i just started reading on how to open and edit files.
when working with ifstream, if the file doesnt exist, it wont be created.
in reference to the code below, when would the condition (!outfile) be false, as if the file doesn't exists it will simply be created by the constructor, hence always making the condition false.
int main()
{
ofstream outfile ("test1.txt");
if (!outfile)
{
cout << "cannot create file test1.txt" << endl;
return 1;
}
outfile << 10 << " " << 345.12 << endl;
outfile << "This is a short text file";
outfile.close();
system("PAUSE");
return 0;
}
One way opening an ofstream could fail is if the file in the given path exists, but you do not have the permission to write to it. Alternatively, if the file does not exist but you do not have permission to create a file in the given path, opening the ofstream should also fail.
Another failing situation could be if the files does not exist, and the underlying device does not have sufficient free space/inodes to create one.
Related
I'm trying to open a C++ .txt file as shown in my code below. This is part of a larger program that I'm working on where I write the contents of one file into another so that it contains the same information as the original but I am required to provide user-input. If the user-provides a .txt file that is not the one we are using, I have to produce an error message and prompt them to re-enter an input until they input the correct one (test.txt):
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
/* Refers to test.txt */
ofstream mainfile;
std::string filename;
std::cout << "Please enter the name of your data file: ";
std::cin >> filename;
mainfile.open(filename.c_str());
mainfile << "test.txt";
if(!mainfile) {
std::cout << "I'm sorry, I could not open '" << filename << "'." << std::endl;
std::cout << "Please enter another name: " <<
std::endl;
std::cin >> filename;
} else {
std::cout << "File '" << filename << "' opened successfully!" << std::endl;
}
return 0;
}
My current issue is that the program is terminating too early, even when I input incorrect inputs such as jaguar.txt or flowers.txt, anything that isn't "test.txt". In fact, when I input just about any .txt file name it will output saying that it opened successfully.
It seems that what you want to do is open up 2 different files, where one is used as the file to copy from (test.txt), and the other is the file to copy to (jaguar.txt). Instead of checking if test.txt exists with std::ofstream, you should instead use std::ifstream.
Using ifstream, if the file does not exist, your code will work properly. Instead, because you are currently using ofstream, the file will open correctly, because you're essentially telling it to make the file for you.
So basically, where you have used ofstream mainfile, instead it should be:
ifstream mainfile;
Later in the code, you can prompt the user for the file to copy to (i.e. jaguar.txt), and this will be the one where you output data using ofstream.
Use ifstream to read from a file, and ofstream to write to a file.
To check whether the source file exists, test the corresponding ifstream after trying to open it:
ifstream mainfile; // ifstream stands for "input file stream"
std::cout << "Please enter the name of your data file: ";
std::cin >> filename;
mainfile.open(filename);
while (!mainfile) { // asking endlessly, until the user inputs a good file
std::cout << "I'm sorry, I could not open '" << filename << "'." << std::endl;
std::cout << "Please enter another name: " <<
std::endl;
std::cin >> filename;
mainfile.open(filename);
}
std::cout << "File '" << filename << "' opened successfully!" << std::endl;
By the way, in the error message "open" is programmer's jargon. It's a general word which includes both reading and writing. If your application copies stuff from one file to the other, the user may get confused: is there a problem with input or output? You might want to say "read" instead of "open", even though technically you didn't read anything yet. That would make a clearer error message.
If you want to copy one file to another, use one of the methods described in a dedicated question.
I'm attempting to create a repeating menu that will allow a user to re-enter a file name if the program is unable to open the file.
Right now it works correctly if I enter the name of an existing file, but if the file doesn't exist it prints the "File not found" then executes the rest of the program. I'm new to file streams and most of the code here was found through references. I'm a bit lost on what exactly is going on and what the best way to handle the situation is. Any guidance would be appreciated.
typedef istream_iterator<char> istream_iterator;
string fileName;
ifstream file;
do {
cout << "Please enter the name of the input file:" << endl;
cin >> fileName;
ifstream file(fileName.c_str());
if (!file) {
cout << "File not found" << endl;
}
} while (!file);
std::copy(istream_iterator(file), istream_iterator(), back_inserter(codeInput));
After constructing the object file will always exist, so your loop condition always fails. Change the condition to whether the file didn't open properly.
do {
...
