Translate3LetterTo1Letter

How to get the 1 letter code AS sequence out of a .pdb file?

#include <BALL/KERNEL/chain.h>
#include <BALL/KERNEL/protein.h>
#include <BALL/KERNEL/system.h>
#include <BALL/STRUCTURE/peptides.h>	//for change of 3 letter code to 1 letter code
#include <BALL/FORMAT/PDBFile.h>

using namespace BALL;

//read in .pdb file containing protein
PDBFile infile("system.pdb");
System S;
infile >> S;
infile.close();

// get the BALL sequence
String ball_seq= "";

// get the protein
if (RTTI::isKindOf<Protein>(*(S.getMolecule(0))))
{
  //we take the first chain for illustration
  Protein* protein = RTTI::castTo<Protein>(*(S.getMolecule(0)));       
  ChainIterator ch_it = protein->beginChain();
  for (ResidueIterator r_it = ch_it->beginResidue(); +r_it; r_it++)
  {
    if ((Peptides::OneLetterCode(r_it->isAminoAcid())) && (r_it->getName() != String("ACE")))
    {
      ball_seq += Peptides::OneLetterCode(r_it->getName());
    }
  }
}
cout << "One letter code: " << ball_seq << endl;