I want to read a file line by line. Did something like
void Parse (string filepath) {
ifstream sourceFile;
sourceFile.open(filepath);
for (string line; getline(sourceFile, line);) {
cout << "1" << endl;
cout << line << endl;
}
}
int main() {
Parse("C:\\test.txt");
getchar();
return 0;
}
Then put some text into C:\test.txt, but when I run, I dont get anything. Why? Not even the "1". I notice no exception if the file is not there too. I suppose that a sign of a problem?
You have to check for success/error manually. Try with ifstream::good():
sourceFile.open(filepath);
if(!sourceFile.good()) {
// do something
If you don't want to check manually, you can enable exceptions:
// call that before open()
sourceFile.exceptions ( ifstream::failbit | ifstream::badbit );
I think you have problems opening the file. I would suggest two things:
check if sourceFile is opened successfully(if (sourceFile))
debug the code and see the code path your code follows.
EDIT: adding the actual solution to the problem in my answer(instead of just a comment) so that people won't miss it:
Here is one more thought - check the file name in its properties. Has happened to me that if windows hides the extension of the file the name is actually test.txt.txt, while what I see displayed is only test.txt.
change your for loop to
for (string line; sourceFile.good();) {
getline(sourceFile, line);
}
This way, you check the validity of your stream in the conditional part of the for, and get the line if the stream good.
Related
As far as I know, stream.ignore(n, 'n') should ignore an (n) amount of characters or if ‘\n’ is reached, and skip over to the next line, however, when I run the next code:
// include...
void insertInfo(int info) {
std::fstream stream("infoFile.txt"); // Open the file
while (!stream.eof()) {
std::string a{};
// getline(stream, a); <--- Tried this, didn't work either
stream.ignore(99, '\n');
} // Skip to the last line without any number, in theory
std::cout << info << std::endl; // Check if the output it's correct (Which is)
stream << info; // Insert the info
stream.close(); // Close the file
}
void main() //Main
{
std::cout << "Enter your name, followed by the info you want to add to infoFile:" << std::endl;
std::string info, temp = "";
std::getline(std::cin, temp); // Get the info input
std::stringstream sstream;
sstream << temp;
sstream >> temp >> info; // Remove the name keeping only the info
temp = ""; // ^
std::string::size_type sz;
insertInfo(stoi(info, &sz)); // Convert info string into an integer and insert it in infoFile
}
The console prints out the "info" correct value, however, when I check info.txt, in which I previously wrote a '0' on, you don't see any change.
I tried removing the "ignore" function and it overwrites the 0, which is exactly what I was trying to prevent.
I also tried using "getline" function but the same thing happens.
What is the error here?
Problem
Cannot write to file.
Why
void insertInfo(int info) {
std::fstream stream("infoFile.txt"); // Open the file
Opens file with default permissions, which includes reading. The C++ Standard says I should expect "r+" behaviour and the C Standard says a file opened with "r+" behaviour must exist in order to be read (Someone please add a link if you have one). You cannot create a new file. This is problem 1. The Asker has dealt with this problem by providing a file.
Note: take care when working with files via relative paths. The program's working directory may not be where you think it is. This is problem 1a. It appears that the Asker has this taken care of for the moment.
while (!stream.eof()) {
Common bug. For more details see Why is iostream::eof inside a loop condition considered wrong? In this case since all you're looking for is the end of the file, the fact that the file hasn't been opened at all or has encountered any read errors is missed. Since a file in an error state can never reach the end of the file this quickly becomes an infinite loop. This is problem 2.
std::string a{};
// getline(stream, a); <--- Tryied this, didn't work neither
stream.ignore(99, '\n');
Always test IO transactions for success. This call can fail unchecked.
} // Skip to the last line without any number, in theory
Assuming nothing has gone wrong, and since we're not checking the error state assuming's all we can do, the file has reached the end and is now in the EOF error state. We can't read from or write to the stream until we clear this error. This is problem number 3 and likely the problem the Asker is struggling with.
std::cout << info << std::endl; // Check if the output it's correct (Wich is)
stream << info; // Insert the info
This can fail unchecked.
stream.close(); // Close the file
This is not necessary. The file will be closed when it goes out of scope.
}
Solution
void insertInfo(int info) {
std::fstream stream("infoFile.txt"); // Open the file
while (!stream.eof()) {
stream.ignore(99, '\n');
} // Skip to the last line without any number, in theory
std::cout << info << std::endl; // Check if the output it's correct (Wich is)
stream.clear(); // Added a call to clear the error flags.
stream << info; // Insert the info
stream.close(); // Close the file
}
Now we can write to the file. But let's improve this shall we?
void insertInfo(int info) {
std::fstream stream("infoFile.txt");
while (stream.ignore(99, '\n')) // moved ignore here. now we ignore, then test the result
{
}
stream.clear();
stream << info << '\n'; // added a line ending. Without some delimiter the file
// turns into one big number
}
Note that this isn't exactly kosher. If any ignore fails for any reason, we bail out and possibly write over data because the code blindly clears and writes. I'm not spending much time here trying to patch this up because we can get really, really simple and solve the problem of creating a non-existent file at the same time.
void insertInfo(int info) {
std::fstream stream("infoFile.txt", std::ios::app);
stream << info << '\n';
}
Two lines and pretty much done. With app we append to the file. We do not need to find the end of the file, the stream automatically points at it. If the file does not exist, it is created.
