C++ Clears file when a new print action takes place - c++

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 );

Related

C++ line justification with I/O

I am creating a program that justifies a paragraph to ensure that each line has a length of 75 char. I have created functions that will insert spaces and create these desired lengths as needed, but I am having problems reading a text file and trying to break it down line by line. Each line provided is less than the 75 char limit, and my functions do properly work when it is given only a line. But I do not know how to read line by line, manipulate it, and then write to my new .txt file. When I output this to the new text file, I am greeted by a justified line of text, not text that is in a paragraph block!
I have tried to create an if else loop that would only run when the string.length() is less than 75 char, and would create a new line when false, but I do not know how to create this new line in the program
string myString;
string line("\n");
while (getline(inFile, myString))
{
cout << myString << endl;
puncLoop(myString);
spaceLoop(myString);
}
}
In Order to output the file with new line you can use "\n".
#include <iostream>
#include <string>
#include <fstream>
int main() {
//in file object
std::ifstream inFile("example.txt");
//out file object
std::ofstream outFile ("example2.txt", std::ios_base::out | std::ios_base::trunc );
//Checking if file exist
if( inFile && outFile )
{
//temp valarable to store each line
std::string mystring;
//Loop through each line
while (getline(inFile, mystring))
{
//... Call Your Business Logic functions here, ( make use of pass by refernce or return to get back the string )
outFile << mystring.c_str() << "\n";
}
//closing file after completing
inFile.close();
outFile.close();
}
else
{
std::cout << "Could not open File to read or write"<<std::endl;
}
return 0;
}

How to write and read a file with `fstream` simultaneously in c++?

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;
}

How to save and also read c++ fstream file without closing it

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.

Error copying text from one file to another c++ fstream

This is my code
#include <iostream>
#include <fstream>
#include <string>
int main()
{
std::fstream file;
file.open("text.txt", std::fstream::in | std::fstream::out |
std::fstream::app);
if(!file.is_open())
{
std::cout << "Could not open file(test.txt)" << std::endl;
} else {
file << "These are words \nThese words are meant to show up in the new file \n" <<
"This is a new Line \nWhen the new fstream is created, all of these lines should be read and it should all copy over";
std::string text;
file >> text;
std::cout << text << std::endl;
file.close();
std::fstream newFile;
newFile.open("text2.txt", std::fstream::in | std::fstream::out |
std::fstream::app);
if(newFile.is_open())
{
newFile << text;
}
}
}
I'm trying to copy the contents of text.txt to text2.txt but for some reason the text string always ends up empty. I've checked the files and text gets populated but text2 is empty. What's going wrong here?
When you append a string to an fstream, the input / output position is set to the end of the file. This means that when you next read from the file, all you will see is an empty string.
You can check what the current input position is by using:
file.tellg()
And set the input / output position to the start by using:
file.seekg(0)
The full reference for std::fstream is here.
You're trying to read from the end of the file. The position is set to the end of the last thing you wrote to the file, so, if you want to read what you wrote, you have to reset it:
file.seekg(0);
This will set the position for the input back to the start of the file. Note however that reading from the file the way you do now will simply get you 1 word (up to the first whitespace). If you want to read it all, perhaps you should look at something like: Read whole ASCII file into C++ std::string.

Simple File-I/O Program C++

#include <iostream>
#include <fstream>
#include <stdlib.h>
#include <process.h>
using namespace std;
int main(){
system("cls");
char mline[75];
int lc=0;
ofstream fout("out.txt",ios::out);
ifstream fin("data.txt",ios::in);
if(!fin){
cerr<<"Failed to open file !";
exit(1);
}
while(1){
fin.getline(mline,75,'.');
if(fin.eof()){break;}
lc++;
fout<<lc<<". "<<mline<<"\n";
}
fin.close();
fout.close();
cout<<"Output "<<lc<<" records"<<endl;
return 0;
}
The above code is supposed to read from the file "data.txt" the following text
"The default behaviour of ifstream type stream (upon opening files ) allows users
to read contents from the file. if the file mode is ios::in only then reading is
performed on a text file and if the file mode also includes ios::binary along with
ios::in then, reading is performed in binary mode. No transformation of characters
takes place in binary mode whereas specific transformations take place in text mode."
and create a file out.txt , in which the same text is stored using line numbers ( A line can have 75 characters or ends at '.' - whichever occurs earlier ).
Whenever I run the program, it just gets stuck at the console - which doesnt respond upon pressing any keys whatsoever.
Can someone tell me what's going on in here ?
If any one of the attempted reads in the file is longer than 74 characters, getline will set the failbit for fin, and you will never reach the end of the file. Change your code to the following:
for (; fin; ++lc) {
fin.getline(mline,75,'.');
if (!fin.eof() && !fin.bad())
fin.clear();
fout<<lc<<". "<<mline<<"\n";
}
This will break your loop if you reach the end of the file or if something catastrophic happens to the stream. You'll also need to think about handling the extra read that is performed if the file ends with a period.
Consider switching to std::string.
#include <iostream>
#include <fstream>
#include <string>
int main()
{
int lc = 0;
std::ofstream fout("out.txt");
std::ifstream fin("data.txt");
for (std::string line; getline(fin, line, '.'); )
fout << ++lc << ". " << line << "\n";
std::cout << "Output " << lc << " records\n";
}