Reordering the index

Question

When I index a molecular formula, I use the syntax \index{C8H12O4} and \index{C10H16O5}. The resulting output will have C10 before the C8 compound. What I understand is: in the second entry, C is followed by 1, whereas in the first case, C is followed by 8. 1 is less than 8, hence C10 comes before C8.

The question is how to reorder so that C with lower number comes first and then the other. In the above case, C8 first, followed by C10.

A related question is: the CAS numbers “100-51-7” will appear before “88-45-6”. How to make it work the other way round?

Here is my MWE

\documentclass[a4paper,10pt]{book}   
\usepackage{imakeidx}  % as i have multiple indexing
\makeindex  % as i have multiple indexing
\begin{document}

The illustrative example to show the problem I'm facing with indexing. C8H12O4\index{C8H12O4} and C10H16O5\index{C10H16O5} are interesting compounds. The resulting output will have C10 above the C8 compound.

Also checked for CAS numbers: 100-51-7\index{100-51-7} and 88-45-6\index{88-45-6} has to be indexed in the reverse order.  First 88-45-6, then 100-51-7.

\printindex
\end{document}

Answer 1

First, you need to do a small amount of planning to come up with the right scheme. Then, the \index command allows a parameter in the form {sortable@printable}, so you just add a sufficient number of leading zeros to the index sort key, like this:

C8H12O4\index{C08H12O4@C8H12O4} and C10H16O5\index{C10H16O5} 
100-51-7\index{100-51-7} and 88-45-6\index{088-45-68@8-45-6}

Take, for example, the case of 88-45-46. If we can assume that the largest number will be 999-999-99, then we code it twice, once for printing, and once for sorting. The print key is "as-is": 88-45-6, but the sort key is padded with appropriate leading zeros: 088-045-06. Now, to index and sort it into the right place, all we need is:

\index{088-045-06@88-45-6}

(I hope that clarifies it a bit better)