How to calculate the true length of a range of residues

Insertion codes pose a problem when calculating length–the number of residues between two residues in the same chain. For example, the following method may return an incorrect result; ```java int getLength(String string) {

   Atom[] ca = cache.getAtoms(string);
   int start = ca[0].getGroup().getResidueNumber().getSeqNum();
   int end = ca[ca.length-1].getGroup().getResidueNumber().getSeqNum();
   return end - start;

} ```

Instead, use AtomPositionMap and ResidueRange: ```java int getLength(String string) {

   AtomPositionMap map = AtomPositionMap.ofAminoAcids(cache.getAtoms(string));
   ResidueRange range = ResidueRange.parse(string, map);
   System.out.println(range.getLength()); // will print the correct length

} ```

The above method won’t give the best performance if you’re calling it for multiple structures from the same PDB Id. Instead, it is better to reuse, creating only one AtomPositionMap per PDB Id.