c++ Reading numbers from text files, ignoring comments - c++

Question

So I've seen lots of solutions on this site and tutorials about reading in from a text file in C++, but have yet to figure out a solution to my problem. I'm new at C++ so I think I'm having trouble piecing together some of the documentation to make sense of it all.
What I am trying to do is read a text file numbers while ignoring comments in the file that are denoted by "#". So an example file would look like:
#here is my comment
20 30 40 50
#this is my last comment
60 70 80 90
My code can read numbers fine when there aren't any comments, but I don't understand parsing the stream well enough to ignore the comments. Its kind of a hack solution right now.
/////////////////////// Read the file ///////////////////////
std::string line;
if (input_file.is_open())
{
//While we can still read the file
while (std::getline(input_file, line))
{
std::istringstream iss(line);
float num; // The number in the line
//while the iss is a number
while ((iss >> num))
{
//look at the number
}
}
}
else
{
std::cout << "Unable to open file";
}
/////////////////////// done reading file /////////////////
Is there a way I can incorporate comment handling with this solution or do I need a different approach? Any advice would be great, thanks.

Answer 1

If your file contains # always in the first column, then just test, if the line starts with # like this:
while (std::getline(input_file, line))
{
if (line[0] != "#" )
{
std::istringstream iss(line);
float num; // The number in the line
//while the iss is a number
while ((iss >> num))
{
//look at the number
}
}
}
It is wise though to trim the line of leading and trailing whitespaces, like shown here for example: Remove spaces from std::string in C++

Answer 2

If this is just a one of use, for line oriented input like yours, the
simplest solution is just to strip the comment from the line you just
read:
line.erase( std::find( line.begin(), line.end(), '#' ), line.end() );
A more generic solution would be to use a filtering streambuf, something
like:
class FilterCommentsStreambuf : public std::streambuf
{
std::istream& myOwner;
std::streambuf* mySource;
char myCommentChar;
char myBuffer;
protected:
int underflow()
{
int const eof = std::traits_type::eof();
int results = mySource->sbumpc();
if ( results == myCommentChar ) {
while ( results != eof && results != '\n') {
results = mySource->sbumpc(0;
}
}
if ( results != eof ) {
myBuffer = results;
setg( &myBuffer, &myBuffer, &myBuffer + 1 );
}
return results;
}
public:
FilterCommentsStreambuf( std::istream& source,
char comment = '#' )
: myOwner( source )
, mySource( source.rdbuf() )
, myCommentChar( comment )
{
myOwner.rdbuf( this );
}
~FilterCommentsStreambuf()
{
myOwner.rdbuf( mySource );
}
};
In this case, you could even forgo getline:
FilterCommentsStreambuf filter( input_file );
double num;
while ( input_file >> num || !input_file.eof() ) {
if ( ! input_file ) {
// Formatting error, output error message, clear the
// error, and resynchronize the input---probably by
// ignore'ing until end of line.
} else {
// Do something with the number...
}
}
(In such cases, I've found it useful to also track the line number in
the FilterCommentsStreambuf. That way you have it for error
messages.)

Answer 3

An alternative to the "read aline and parse it as a string", can be use the stream itself as the incoming buffer:
while(input_file)
{
int n = 0;
char c;
input_file >> c; // will skip spaces ad read the first non-blank
if(c == '#')
{
while(c!='\n' && input_file) input_file.get(c);
continue; //may be not soooo beautiful, but does not introduce useless dynamic memory
}
//c is part of something else but comment, so give it back to parse it as number
input_file.unget(); //< this is what all the fuss is about!
if(input_file >> n)
{
// look at the nunber
continue;
}
// something else, but not an integer is there ....
// if you cannot recover the lopop will exit
}