Next improvement: Let people know if the write failed.
bool insertInfo(int info) {
std::fstream stream("infoFile.txt", std::ios::app);
return static_cast<bool>(stream << info << '\n');
}
If the file was not written for any reason, the function returns false and the caller can figure out what to do. The only thing left is to tighten up the stream. Since all we do is write to ti we don't need the permissiveness of a fstream. Always start with the most restrictive and move to the least. This helps prevent some potential errors by making them impossible.
bool insertInfo(int info) {
std::ofstream stream("infoFile.txt", std::ios::app);
return static_cast<bool>(stream << info << '\n');
}
Now we use an ofstream and eliminate all the extra overhead and risk brought in by the ability to read the stream when we don't read the stream.
#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
string line = "test";
ifstream myfile("example.txt");
myfile.open("example.txt");
if (myfile.is_open())
{
cout << line << "\n";
cout << "File Opened\n";
getline(myfile, line);
cout << line;
while (getline(myfile, line))
{
cout << line << '\n';
cout << "test";
}
myfile.close();
}
else cout << "Unable to open file";
//return 0;
//getchar();
}
Apologies in advance if this has been answered, but while I've found several answers that are very close to what I need, I can't find an answer to this specific problem.
I'm new to Visual Studio, but have dabbled in c++ in the past. I'm trying to read in data from a text file and (for now) simply print that back out with cout. But, I'm not seeing any results.
At first I figured I just had my txt file in the wrong place - and I did. Initially I would receive the line "Unable to open file", indicating that the file could not be opened. So I moved it around and found out where Visual Studio wanted me to put the file.
So now I successfully see the "File Opened\n" line get printed to the screen, followed by nothing. I thought I might be using getline wrong, but if I replace the file input "myfile" with a "cin" instead, getline will happily read in keyboard input all day, so that's not it either.
So I've put in some test cout statements that print out the value of my string, line. The first one prints out "test" as it should. Then I read in a line of the txt file to that string variable, and when I cout it again I get nothing. It's a blank string.
Also, the line " cout << "test"; " From within the loop does NOT print either. So the loop's not even happening, it seems.
So, as near as I can tell, the program is able to find my textfile, example.txt. But it's not actually seeing the contents within.
The contents of the textfile (and what I'd like the program to print out) are as follows:
"This is the first line
This is the second line
Third
Fourth
Fifth"
Any and all help is much appreciated.
Figured it out.
What went wrong is this line:
ifstream myfile("example.txt");
I don't know exactly why, but since I specify the file to open in the next line down ( myfile.open("example.txt"); ), specifying the filepath in the ifstream declaration caused the issue.
I don't entirely get it, as others have said that the code runs fine for them. But this seems to work, anyway.
If there's any reason why I shouldn't use this solution, please let me know.
you might want to take a look at your file open
"myfile.open("example.txt");" i found if you don't give a file path weird things happen.
myfile.open("c:\test\example.txt");
is the adjustment I made to the code and it work like a dream.
I've been having a nightmare this evening trying to get some very simple I/O functionality going. As embarrassing as it is, I've had some great help from people on here!
My current issue is that I'm attempting to use ifstream.open() and it simply is not opening the file. This is confirmed by getline(ifstream,line); returning false on it's first call.
Here is a copy paste of the current code:
std::string FSXController::readLine(int offset, FileLookupFlag flag)
{
// Storage Buffer
string line;
streampos sPos(offset);
try
{
// Init stream
if (!m_ifs.is_open())
m_ifs.open("C:\\Users\\guyth\\Documents\\test.txt", fstream::in);
}
catch (int errorCode)
{
showException(errorCode);
return "";
}
// Set stream to read input line
m_ifs.seekg(sPos);
if (!getline(m_ifs, line))
return "";
// Close stream if no multiple selection required
if (flag == FileLookupFlag::single)
m_ifs.close();
return line;
}
This code is in 'bug fix mode' and so therefore is pretty messy, don't worry too much about that, cleanup will happen when this method is finally working.
I have tried:
Absolute file path
Saving path into string and then calling the .c_str() method.
Running VS 2015 in Administrator mode
Ensuring file has read/wright access
Ensuring no duplicate file extensions
Yes the file definitely has content! :D
I'm kinda out of ideas now and am really not sure why this file is refusing to load.
