Reading data from a file into a dynamically allocated array

[emaghero]

Afternoon all,

I am trying to read data from a file into an array. The array size is uknown before I open the file and count the number of data points. The function I have in mind is the following

void read_data(char *file,int &ndata,double *x,double *y)
{
	//Read the data from the file and store it in memory
	//This reads the data stored in the two column file and stores it in memory

	ifstream data;

	data.open(file,ios_base::in);
	
	if(!data){
		cerr<<"Could not open the file"<<endl;
		cerr<<"Calculation will not occur\n";
	}
	else{
		ndata=0; // Read the number of data points from the file

		while(data.ignore(1280,'\n'))
			++ndata;
		data.clear();
		data.seekg(0);

		ndata--; // Overcounts the number of data points by one

		cout<<"There are "<<ndata<<" lines of data\n";

		x=new(double [ndata+1]); // Resize the arrays according to the number of data points
		y=new(double [ndata+1]);

		for(int i=1;i<=ndata;i++){ // Read the data into the arrays
			data>>x[i];
			data.ignore(100,',');
			data>>y[i];
		}

		data.close();
	}
}

This works fine but when I try to run the function inside main I get an out of bounds access error.

int _tmain(int argc, _TCHAR* argv[])
{
	//Step One: Read the data in from the file
	int ndpts=0;
	
	double *x=0; // Declare the pointers for the arrays to store the data
	double *y=0;

        // Some file names
	char *f1="Rib_InP_W_1.0_E_0.2_T_0.3_overlap_versus_offset_straight_bend_1.txt";
	char *f2="Rib_InP_W_1.0_E_0.2_T_0.3_overlap_versus_offset_straight_bend_10.txt";
	char *f3="Rib_InP_W_1.0_E_0.2_T_0.3_overlap_versus_offset_straight_bend_21.txt";
	
	
	read_data(f1,ndpts,x,y); // Read the data from the file

	for(int i=1;i<ndpts;i++){
		cout<<i<<" , "<<x[i]<<" , "<<y[i]<<endl;	//Crash and burn!!
	}

	delete[] x;
	delete[] y;
	
	system("PAUSE");
	return 0;
}

I think the arrays x and y go out of scope once the call read_data is complete, any ideas on how to keep the data in memory? I know I can do this using vector<double> x instead of double *x, but I don't want to have to re-write another function later on to be compatible with vector.

Thanks.

[Banfa]

Check the value of x and y after line 15 has run in main. You will find they are still 0 because you do not pass back the the allocated pointers. The value returned by new remains local ro read_data.

This code is very unstructured and you could avoid the whole thing by applying a little structure.

Instead of 2 arrays of double which are clearly related you should declare a structure that holds 2 doubles and then have an array of those structures.

However then instead an array of those structures you should have a vector of those structures. The vector will handle the memory management for you.