Read File Line by Line Using Ifstream in C++

Read file line by line using ifstream in C++

First, make an ifstream:

#include <fstream>
std::ifstream infile("thefile.txt");

The two standard methods are:

  1. Assume that every line consists of two numbers and read token by token:

    int a, b;
    while (infile >> a >> b)
    {
    // process pair (a,b)
    }
  2. Line-based parsing, using string streams:

    #include <sstream>
    #include <string>

    std::string line;
    while (std::getline(infile, line))
    {
    std::istringstream iss(line);
    int a, b;
    if (!(iss >> a >> b)) { break; } // error

    // process pair (a,b)
    }

You shouldn't mix (1) and (2), since the token-based parsing doesn't gobble up newlines, so you may end up with spurious empty lines if you use getline() after token-based extraction got you to the end of a line already.

C++ reading file line by line [duplicate]

    string line;
ifstream myfile;
myfile.open("myfile.txt");

if(!myfile.is_open()) {
perror("Error open");
exit(EXIT_FAILURE);
}
while(getline(myfile, line)) {
cout << line << endl;
}

You just need to add a loop to get all lines of the file

C++ Reading text file line by line

Working C++ Code

To clear doubts Test it in https://www.tutorialspoint.com/compile_cpp11_online.php
Just copy-paste execute

#include <iostream>
#include <string>
#include <sstream>
#include <fstream>
int main ()
{
//code create a file with numbers
std::ofstream myfile;
myfile.open ("file.txt");
myfile << "123 12 32323\n 4444 55 535\n";
myfile.close();

std::ifstream input( "file.txt" );
for( std::string eachLine; getline( input, eachLine ); )
{
std::istringstream strm(eachLine);
std::string splitedLines;
// loop for each string and add to the vector
while ( strm >> splitedLines )
{
std::stringstream geek(splitedLines);
int num; // can be int, float or double
geek >>num;
//perform action on all num in each line one by one
std::cout<<num<<std::endl;
}
}
return 0;
}

Edit: PythonCode read numbers one by one in each line

fileName = open('a.txt', 'r')
line = fileName.readline() //reading first line
while(line):
for eachStringNumber in line.split():
number = int(eachStringNumber)
/// Methods will be applied to each number in the line ///
line = fileName.readline() // reading lines one by one in each loop
fileName.close()

Read a file line by line in C++

Apart from the errors mentioned in the comments, the program has a logical error because istream& istream::get(char* s, streamsize n) does not do what you (or I, until I debugged it) thought it does. Yes, it reads to the next newline; but it leaves the newline in the input!

The next time you call get(), it will see the newline immediately and return with an empty line in the buffer, for ever and ever.

The best way to fix this is to use the appropriate function, namely istream::getline() which extracts, but does not store the newline.

The EOF issue

is worth mentioning. The canonical way to read lines (if you want to write to a character buffer) is

  while (f.getline(buf, bufSz))
{
cout << buf << "\n";
}

getline() returns a reference to the stream which in turn has a conversion function to bool, which makes it usable in a boolean expression like this. The conversion is true if input could be obtained. Interestingly, it may have encountered the end of file, and f.eof() would be true; but that alone does not make the stream convert to false. As long as it could extract at least one character it will convert to true, indicating that the last input operation made input available, and the loop will work as expected.

The next read after encountering EOF would then fail because no data could be extracted: After all, the read position is still at EOF. That is considered a read failure. The condition is wrong and the loop is exited, which was exactly the intent.

The buffer size issue

is worth mentioning, as well. The standard draft says in 30.7.4.3:

Characters are extracted and stored until one of the following occurs:

  1. end-of-file occurs on the input sequence (in which case the function calls setstate(eofbit));
  2. traits::eq(c, delim) for the next available input character c
    (in which case the input character
    is extracted but not stored);
  3. n is less than one or n - 1 characters are stored
    (in which case the function calls setstate(
    failbit)).

The conditions are tested in that order, which means that if n-1 characters have been stored and the next character is a newline (the default delimiter), the input was successful (and the newline is extracted as well).

This means that if your file contains a single line 123 you can read that successfully with f.getline(buf, 4), but not a line 1234 (both may or may not be followed by a newline).

The line ending issue

Another complication here is that on Windows a file created with a typical editor will have a hidden carriage return before the newline, i.e. a line actually looks like "123\r\n" ("\r" and "\n" each being a single character with the values 13 and 10, respectively). Because you opened the file with the binary flag the program will see the carriage return; all lines will contain that "invisible" character, and the number of visible characters fitting in the buffer will be one shorter than one would assume.

The console issue ;-)

Oh, and your Console was not entirely empty; it's just that modern computers are too fast and the first line which was probably printed (it was in my case) scrolled away faster than anybody could switch windows. When I looked closely there was a cursor in the bottom left corner where the program was busy printing line after line of nothing ;-).

The conclusion

  • Debug your programs. It's very easy with VS.
  • Use getline(istream, string).
  • Use the return value of input functions (typically the stream)
    as a boolean in a while loop: "As long as you can extract any input, use that input."
  • Beware of line ending issues.
  • Consider C I/O (printf, scanf) for anything non-trivial (I didn't discuss this in my answer but I think that's what many people do).

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

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');
}

How to read lines from a file using the ifstream?

There are two basic problems with your code:

  1. You are returning a local variable. The statement return buffer; results in a dangling pointer.

  2. You are using a char buffer. C-style strings are discouraged in c++, you should always prefer std::string instead.

A far better approach is this:

string FSXController::readLine(int offset, FileLookupFlag flag) {
string line;
//your code here

getline(m_ifs, line) //or while(getline(my_ifs, line)){ //code here } to read multiple lines
//rest of your code
return line;
}

More information about std::string can be found here

Reading from file line by line

Instead of using a for loop you can use a while loop:

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

using namespace std;

int main() {

string line;
ifstream out("note.txt");
while(getline(out, line)) {
cout << line << endl;
}
out.close();
}

If you are forced not to use strings then you can try a char buffer char buf[1024]. It must be pointed out that this approach is dangerous and error prone. If a line has more than 1024 characters then a buffer overflow will occur. Buffer overflow is the cause of many vulnerabilities and crashes. That being said, if you really have to use this method I would suggest you to be very careful by making the appropriate checks.

How to read file having different line ending in C++

As Sebastian said, we will need to read the block and then find out the appropriate line-ending.

So, we will need to open the file in binary mode and read the last characters.

#include<iostream>
#include<fstream>
#include<string>
using namespace std;
void SetLineEnding(char *filename, std::string &newline, char &delimiter)
{
std::string str;
std::ifstream chk(filename,std::ios::binary);
if(getline(chk, str))
{
if(str.size() && str[str.size()-1] == '\r')
{
//It can be either \r or \r\n
if(getline(chk, str))
{
delimiter = '\n';
newline = "\\r\\n";
}
else
{
delimiter = '\r';
newline = "\\r";
}
}
else
{
delimiter = '\n';
newline = "\\n";
}
}
}
int32_t main()
{

string newLine;
string delimiter;
char filename[256];
in>>filename;
SetLineEnding(filename,newLine,delimiter);
std::ifstream inp(filename,ios::in);
if(!inp.is_open())
{
cout<<"File not opened"<<endl;
return 0;
}
//getline() function with delimiter
string str;
getline(inp,str,delimiter);

return 0;
}

Now you can pass delimiter to getline() and you will be able to read according to line-ending.



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