In general I read and edit a file in C++ with following code
#include <iostream>
#include <fstream>
using namespace std;
int main(){
char text[200];
fstream file;
file.open ("example.txt", ios::out | ios::in );
cout << "Write text to be written on file." << endl;
// Reding from file
file >> text;
cout << text << endl;
// Writing on file
cin.getline(text, sizeof(text));
file << text << endl;
//closing the file
file.close();
return 0;
}
but if the text file is very huge; say 2GB, and i just want to change a single word and save it, how to write a file so that only the sector with the edit is effected on the disc instead of rewriting whole file. How to inplace edit a file?
I have seen app called subler on macos to just add subtitles to a huge movie file and save it without rewriting entire movie file in just few seconds! so I knew inplace editing a file is possible.
Related
My question is how can I get my console to properly display the contents of fileB based on the below information.
Below is the code I have created for basic file input/output operations. I am trying to copy the content from fileA over to fileB. After this is done I am trying to display the contents of fileB to cout. The code runs and updates the contents of fileB to whatever was stored in fileA. However, the console does not display the new content of fileB. It just shows a blank box.
#include <iostream> // Read from files
#include <fstream> // Read/Write to files
#include <string>
#include <iomanip>
void perror();
int main()
{
using std::cout;
using std::ios;
using std::ifstream;
ifstream ifile; // ifile = input file
ifile.open("fileA.txt", ios::in);
using std::ofstream;
ofstream ofile("fileB.txt", ios::out); // ios::app adds new content to the end of a file instead of overwriting existing data.; // ofile = output file
using std::fstream;
fstream file; // file open fore read/write operations.
if (!ifile.is_open()) //Checks to see if file stream did not opwn successfully.
{
cout << "File not found."; //File not found. Print out a error message.
}
else
{
ofile << ifile.rdbuf(); //This is where the magic happens. Writes content of ifile to ofile.
}
using std::string;
string word; //Creating a string to display contents of files.
// Open a file for read/write operations
file.open("fileB.txt");
// Viewing content of file in console. This is mainly for testing purposes.
while (file >> word)
{
cout << word << " ";
}
ifile.close();
ofile.close();
file.close();
getchar();
return 0; //Nothing can be after return 0 in int main. Anything afterwards will not be run.
}
fileA.txt
1
2
3
4
5
fileB.txt (file is initially a blank text document).
fileB.txt (after code runs)
1
2
3
4
5
ofile will have an internal buffer and if it isn't flushed and you only write a small amount of data (possibly as much as 64kb) then no data will be written to your output file until you call ofile.close() or at the end of main().
Simply move ofile.close() to before file.open("fileB.txt").
This happens because you haven't closed the oFile object before you open the FileB.
ofile.close();
file.open("fileB.txt");
By doing this you will have access to the updated file.
I hope that this helps you.
I'm trying to write some text to a file and then read it using only 1 fstream object.
My question is very similar to this question except for the order of the read/write. He is trying to read first and then write, while I'm trying to write first and then read. His code was able to read but did not write, while my code is able to write but not read.
I've tried the solution from his question but it only works for read-write not write-read.
Here is my code:
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
fstream fileObj("file.txt", ios::out|ios::in|ios::app);
// write
fileObj << "some text" << endl;
// read
string line;
while (getline(fileObj, line))
cout << line << endl;
}
The code writes some text to file.txt successfully but it doesn't output any text from the file. However, if I don't write text to the file (remove fileObj << "some text" << endl;), the code will output all text of the file. How to write first and then read the file?
This is because your file stream object has already reached the end of the file after the write operation. When you use getline(fileObj, line) to read a line, you are at the end of the file and so you don't read anything.
Before beginning to read the file, you can use fileObj.seekg(0, ios::beg) to move the file stream object to the beginning of the file and your read operation will work fine.
int main()
{
fstream fileObj("file.txt", ios::out | ios::in | ios::app);
// write
fileObj << "some text" << endl;
// Move stream object to beginning of the file
fileObj.seekg(0, ios::beg);
// read
string line;
while (getline(fileObj, line))
cout << line << endl;
}
Although this answer doesn't qualify for your requirement of "reading and writing a file simultaneously", keep in mind that the file will most likely be locked while being written to.
Here the simple example to write and read the file.
Hope it will help you.
#include<fstream>
using namespace std;
int main ()
{
ofstream fout ("text.txt"); //write
ifstream fin ("text.txt"); // read
fout<<"some text";
string line;
while (fin>> line) {
cout<<line;
}
return 0;
}
I opened a file both read and write mode
using the following statement
file.open(fileName, ios::in | ios::out | ios::trunc);
my main purpose for opening the file in both mode is, read and write the file at the same time.
