Reading a particular set of lines from a .txt file

[mugga]

Hi,
I have a .txt file which contains the x, y and z co-ordinates of particles which I am trying to cast for a particular compound. The no. of particles present is of the order of 2 billion and hence the size of the text file is of the order of a few Gigabytes. The particles have been casted layer wise - thus, if there are 15000 layers in which I have casted the particles, there are approx. 2 billion/15000 particles in each layer. Thus, every 2 billion/15000 particles have the same Y co-ordinate. Now, I need to read the particles at a given value of Y (say y = 10). I wrote a small program, where I had used fin.seekg( ). However I realized that the seeking of the position from where the file has to be read is not done line-wise, but is done character-wise. Could someone please tell me how I could read the co-ordinates of all the particles at a particular value of Y using a simple C++ program. I know that this can be done on the command line usind 'sed'. Is there a way possible to use sed, but write the output to another file?

[Galik]

What is the exact format of your data? Can you give is one or two lines?

[mugga]

Hi,
The following is the format of the data:

21.2342 (\t) 11.2430 23.5453 0.005 2.25 86 (\n)
49.2429 (\t) 11.2430 45.4353 0.005 2.25 86 (\n)

So basically the first 3 nos. are the x, y and z co-ordinates respectively of the particles. The other 3 nos. are the radius, density and type. Each of the nos. are floating point numbers (except 'type' which is int). And the data in each line is separated by a TAB spacing.

[Galik]

I can't say that this is fully tested but this is the approach I would take.

You would run this by piping in your data file and specifying the required value for y as a parameter. Then redirect the putput to the target file:


cat input_file.txt | ./filter 11.2430 > output_file.txt


The output file should contain all the matches in the same format as the input data.

// filter.cpp

#include <string>
#include <sstream>
#include <iostream>

struct particle
{
	double x;
	double y;
	double z;

	double radius;
	double dencity;

	int type;
};

int main(int argc, char* argv[])
{
	if(argc < 2)
	{
		std::cerr << "Error, need to supply y as argument." << std::endl;
		return 1;
	}

	// get input parameter y
	double y;
	std::istringstream iss(argv[1]);
	if(iss >> y) // this will fail for bad input
	{
		std::string line;
		while(std::getline(std::cin, line)) // one line at a time
		{
			// Then put the line into a stringstream
			// to read out the individual values
			std::istringstream iss(line);
			particle p;
			iss >> p.x;
			iss >> p.y;
			iss >> p.z;
			iss >> p.radius;
			iss >> p.dencity;
			iss >> p.type;

			if(p.y == y) // do we have a match?
			{
				// output the matching record.
				std::cout << p.x << '\t';
				std::cout << p.y << '\t';
				std::cout << p.z << '\t';
				std::cout << p.radius << '\t';
				std::cout << p.dencity << '\t';
				std::cout << p.type << std::endl;
			}
		}
	}
	else
	{
		std::cerr << "Argument y was not valid." << std::endl;
		return 1;
	}

	return 0;
}

[mugga]

Hey,
That really helped a lot. Thanks a ton! :)