How to read a file line by line or a whole text file at once?

Question

I'm in a tutorial which introduces files (how to read from file and write to file)

First of all, this is not a homework, this is just general help I'm seeking.

I know how to read one word at a time, but I don't know how to read one line at a time, or how to read the whole text file.

What if my file contains 1000 words? It is not practical to read entire file word after word.

My text file named "Read" contains the following:

I love to play games
I love reading
I have 2 books

This is what I have accomplished so far:

#include <iostream>
#include <fstream>

using namespace std;
int main (){
   
  ifstream inFile;
  inFile.open("Read.txt");

  inFile >>

Is there any possible way to read the whole file at once, instead of reading each line or each word separately?

Answer 1

Read whole file at once

#include <fstream>
#include <string>

#include <iostream> // only needed for testing

int main()
{
    std::ifstream myfile("file.txt");
    std::string file_text(
        std::istreambuf_iterator<char>(myfile),
        std::istreambuf_iterator<char>()
    );
    
    // test if it worked
    std::cout << file_text;
}

Read file line by line

#include <fstream>
#include <string>

#include <iostream> // only needed for testing

int main()
{
    std::ifstream myfile("file.txt");
    std::string file_text, line;

    while(getline(myfile, line))
        file_text += line + '\n';

    // test if it worked
    std::cout << file_text;
}

Obviously, you may want to do some work for each line instead of just appending it. This is just a minimal example.

Answer 2

#include <iostream>
#include <filesystem>
#include <fstream>
#include <optional>
#include <string>

// Function to read the contents of a file into a string
// Returns std::nullopt if the file cannot be opened
std::optional<std::string> readFileContents(const std::filesystem::path& path)
{
    // Open the file as an input file stream
    std::ifstream fileStream(path);

    if (!fileStream.good())
    {
        return std::nullopt;
    }

    // Read the whole chunk
    std::string fileContents((std::istreambuf_iterator<char> 
    (fileStream)),std::istreambuf_iterator<char>());

    return fileContents;
}

int main()
{
    const std::filesystem::path filePath("C:/Day1/file.txt");

    auto fileContents = readFileContents(filePath);

    if (fileContents)
    {
        std::cout << *fileContents << '\n';
    }
    else
    {
        std::cerr << "Unable to open the file for reading." << '\n';
    }
}

Answer 3

The above solutions are great, but there is a better solution to "read a file at once":

ifstream ifs(filename);
ostringstream oss;
oss << ifs.rdbuf();
string entireFile = oss.str();

Answer 4

The below snippet will help you to read files which consists of unicode characters

CString plainText="";
errno_t errCode = _tfopen_s(&fStream, FileLoc, _T("r, ccs=UNICODE"));
    if (0 == errCode)
    {
        CStdioFile File(fStream);
        CString Line;
        while (File.ReadString(Line))
        {
            plainText += Line;
        }
    }
    fflush(fStream);
    fclose(fStream);

you should always close the file pointer after you read, otherwise it will leads to error

Answer 5

hello bro this is a way to read the string in the exact line using this code

hope this could help you !

#include <iostream>
#include <fstream>

using namespace std;


int main (){


    string text[1];
    int lineno ;
    ifstream file("text.txt");
    cout << "tell me which line of the file you want : " ;
    cin >> lineno ; 



    for (int i = 0; i < lineno ; i++)
    {
        
        getline(file , text[0]);

    }   

    cout << "\nthis is the text in which line you want befor  :: " << text[0] << endl ;
    system("pause");

    return 0;
}

Good luck !

Answer 6

you can also use this to read all the lines in the file one by one then print i

#include <iostream>
#include <fstream>

using namespace std;



bool check_file_is_empty ( ifstream& file){
    return file.peek() == EOF ;
}

int main (){


    string text[256];
    int lineno ;
    ifstream file("text.txt");
    int num = 0;

    while (!check_file_is_empty(file))
    {    
        getline(file , text[num]);
        num++;
    }
    for (int i = 0; i < num ; i++)
    {
        cout << "\nthis is the text in " <<  "line " << i+1 << " :: " << text[i] << endl ;


    }
    
    system("pause");

    return 0;
}

hope this could help you :)

Answer 7

I know this is a really really old thread but I'd like to also point out another way which is actually really simple... This is some sample code:

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main() {

    ifstream file("filename.txt");
    string content;

    while(file >> content) {
        cout << content << ' ';
    }
    return 0;
}

Answer 8

Well, to do this one can also use the freopen function provided in C++ - http://www.cplusplus.com/reference/cstdio/freopen/ and read the file line by line as follows -:

#include<cstdio>
#include<iostream>

using namespace std;

int main(){
   freopen("path to file", "rb", stdin);
   string line;
   while(getline(cin, line))
       cout << line << endl;
   return 0;
}

Answer 9

You can use std::getline :

#include <fstream>
#include <string>

int main() 
{ 
    std::ifstream file("Read.txt");
    std::string str; 
    while (std::getline(file, str))
    {
        // Process str
    }
}

Also note that it's better you just construct the file stream with the file names in it's constructor rather than explicitly opening (same goes for closing, just let the destructor do the work).

Further documentation about std::string::getline() can be read at CPP Reference.

Probably the easiest way to read a whole text file is just to concatenate those retrieved lines.

std::ifstream file("Read.txt");
std::string str;
std::string file_contents;
while (std::getline(file, str))
{
  file_contents += str;
  file_contents.push_back('\n');
}  

Answer 10

Another method that has not been mentioned yet is std::vector.

std::vector<std::string> line;

while(file >> mystr)
{
   line.push_back(mystr);
}

Then you can simply iterate over the vector and modify/extract what you need/

Answer 11

I think you could use istream .read() function. You can just loop with reasonable chunk size and read directly to memory buffer, then append it to some sort of arbitrary memory container (such as std::vector). I could write an example, but I doubt you want a complete solution; please let me know if you shall need any additional information.