But In my code scenario,
when I am reading the file after writing it, the ouput showing blank that means,
it is not saving my writing contents because I am not closing it.
And I want to close the file after finishing both write and read the operation
I found a solution in Stack Overflow,
to use flush() function to save the file without closing
file.flush();
but, the problem is it's not working for my case
So, how can I save c++ fstream file without closing?
Here's my full code for better understanding
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
int main(int argc, char const *argv[])
{
string fileName = "text.txt";
fstream file;
file.open(fileName, ios::in | ios::out | ios::trunc);
if (file.is_open())
{
file << "I am a Programmer" << endl;
file << "I love to play" << endl;
file << "I love to work game and software development" << endl;
file << "My id is: " << 1510176113 << endl;
file.flush(); // not working
}
else
{
cout << "can not open the file: " << fileName << endl;
}
if (file.is_open())
{
string line;
while(file)
{
getline(file, line);
cout << line << endl;
}
}
else
{
cout << "can not read file: " << fileName << endl;
}
file.close();
return 0;
}
Actually, if you want to save any file immediately without closing the file, then you can simply use
file.flush();
But if you are want to read the file without closing just after writing it, you can simply use
file.seekg(0);
actually seekg() function resets the file pointer at the beginning, for this, it is not mandatory to save the file. so, this has nothing to with flush() function
but you can do both if you want to
Before reading from that file you need to make sure to place the pointer to that file to the beginning of the file. After writing to the file it'll point to the end. Therefore you won't be able to read anything.
You need to use file.seekg(0); somewhere after the file.flush() but before starting to read to place the file pointer to the very beginning.
Update
This should work without the flush. However this will depend on the implementation of the std library. Although I'd consider this as bug if it doesn't work without calling flush() imho it does not hurt to call it explicitly.
I have a C++ (Made in Netbeans 8 in Mac OS X 10.9) that in short writes to a file then ends the program. Here is the Meat of my program.
#include <iostream>
#include <fstream>
#include <sstream>
ofstream outfile("NETBEANS_PMCL.txt");
if(outfile.fail())
{
cout << "Cant open that file." << endl;
return 0;
}
outfile << "name" << endl;
outfile.close();
return 0;
But each time I run the program it rewrites on top of the name that was printed the last time I ran the program. How do I make it so that if there is content on the first line then skip two lines and print and if there is content there skip two lines and print and so on.
The problem is that ofstream is to write to a file, and what you want is to read to see if there is content, skeep 2 lines, and write new content.
std::fstream file("NETBEANS_PMCL.txt", std::fstream::in | std::fstream::out | std::fstream::app);
After that you may try to read 2 line and write content. Consider the function member getline to read the content.
getline (char* s, streamsize n, char delim );
I'm trying to write up a program that will display the contents of a text file to the screen for a user. Specifically, the text file will be a list of names that the program will read and display each name to the user individually. The user will then either like the name and keep it or dislike the name and remove it.
My dilemma is: if the user elects to keep the name, the program will need to go from reading the file to "writing" (deleting the name) the file and then back to reading the file again! I found the following relevant code on http://www.tutorialspoint.com/cplusplus/cpp_files_streams.htm. It shows that one must use .close() to switch from reading to writing, but this seems funky to a newbie like me. Is there a better way to do it or is the code below just fine?
#include <fstream>
#include <iostream>
using namespace std;
int main ()
{
char data[100];
// open a file in write mode.
ofstream outfile;
outfile.open("afile.dat");
cout << "Writing to the file" << endl;
cout << "Enter your name: ";
cin.getline(data, 100);
// write inputted data into the file.
outfile << data << endl;
cout << "Enter your age: ";
cin >> data;
cin.ignore();
// again write inputted data into the file.
outfile << data << endl;
// close the opened file.
outfile.close();
This is where the file goes from write mode to read mode.
// open a file in read mode.
ifstream infile;
infile.open("afile.dat");
cout << "Reading from the file" << endl;
infile >> data;
// write the data at the screen.
cout << data << endl;
// again read the data from the file and display it.
infile >> data;
cout << data << endl;
// close the opened file.
infile.close();
return 0;
}
Also, I'm having a hard time finding how to read and modify individual characters in the file. I need to do this too, as the file needs to follow a specific pattern, with five names per line and one space between each name (newline at end of fifth name, obviously). Help with this would be appreciated.
Changing things mid-file is complicated.
What I would do is either create a temporary file, write the kept names to that file and replace the original file with this temporary file, (or just store the kept names in a vector and rewrite the file)
try
std::fstream ff("io.txt", std::fstream::in | std::fstream::out);
and fstream::read, fstream::write, fstream::seekg
If the file is small then load it into memory.
Otherwise use fopen with "a+" or "r+" mode and fseek.