Reading in numbers from a file into an array

Question

So I have a file with three columns in e.g:

1 1 750

These are x,y,intensity values.

Then I try to read in this long file into arrays. This is the code so far:

using std::cout;
using std::endl;
using std::string;

string fOutFileName("gaintest.root"); 


int main()
{

  std::ifstream file1("data_p30.dat");

  double intensity;
  int i;
  int j ;
  double c[1000][1000];
if (file1.is_open()) {
    file1.seekg(0);
    while (!file1.eof()) {
      file1 >> i >> j >> intensity;
      c[i][j]=intensity;
      cout<<c[i][j]<<'/n'; 
    }
    file1.close();
  } else cout << "Error, cannot open file 1";
}

So ultimately I want to be able to have a 2D array that is linked to the intensity. Any ideas on why I am failing? It compiles fine but when it run, it looks like this:

root [0] 
 *** Break *** segmentation violation



===========================================================
There was a crash.
This is the entire stack trace of all threads:
===========================================================
#0  0x0000003701098c05 in waitpid () from /lib64/libc.so.6
#1  0x000000370103c481 in do_system () from /lib64/libc.so.6
#2  0x00002b036f5ebc6a in TUnixSystem::StackTrace() ()
   from /batchsoft/root/root528-x86_64-slc5-43/lib/libCore.so
#3  0x00002b036f5eb63c in TUnixSystem::DispatchSignals(ESignals) ()
   from /batchsoft/root/root528-x86_64-slc5-43/lib/libCore.so
#4  <signal handler called>
#5  0x00002b0370acd515 in TRint::Run(bool) ()
   from /batchsoft/root/root528-x86_64-slc5-43/lib/libRint.so
#6  0x000000000040106d in main ()
===========================================================


The lines below might hint at the cause of the crash.
If they do not help you then please submit a bug report at
http://root.cern.ch/bugs. Please post the ENTIRE stack trace
from above as an attachment in addition to anything else
that might help us fixing this issue.
===========================================================
#5  0x00002b0370acd515 in TRint::Run(bool) ()
   from /batchsoft/root/root528-x86_64-slc5-43/lib/libRint.so
#6  0x000000000040106d in main ()
===========================================================

Answer 1

In C++

double c[1000, 1000];

The first 1000 is thrown away and the compiler generates code for:

double c[1000];

What you wrote [1000, 1000] is a multi-dimensional array in Pascal, but not in C++. In C++ you would use:

double c[1000][1000];

Answer 2

The errors you have displayed are generated by ROOT. Since this is not a ROOT forum, ROOT is a c++ data analysis framework developed by scientists at CERN. For the curious, their website is here. Your problem however, is not related to ROOT.

2D arrays are declared by

double a[2][2];

and this will work for you.

Perhaps more safe is to use the template class

std::vector<double> v;

For a 2D application this would look like

std::vector<std::vector<double> > v2;

The advantage of this is that its size can be adjusted as needed and

v.push_back(d);

will add an element to the end of the vector, elongating it if necessary. Elements can also be accessed with array syntax like

v[1] 

or

v2[1][2]