(); // magically read all of the file!
    auto entire_file_contents = sstr.str();
    const char* arr = entire_file_contents.c_str();
    // Now do whatever you like with the char array arr
}

See also this: What is the best way to read an entire file into a std::string in C++?

By the way, it's kind of tricky to determine the size of a file given an std::ifstream without sort-of reading the whole file, see this answer.

@davidshen84: [`not` is the same as `!`](http://en.cppreference.com/w/cpp/language/operator_alternative) - and this equivalence is very old actually. Also see [here](http://en.cppreference.com/w/cpp/io/basic_ios/operator!) about `ifstream`'s `operator!`. — einpoklum, Jun 11 '18 at 23:44
@M.M: Already fixed a while ago I think. Plus, feel free to edit. — einpoklum, Jun 12 '18 at 00:15
@Barmar: No-but-I-should-fix-the-indentation, and yes, respectively. Edited. — einpoklum, Jun 12 '18 at 07:33

Pete Becker · Answer 2 · 2018-06-11T23:52:44.570

2

Here's another approach:

#include <fstream>
#include <iterator>
#include <algorithm>

int main() {
    std::ifstream file("text.txt");
    std::vector<char> text;
    std::copy(std::istream_iterator<char>(file),
        std::istream_iterator<char>(),
        std::back_inserter(text));
    // now text holds the contents of the file
    // if you really need a char array, you can use text.data()
}

Note that the code does not test whether the file was opened successfully. I/O operations on streams are no-ops when the stream is in an error state. Yes, you can check if you prefer, but it's not strictly necessary.

edited Jun 11 '18 at 23:52

answered Jun 11 '18 at 23:48

Pete Becker

69,019
6
64
147

+1, but I suggest you include some token error handling code anyway, as otherwise you're surreptitiously legitimizing failure to check errors generally. – einpoklum Jun 11 '18 at 23:52
Also, how fast is this? Are you sure it doesn't end up copying one character at a time? – einpoklum Jun 12 '18 at 12:05

score 1 · Answer 3 · answered Jun 11 '18 at 23:57

1

Here's one way to do it:

std::ifstream file("text.txt", std::ios::binary | std::ios::ate);
std::streamsize size = file.tellg();
file.seekg(0, std::ios::beg);

std::vector<char> buffer(size);
if (file.read(buffer.data(), size))
{
    //...
}

answered Jun 11 '18 at 23:57

Andreas DM

9,654
6
31
58

This has different behaviour to the original intent of opening the file in text mode, on some operating systems – M.M Jun 12 '18 at 00:08

Convert text document to a char array c++

3 Answers3