I am looking for an answer to my question, but i didn't find it in any other place.
I'm trying to read from a .txt file, that is located in the same directory as my project files.
I wrote this simple code:
ifstream file("file.txt");
std::string line;
std::getline(file, line);
cout << line;
...but unfortunately, nothing happened, not even an error or crashing.
Upon exploring a little further... even if I change the name of the txt("file") file, to the name of a file that doesn't exist, nothing happens.
What am I missing?
How do you know there were no errors? You did not check.
#include <cerrno>
and then
ifstream file("file.txt");
if (file) // is the file readable?
{
std::string line;
if (std::getline(file, line)) // did we manage to read anything?
{
cout << line;
}
else
{
cout << "File IO error";
}
}
else
{
cout << "error opening file: " << strerror(errno);
}
performs rudimentary checking.
if your error is due to opening file then provide full path to the file and check.
in your code you are reading the first line so if it is a white space then you can see nothing as output.
you must to iterate over each line until the last line (reaching the end of file EOF).
// let's say your file is "test.txt" which is located in D\\MyDB
// ifstream file("file.txt");
ifstream file("D:\\MyDB\\test.txt"); // use full path instead and check manully whether the file is there or not
std::string line;
if(file.fail())
cout << "Opening file failed!" << endl;
else
while(std::getline(file, line))
{
cout << line;
}
if it works when providing the full path then your current path is not the same as your project.
you can change the current directory using some API so if you are on windows then use: SetCurrentDirectory(path); and on linux use: chdir(sDirectory.c_str());
** I mean compilers not OS
Related
Note: I am using the C++11 standard, so I don't see why this isn't working with or without c_str() appended.
I have the following code:
// open streams
ifstream in(input);
ofstream out(output);
// get which file to open
in.ignore(INT_MAX, ':'); // we don't need the beginning part
in.ignore(); // remove trailing whitespace
string fileLocation;
getline(in, fileLocation);
out << "Loading: " << fileLocation << endl;
cout << "Loading: " << fileLocation << endl;
// now that we know where the file is, load it:
ifstream file(fileLocation);
which reads from a file that looks vaguely like this
File: file.txt
(Subcommands below)
I know that I am pulling the correct filename because of the terminal output.
Anyway, I noticed that the stream wasn't opening properly, so I added this conditional to check:
if ( !file )
{
cout << "File wasn't loaded properly." << endl;
}
And sure enough, I see that message when running the program.
My question is this: how come, when I hard-code the file location, e.g. ifstream file("file.txt") it opens up no problem? How do I get this working properly?
I wrote a code in C++ that writes a .txt file.
Then I want to open the code again and give some information, so I can get a new text depending on what I gave as an input.
For example I want to give the name of a month, and print in another .txt file all the lines that came after the word "November".
I found some solutions, but none of them worked for me!
One solution that I found on stack overflow is the following:
void Keyword(ifstream & stream, string token) {
string line;
while (getline(stream, line)) {
if (line.find(token) != string::npos) {
cout << line << endl;
}
}
cout << token << " not found" << endl;
}
I can't print the next lines with the code above.
Any suggestion would be helpful!
Thanks!
If you want to perform operations on files such as 'Read' and/or 'Write',you might want to search on the net(or if you have a C++ book) on topics such as "File I/O operations using C++". Anyways moving on, C++ has 2 basic classes to handle files which are ifstream and ofstream. And to use them you have to include ethier the header fstream(i.e #include<fstream>) or include them separately as #include<ifstream> and #include<ofstream>. ifstream is basically used for all input operations such as reading files etc. Similarly ofstream is used for all output operations such as writing data to files.
