Erase empty lines from a text file in C++ - c++

Question

I have a text file, which may contain some empty lines. I want to open this file and look for the empty lines. If an empty line exists then, I want to delete that line. I do not want to create a temporary file for this purpose. I want to edit the same file and close it afterwards.
I have already seen a couple of posts about similar problem but none of them worked for me.
void DeleteEmptyLines(const string& FilePath)
{
//EXISTING File
std::fstream FileStream;
std::string CurrentReadLine;
if (doesFileExist(FilePath))
{
FileStream.open(FilePath, std::fstream::in | std::fstream::out | std::fstream::app);
//Read all the lines till the end of the file
while(getline(FileStream, CurrentReadLine))
{
//Check if the line is empty
if(CurrentReadLine.empty())
{
cout<<"Empty line found"<<endl;
//DELETE THIS EMPTY LINE
}
}
FileStream.clear();
FileStream.close();
}
// else --->> do nothing
}
Current File (MyFile.txt):
Line1
Line2
Line3
What I need (MyFile.txt):
Line1
Line2
Line3
PS: I am using VS2010 on windows machine.

Answer 1

Simple solution. Reads file into a string skipping empty lines, then overwrites the file with the contents of the string.
void DeleteEmptyLines(const std::string &FilePath)
{
std::ifstream in(FilePath);
std::string line, text;
while (std::getline(in, line))
if !(line.empty() || line.find_first_not_of(' ') == std::string::npos)
text += line + "\n"
in.close();
std::ofstream out(FilePath);
out << text;
}
EDIT: #skm The new answer you posted does not erase lines with empty spaces, as you stated.
To fix this use this condition to make sure a line is not "empty":
!(CurrentReadLine.empty() || CurrentReadLine.find_first_not_of(' ') == std::string::npos)

Answer 2

You can't do it in all currently used filesystems, because data is stored successively. So do 'delete' some bytes from file you have to move all subsequent bytes at that amount of bytes. And you can't open file for reading and at that moment rewrite it from beginning.
So you have 3 options:
Read all file into memory and then rewrite lines to the original
file skipping empty lines.
Read file line by line, save only
non-empty lines into memory, and then write then down to the same file.
Use temporary file, which is actually a good choice because you
don't have to have a lot of RAM and file moving operation is
low-cost at most filesystems (if source and destination are at the
same partition).

Answer 3

I have modified my function as following.
void DeleteEmptyLines(const string& FilePath)
{
std::string BufferString = "";
//File
std::fstream FileStream;
std::string CurrentReadLine;
if (doesFileExist(FilePath))
{
FileStream.open(FilePath, std::fstream::in); //open the file in Input mode
//Read all the lines till the end of the file
while(getline(FileStream, CurrentReadLine))
{
//Check if the line is empty
if(!CurrentReadLine.empty())
BufferString = BufferString + CurrentReadLine + "\n";
}
if(DEBUG) cout<<BufferString<<endl;
FileStream.close();
FileStream.open(FilePath, std::fstream::out); //open file in Output mode. This line will delete all data inside the file.
FileStream << BufferString;
FileStream.close();
}
// else --->> do nothing
}
This function does the following steps:
Open the file in input mode
Read all the lines which are not empty
Closes the file
Open the file again in Output mode (deletes all the data inside file)
Put the string into the file
Closes the file.