stored dynamic array values incorrect

[NSH]

So I've been trying to figure out dynamic arrays for a Quantum Mechanics project I've been working on and am currently having the issue that when I try to have a value called f_theta computed by my program stored into the array I get an output that is not the value computed.

In order to try and diagnose the problem I commented out various parts of the code that deals with the array and I have determined that the problem comes from line 76. When that piece (the actual part that is supposed to store the f_theta into the array) is commented out the array output are all 0 which is what I would expect since I zeroed all the elements in the array prior to that. but when that line of code is in the program I get the output:

"The array stored: -2147483648"

This is clearly not correct as each value that is supposed to be stored in the array is on the order of 10^24.

The code:

#include <iostream>
#include <ctime>
#include<cmath>
#include <new>

using namespace std;

#define m1 10 //wimp mass in GeV/c^2
#define m2 73 // germanium nucleus mass in GeV/c^2
#define u 91 // z boson mass in GeV/c^2 this is the mediating particle
#define g 2.00976e-22 // coupling constant in GeV*m
#define h 6.58211899e-25 // plancks constant h-bar in GeV*s

int main ()

{
	
	//number of iterations for algorithim or "data points" in the simulated experiment
	//n_iter is user defined, i and k are indexes for the computer
	long int n_iter, i, k;
	
	//p_initial is momentum of neutralino
	float p_initial;
	
	//q is momentum transfer to germanium nucleus
	//E_observed is a calculated quantity for the "observed" phonon energy in a germanium detector
	// which will be taken to be related to the magnitude of q by E= q^2/2m where m is the mass of germanium nucleus
	//scale is the random float from 0..1 that defines the observed energy. note |q|<= |p_initial|
	//f_theta is the calculated scattering amplitude
	
	double scale,E_observed,q,f_theta,E_neutralino;
	
	//Here we will declare f_array which will contain a list of scattering amplitudes (uses pointer?)
	long int *f_array;
	
	//user sets number of calculations of scattering amplitude
	cout<<"How many data points would you like to use to calculate the scattering amplitude? (n<=1000) \n";
	cin>>n_iter;
	
	//user defines an incident momentum
	cout << "What is the incident momentum of the neutralino?  \n";
	cin>> p_initial;
	
	//resize array to match user data set input for n_iter (number of data points that the program will compute)
	f_array= new () long int[n_iter];
	
	//zeroes all elements in the array
	for(k=0;k<n_iter;k++)
		f_array[k]=0;
	
	//seed random number generator
	srand(time(NULL));
	
	for(i=0; i<n_iter; i++)
	{
		//creates a random float between 0..1 for every trail
		scale=float(rand())/float(RAND_MAX); 
		cout<<"scale is: " << scale << "\n"; 
		
		//calculates energy of neutralino based on user input for momentum
		E_neutralino=((p_initial*p_initial)/(2*m1));
		
		//creates an observed phonon energy based on the energy of the neutralino, Note E_observed<=E_neutralino
		E_observed=(scale*(E_neutralino)); 
		cout<<"The observed energy: " << E_observed<< " GeV/c^2 \n";
		
		//calculate momentum transfer to Germanium
		q=sqrt((2*m2*E_observed));
		cout<<"q is: " << q << " GeV/c \n";
		
		//calculate scattering amplitude formula derived on poster or in paper
		f_theta=((2*m1*g)/((h*h)*(((u*u)+(q*q)))));
		cout << "The scattering amplitude was calculated to be:" << f_theta<<" \n";
		
		//stores array values for each computed f_theta
		f_array[i]=f_theta;
		cout<<"The array stored:" << f_array[i]<<"\n";
	}
	//calculate the differential cross section
	
}

[slicedpan]

Hi i had a look at your code, whats happening is that you are trying to assign f_theta (which is a floating point number) to f_array[INDEX] which is an integer. if you change the type of f_array to a float or a double this should work fine

[NSH]

ok, so even though the index is an int type i still have to declare float type for the array? I did what you suggested and it works (thank you!) but I want to know for future reference.

[slicedpan]

yeah the type of the array should be the type of data you want to store in it. arrays always use integers to access elements.

[slicedpan]

actually if you want to use dynamic arrays that you can resize on the fly consider using <vector>.
e.g. vector<double> f_array;
there's plenty of info on their usage on the net.