You can open a file and write data to it by doing the following,
ofstream myFile("filename");// Create an instance of ofstream and open file for writing data
and to write data to the file use the << operator like below,
myFile<<data;
Similarly, You can open a file and read data as follows,
ifstream myFile("filename");//Create an instance of ifstream and open file to read data
and to read data from the file use the >> operator as shown below,
myFile>>data;
You can also open a file using the method void open(const char *filename, ios::openmode mode); as shown below,
//Writing only
ofstream outFile;
outFile.open("filename.txt",ios::out);
//Reading only
ifstream inFile;
inFile.open("filename.txt",ios::in);
//For reading and writing
fstream file;
file.open("filename.txt",ios::in|ios::out);
//For closing File
outFile.close();
//or
inFile.close();
//or
file.close();
Note the open() method takes various flags such as ios::in for reading mode, ios::out for writing mode, ios::app for adding data to the end etc.
All of these can also combined by using the bit OR operator | as shown below,
outFile.open("filename.txt",ios::out|ios::app);
There is a lot more in IO. I just covered the things required to start.
Here is the solution to your problem. Try to understand it.
#include<iostream>
#include<fstream>
#include<cstring>
using namespace std;
int main()
{
ofstream outFile;
ifstream inFile;
char fileName[10],data[50];
int noLines;
cout<<"Enter Month:"<<endl;
cin>>fileName;
cout<<"Enter Number of lines you want to enter:"<<endl;
cin>>noLines;
outFile.open(fileName,ios::out);
cout<<fileName<<"(Enter Data):";
for(int i=0;i<=noLines;i++)
{
cin.getline(data,50);
outFile<<data<<endl;
}
outFile.close();
cout<<"Openening "<<fileName<<" :"<<endl;
inFile.open(fileName,ios::in);
for(int i=0 ;i<=noLines ;i++)
{
inFile.getline(data,50);
cout<<data<<endl;
}
inFile.close();
return 0;
}
OP has found most of the solution already:
string line;
while (getline(stream, line)) {
if (line.find(token) != string::npos) {
cout << line << endl;
}
}
cout << token << " not found" << endl;
But this only prints the lines with the keyword. And always prints the "not found" message. Ooops.
Instead I pitch:
string line;
bool found = false;
while (!found && getline(stream, line))
{ // search for keyword
if (line.find(token) != string::npos)
{
found = true; // found keyword. Stop looking
}
}
if (found)
{ // print out all remaining lines in the file
while (getline(stream, line))
{
cout << line << endl;
}
}
else
{
cout << token << " not found" << endl;
}
The above splits the finding of the token and the printing of the remaining file into two stages for readability. It can be compressed into one loop, but two things make this a sucker bet:
this program will be IO bound. It will spend the vast majority of its time reading the file, so little tweaks that do not address getting the file into memory are wasted time.
combining the loops would require the addition of logic to the loop that would, over along run, dwarf the minuscule cost of switching loops.
Try this:
http://www.cplusplus.com/doc/tutorial/files/
and this:
http://www.cplusplus.com/forum/beginner/14975/
It's about reading and writing files in c++ and about searching in files.
While testing my code, I faced an issue, that ifstream doesn't get updated when more data is written to its file. So here is a sample code that demonstrates the problem:
ifstream is(filename);
string line;
while (getline(is, line))
cout << "line: " << line << endl;
ofstream os(filename, ofstream::out | ofstream::app);
string additional("additional");
os << additional;
os.flush();
while (getline(is, line))
cout << "line additional: " << line << endl;
No additional lines were written to stdout, though they are written to the file.
I'm not using fstream instead of a couple of if/ofstream because I need it like this for testing purposes.
How to make ifstream "see" the changes in the file?
UPDATE: I cleared the bits using clear method. It works OK on my Ubuntu machine with gcc. But it doesn't work on my Mac OSX with llvm. Do you know how to do it platform independently?
You need to call std::ios::clear on the input stream after the first read.
