Re: How to perform different sorts on multiple sections of items in the same array

[Athanasius]

Hello estreb,

This sounds like homework, so I’ll give advice rather than code. First, you need to nail down the requirements. What sort of output is wanted? If it’s just three separate lists, sorted according to the criteria, I would proceed as follows:

1. Partition the array into two: (i) strings containing the letter “r” and (ii) all others.
2. Partition array (ii) into two further arrays: (iii) strings starting with the letter “e” and (iv) all others.
3. Sort array (i) alphabetically, ascending.
4. Sort array (iii) using a sort routine that compares third characters alphabetically, ascending.
5. Sort array (iv) using a sort routine that compares final characters alphabetically, descending.
6. Output arrays (i), (iii), and (iv).

For (1) and (2), look at using grep or the part function from List::MoreUtils. For (3), just use sort. For (4) and (5), use sort together with a custom sort routine employing substr.

Now, there are parts of the specification that make me think the desired output may be more complicated than three separate sorted lists:

How do I “pick and choose” which items in the array I will sort, and make sure I leave the other items alone?

If you really do need to output a single array, with the original elements sorted “in place” and all interleaved back together (whatever that would really mean), a more complex approach will be needed. But any thought along those lines will be wasted effort until the specification has been clarified. As always, the best way to clarify a specification is to provide sample input together with the corresponding desired output (and this is just as important when talking to lecturers as when presenting problems to PerlMonks).

Hope that helps,

Update: fixed typo.