Pre- versus post-incrementing - most of the time it's a matter of style, what the coder prefers. When used in a standalone manner ( nothing else occurring in the statement), it doesn't matter.
will give exactly the same results. On the other hand, there's a difference in
The question is, what do you really mean to occur? This is an easy way to get OBO errors (Off By One) if you don't use the version that correctly implements your algorithm. In code like this, I advise students to separate the increment/decrement from the other actions, that way it's perfectly clear when you want the change to occur.

As I mentioned earlier, to be absolutely clear what you're doing in your function call, just use index-1 as the argument, as you're not really seeking to change the value of index, just passing a value that is one less.

I didn't get any seg fault or other runtime error with your original code, just incorrect result.

Time is never wasted when it's spent helping someone learn something.
(hmm, might make that my new sig)