index or position in std::set

Question

I have a std::set of std::string. I need the \"index\" or \"position\" of each string in the set, is this a meaningful concept in the context?

I guess find() will re

Answer 1

Despite what others have written here, I don't think that "index" or "position" has meaning with respect to a set. In mathematical terms, a set exposes only its members and maybe its cardinality. The only meaningful operations involve testing whether an item is a member of the set, and combining or subtracting sets to yield new sets.

Some people talk about sets as data structures in looser terms, by facets of being "ordered" or "unordered", and whether they permit duplicates or enforce uniqueness. The former facet distinguishes an array with an O(n) insertion guard, where an attempt to insert an item first scans the existing members to see if the new item exists and, if not, inserts the new item at the end, and a hash table, that might retain such order only within a bucket's chain. A tree such as the Red-Black Tree used by std::set is somewhere in between; its traversal order is deterministic with respect to the strict weak order imposed by the comparator predicate, but, unlike the array sketched above, it doesn't retain insertion order.

The other facet — whether the set permits duplicate elements — is meaningless in mathematics, and is more accurately described as a bag. Such a structure acknowledges the difference between identity and value-based "sameness."

Your problem may involve caring about some position; it's not clear what that position means, but I expect you're going to need some data structure separate from std::set to model this properly. Perhaps a std::map mapping from your set of elements to each position would do. That would not guarantee that the positions are unique.

It may also help clarify the problem to think how you'd model it as relations, such as in a relational database. What comprises the key? What portions of the entities can vary independently?

Answer 2

std::distance is what you need. You will want, I guess std::distance(set.begin(), find_result)

Answer 3

I don't think it is meaningful - set's are 'self keyed' and sorted thus the 'index' would be invalidated when the set is modified.

Of course it depends upon how you intend to use the index and if the set is essentially static (say a dictionary).