When you read the whole file, it sets the failbit in the stream and will refuse to keep reading, even if the file actually changed in the meantime.
ifstream is(filename);
string line;
while (getline(is, line))
cout << "line: " << line << endl;
ofstream os(filename, ofstream::out | ofstream::app);
string additional("additional");
os << additional;
os.flush();
is.clear(); //< Now we can read again
while (getline(is, line))
cout << "line additional: " << line << endl;
When you hit the end of the file while reading is the first time, it sets the eofbit in the stream's internal error state.
You need to clear it before you can continuing reading, by calling the is.clear() function, which resets the internal error state.
I think the reason is that you have opened output stream file using "ofstream::out"
ofstream os(filename, ofstream::out | ofstream::app);
now, the "ofstream::out" truncates the file as soon as it opens it. That is, all the previous contents of your file will be deleted once it is opened. Please try using this instead:
ofstream os(filename, ios::app);
So I am working on a program for class in which we have to open two different text files to retrieve the appropriate text to be displayed in the console. My code is not opening the file and keeps outputting the else statement ".txt file cannot be open". I've tried several different ways to open the file but with no luck. Any help here would be greatly appreciated.
//
// main.cpp
// PunchLine program
// Page 896 Problem 3
//
//
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
//File stream objects
fstream inFile;
string line;
//Open joke file to read lines to console
inFile.open("joke.txt", ios::in);
if (inFile.is_open())
{
//Read lines from file to console
while (getline(inFile, line))
{
cout << line << endl;
inFile.close();
}
}
else
cout << "joke.txt file cannot be open.\n";
//Open punchline file to read last line joke to console
inFile.open("punchline.txt", ios::in);
if (inFile.is_open())
{
//Read last line from file to console
inFile.seekp(-52L, ios::end);
getline(inFile, line);
}
else
cout << "punchline.txt file cannot be open.\n";
return 0;
}
When declaring an input file use
ifstream inFile;
Also make sure the input file is in the same folder as your .exe
Edit: http://www.cplusplus.com/doc/tutorial/files/ also, this link should help with working with files.
Edit 2: I already posted this in a comment, but I'll just add it to the official answer: "Change your while loop as well. Instead of the if test, use while(inFile.is_open()) and then use your getline statement inside the loop. Because right now your code reads like while get this line from the file is true cout line. So it might not even be doing the while loop."
I don't think you should close the file inside the while loop. Otherwise, your file gets closed after only the first line is read in. Move the close statement outside the loop. Same for the second block.
if (inFile.is_open())
{
//Read lines from file to console
while (getline(inFile, line))
{
cout << line << endl;
}
inFile.close();
}
else
cout << "joke.txt file cannot be open.\n";
Check that your file exist. If it does, check whether you have the correct path when you open it (check if your .txt files are in the same directory as your .exe file, or specify the full path in your code). If yes, check if the files are read-only.
use
if(!infile)
{
cout<<"cannot open file";
}
I think you need to flush the screen. Once you have flushed and closed the stream. The next time you run an application it should open the file.
e.g.
inFile.flush();
inFile.close();
I'm trying to write a simple program that will print the contents of a text file one line at a time. However, whenever I run the program i just get a blank screen. I'm certain the file I am trying to read contains text over several lines. Any help as to why this isn't working would be super helpfull.
bool show() {
string line;
ifstream myfile;
myfile.open("tasks.txt", ios::app);
while (!myfile.eof()) {
getline (myfile, line);
cout << line << endl;
}
myfile.close();
return true;
}
The problem might be that you are using ios::app with ifstream (input stream), which makes no sense.
According to this,
ios::app: All output operations are performed at the end of the file, appending the content to the current content of the file. This flag can only be used in streams open for output-only operations.
Try this:
std::string line;
ifstream myfile ("tasks.txt");
if (myfile.is_open())
{
while ( getline (myfile,line) )
{
std::cout << line << std::endl;
}
myfile.close();
}
Did you check return value of myfile.isopen()? Perhaps the file isn't there or you don't have read permission.
Oh yes, I missed that - the append flag. Should be ios::in