Hi Nick,

On Mar 19, 2013, at 5:00 PM, Nicholas Firth <[email protected]> wrote:

> Hi RDKitters,
> 
> I'm sure this must of been covered somewhere else but I can't find it in the 
> forum or the source.
> 
> I am manipulating SMILES strings and wonder whether I can use RDKit as a 
> shortcut, so my question is does the order of occurrence in a SMILES string 
> of an atom match the atom index that is given by RDKit. It seems to me that 
> it does but I'd like to know for all cases. For example
> 
> >>> from rdkit import Chem
> >>> mol=Chem.MolFromSmiles('CSNOC')
> >>> for atom in mol.GetAtoms(): print atom.GetAtomicNum(),atom.GetIdx()
> ... 
> 6 0
> 16 1
> 7 2
> 8 3
> 6 4
> 
> This is what I would expect, but is there a case when this order might not be 
> conserved?

The atom and bond order will be preserved unless the smiles contains hydrogens 
as explicit atoms, i.e. as [H]. In that case, the hydrogens are removed and the 
remaining indices are what your would expect.


> If the order isn't always conserved, how does the indexing cope with RDKit 
> canonical smiles?

I don't understand the question but maybe that doesn't matter since the order 
is preserved?

-greg


> Thanks in advance.
> 
> Best,
> Nick
> 
> Nicholas C. Firth | PhD Student | Cancer Therapeutics
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