}
while (!file.is_open())
this code will work.
do {
std::cout << "Please enter the name of the input file:" << std::endl;
std::cin >> fileName;
file = std::ifstream(fileName.c_str());
if (!file) {
std::cout << "File not found" << std::endl;
}
} while (!file);
your error was that you have 2 definition of the file variable.
the variable in while (!file) that is used is the one defined outside the do-while loop, and it is valid state is set to true by default.
In addition to #acraig5075 answer:
Writing a type then a variable name (ifstream file) is to create a new variable. Obviously you know this, but if you use the same name again in, for example, a loop, it makes a new and distinct variable.
ifstream file; // a unique variable
...
do {
...
ifstream file(fileName.c_str()); // another unique variable
...so change the usage inside the loop to:
file.open(fileName.c_str());
When using <fstream> library to open and add a stream to an existing file test.rtf and I use the following lines:
char data[100];
// open a file in write mode.
ofstream outfile;
outfile.open("test.rtf");
if (outfile.is_open()) { cout << "file is open" << endl; }
cout << "Writing to the file" << endl;
cout << "Enter your name: ";
cin.getline(data, 100);
// write inputted data into the file.
outfile << data << endl;
And when reading it by using ifstream, the lines input are displayed correctly. The problem is the output file is not modified and lines I have added are not saved. The question might sound very stupid but it's a problem I could not resolve.
When you << to your file you are just writing to a buffer, not actually "flushing" it to the file itself. If you just close your file you should be fine.
So:
outfile.close()
Also in the future you can flush (actually write from buffer to the file) when you want to write to a file but not close it. .close() flushes then closes for you automatically.
After the program reads the file, gets characters from the file, and finishes, the user is asked if they want another file read or not. If the user say yes, then the program asks for the file name, but then automatically says the file could not be opened and exits the loop. Please help me.
Here is the code:
do //do while opening the source file fails
{
cout << "Enter filename of source file: ";
cin.getline (filename,51);
sourceFile.open(filename); //opens the file with given filename
if (sourceFile.fail())
cout << "File could not be opened" << endl; //error if can't open
sourceFile.clear();
}
while (sourceFile.fail()); //exits if source file doesn't fail
This test:
while (sourceFile.fail())
will never be true because just before you get there you call:
sourceFile.clear()
which will clear any problem bits in the iostate for the stream.
I think you just want to get rid of the call to clear().
The canonical way to check if opening the file failed is to use std::basic_ios::operator !():
do
{
cout << "Enter filename of source file: ";
std::getline(std::cin, filename);
sourceFile.open(filename.c_str());
if (!sourceFile)
{
cout << "File could not be opened" << endl;
}
}
while (!sourceFile);
I am trying to open these two files and read their contents into two different arrays, but whenever I try and open them I get the unable to open file dialog? I don't see anything incorrect but I am not a strong c++ user.
std::ifstream inFile;
inFile.open("fives.txt");
if (inFile.is_open())
{
while (! inFile.eof() )
{
getline (inFile,line);
fives[loop] = line;
cout << fives[loop] << endl;
loop++;
}
inFile.close();
}
else cout << "Unable to open file";
inFile.open("search.txt");
loop=0;
if (inFile.is_open())
{
while (! inFile.eof() )
{
getline (inFile,line);
search[loop] = line;
cout << search[loop] << endl;
loop++;
}
inFile.close();
}
else cout << "Unable to open file";
The files must exist in the current directory, where the current directory is the directory from which the program was executed (not necessarily the one where the executable is saved at).
In your case, you saved the files with the resources, not with the resulting binary (I'm guessing you're running from within the VC++, by default it sets the current directory to where the binary is stored), so the program cannot find them. Use either relative path to where the resources are, or copy the files you're looking for into the directory you're running from.