The condition: if (!getline(m_ifs, line)) Repeatedly returns true... :(
EDIT: I've just tried checking m_ifs.fail() immediately after the open and it returns true, so we know the fail flag was triggered :/
Thanks
Guy
Enable exceptions before opening the stream:
m_ifs.exceptions ( std::ifstream::failbit | std::ifstream::badbit );
Otherwise m_ifs.open won't throw.
And you have to catch std::ifstream::failure:
try {
m_ifs.open("C:\\Users\\guyth\\Documents\\test.txt", fstream::in);
}
catch (std::ifstream::failure e) {
std::cerr << "Exception opening file: " << std::strerror(errno) << "\n";
}
See ios::exceptions for more details.
I am writing an MFC program that has a dialog with an "Export" button that will take all of the data that has been entered into the file and export it to a .txt (at some point I want to change this to a .msg file...but that's a question for another day).
However, when I click the button, it creates the file but doesn't write anything inside the file. For testing, I removed everything except just a simple literal string and even that isn't printing. Here is the current code for that event: The myfile.flush() statement is leftover from when I had a loop that I was trying to print to the file.
void CEHDAHTTimerDlg::OnBnClickedExport()
{
// in this function, we want to export the items to a text file
std::ofstream myfile("TodayTime.txt");
myfile.open("TodayTime.txt");
if (myfile.is_open())
{
myfile << "The average call time is ";
myfile.flush();
myfile.close();
}
else
{
SetDlgItemText(IDC_EXPORT, L"Export Unsuccessful! -- No File");
}
}
Is there anything you all can think of that could be causing this? I've been at it for a few hours trying different things, like utilizing a myfile.write() function instead. I've searched a lot around here, reddit, and google in general trying to find out why this isn't writing.
I appreciate your help.
EDIT:
Okay, calling the myfile constructor the way that I did, by including the file name, went ahead and did what open file would have done
Thanks for your help!
commenting out the redundant "open" solves it.
#include <iostream>
#include <fstream>
int main()
{
// in this function, we want to export the items to a text file
std::ofstream myfile("TodayTime.txt");
// myfile.open("TodayTime.txt");
if (myfile.is_open())
{
myfile << "The average call time is ";
myfile.flush();
myfile.close();
}
else
{
std::cerr << "didn't write" << std::endl;
}
}
I strongly suspect that you're invoking undefined behaviour by opening and already-open stream.
Here is the explanation:
The call to myfile.open("TodayTime.txt"); will fail because the stream is already associated with the file, setting the failbit.
The call to is_open() will succeed, as the file is open.
Then the call to streaming operator << will fail (because of the failbit).
this is because myfile << "The average call time is "; not working
to fix that
std::ofstream myfile;
myfile.open("TodayTime.txt",std::ios:app) //app for appending you can use trunc for
//truncating file
//for flushing content's of existing file use myfile.flush();
if (!data_pack.is_open())
{
std::cerr << "unable to open the file for writing";
}
myfile << "some stuff tho write you can replace the string with variable"
<<std::endl; //for next line
//at last close file
myfile.close();
I want to edit a text file, but I'm stuck in finding the correct functions or methods to do so.
So far I'm able to open a text file and look for a certain string, but I have no idea on how to move the cursor, add or replace information, steps 4 - 7 in my pseudocode shown below.
Can you provide some guidance? Which functions should I use (in case they already exist)?
A sample 'easy' code would be appreciated as well.
Pseudocode:
1. Open file.
2. While not eof
3. Read file until string "someString" is found.
4. Position the cursor at the next line (to where the someString was found).
5. If "someString" = A go to step 6. Else go to step 7.
6. Replace the information in whole line with "newString". Go to step 8.
7. Add new information "newString_2", without deleting the existing.
8. Save and close the text file.
Thanks.
I would recommend to put the getline command into the while loop because then it won't stop only because of EOF but when getline is not able to read anymore.
Like when the error bad occurs (which happens when someone deleted the file while your program was reading it).
It seems like you want to search inside a string, so "find" might be quite helpful.
#include <iostream>
#include <fstream>
#include <string>
int main (){
std::fstream yourfile;
std::string line, someString;
yourfile.open("file.txt", ios::in | ios::app); //The path to your file goes here
if (yourfile.is_open()){ //You don't have to ask if the file is open but it's more secure
while (getline(line)){
if(line.find(someString) != string::npos){ //the find() documentation might be helpful if you don't understand
if(someString == "A"){
//code for replacing the line
}
else{
yourfile << "newString_2" << endl;
}
} //end if
} //end while
} //end if
else cerr << "Your file couldn't be opened";
yourfile.close();
return 0;
}
I can't tell you how to replace a single line in a text file but I hope you can work with that little I can give you.
This should be a good start:
// basic file operations
#include <string>
#include <fstream>
int main ()
{
std::fstream myfile;
std::string line;
while (!myfile.eof())
{
std::getline(myfile,line); // Check getline() doc, you can retrieve a line before/after a given string etc.
//if (line == something)
//{
// do stuff with line, like checking for content etc.
//}
}
myfile.close();
return 0;
}
More informations here