Automatically create a filename for a datafile based on input

[hai2410]

Hello,

I have a number of command line arguments for my int main() program which are used in my program.
I would like to pass these input arguments to the fstream blah.open("filename") member function in some manner such that the output file indicates which arguments were inputted. You may assume all the input arguments are doubles.

example below that i'd hoped at least let me pass a single letter- though it doesn't

#include <fstream>
using namespace std;
int main(int argc, char * argv[]){

ofstream file;
 char* filename= argv[1];
 file.open(*filename);
file <<"hello";
}

Thank you for any help

[wbrscott]

I guess I don't really understand your problem. I ran this code and it created an output file with the name of my first argument with the text hello in the file.

int main(int argc, char * argv[])
{
	ofstream fileOut;
	char* filename = argv[1];
	fileOut.open(filename);
	fileOut << "hello";
}

[hai2410]

Ah, well its good to know that I was only a dereference out! Cheers

However, really what I want to do is include more than just one letter in the filename. In particular I want doubles in the name.

Imagine I run my program with 4 arguments, 1 char, and 3 doubles:

./program H 1.0 2.0 3.0

I'd like to get a filename out which is something like

H_1.0_2.0_3.0.data

Currently I can't see any way of generalising the above beyond a single char (which is why I tried the char in the first place- to indicate that I have at least tried).

[cire]

#include <fstream>
#include <sstream>
#include <iostream>

int main( int argc, char* argv[])
{
    ostringstream fileName ;

    for ( int i=1; i < argc ; ++i )
        fileName << argv[i] <<  (i+1==argc?"":"_") ;

    fileName << ".dat" ;

    cout << fileName.str() ;

    ofstream fileOut(fileName.str()) ; // or fileOut(fileName.str().c_str()) if no c++11.
    // use fileOut 
}

Warning: Did not test.

[hai2410]

Thank you cire. There is only one error left with the small additions i've made: but I can't seem to fix it.

filename.cc:19:33: error: no matching function for call to ‘std::basic_ofstream<char>::basic_ofstream(std::basic_ostringstream<char>::__string_type)’

Using:

#include <fstream>
#include <sstream>
#include <iostream>

using namespace std;
int main( int argc, char* argv[])
{

  // ofstream fileOut;                                                                                                                                        
  ostringstream fileName ;

  for ( int i=1; i < argc ; ++i )
    fileName << argv[i] <<  (i+1==argc?"":"_") ; //not sure what this second part is?                                                                         

  fileName << ".dat" ;

  cout << fileName.str() ; //is this just meant to output the filename to the terminal?                                                                        
  
 ofstream fileOut(fileName.str()) ; // is this a constructor for specific use in conjunction with ostringstream? I thought the syntax was fileOut.open("name"); 
//but trying fileOut.open(fileName.str()); didn't work either?                                                                                                      

  fileOut<< "hello";
  fileOut.close();
}

[cire]

Line 17 - yes.
Line 19 - ofstream fileOut(fileName.str().c_str()); - C++11 fstreams have a constructor that works with std::string. Specifying the file name and open mode in the constructor is pretty much the same as default construction followed by an open.

Line 13: (i+1==argc?"":"_") is shorthand for (pseudocode:)

if ( i+1 == argc)
    ""
else
   "_"

Functionally equivalent:

    int i ;
    for ( i=1; i<argc-1; ++i )
        fileName << argv[i] << '_' ;
    fileName << argv[i] ;

it just keeps the '_' from being appended on the last iteration of the for loop.

[hai2410]

Thanks very much! :)

[wbrscott]

1 argument to create a single file.

ofstream fileOut;
	char* filename = argv[1];
	strcat( filename, ".txt") ; // append .txt to the filename
	fileOut.open(filename); // create output file with name = to first argument + .txt
	fileOut << "hello"; // write some text to output file 

multiple arguments to create a single file.

int main(int argc, char * argv[])
{
	//use multiple arguments to create a file name - sample input argument list is F 1.0 2.0 3.0
	
	ofstream fileOut;
	char* filename = argv[1];

	for( int i = 2; i < argc; i++ )
	{
		strcat (filename, argv[i]);
	}	
	
	strcat (filename, ".txt"); // append .txt to the filename
	fileOut.open(filename);
	fileOut << "hello"; // write some text to output file 

[cire]

You don't own the memory pointed to by filename, so that's very unlikely to end well.