how to convert std::vector to std::set without losing the order

Question

I am using the below code for conversion:

std::set<ObjectType> s(v.begin(), v.end());

However, I need to keep the order of the vector elements. How can I do this?

The *only reason* to use a `set` is that it is *ordered*. If you wanted to keep the order of elements in the `vector`, you should keep the elements in the `vector`. — Nicol Bolas, Jan 26 '16 at 19:43
`std::unique` may be the think you want instead of `std::set`. — Jarod42, Jan 26 '16 at 19:44
@Jarod42 I was thiking of suggesting that but `std::unique` requires the container is sorted. — NathanOliver, Jan 26 '16 at 19:45
Sounds like an xy-problem http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem — , Jan 26 '16 at 19:46
Also, I need to eliminate dublicate values. This is why I am converting vector to set. — eaytan, Jan 26 '16 at 19:52
Boost::Multiindex may be a solution using a sequenced index and a unique index. — drescherjm, Jan 26 '16 at 19:57
So, your question might be: How to eliminate duplicates of an unsorted vector without loosing the order of elements? — , Jan 26 '16 at 19:58

score 3 · Answer 1 · edited May 23 '17 at 10:27

3

You can't if the vector is not ordered. A std::set keeps it contents in either an ascending or descending order.

If the vector is ordered then you just need to set the comparison function of the std::set to whatever was used to order the vector.

You may want to see: How to remove duplicates from unsorted std::vector while keeping the original ordering using algorithms?

edited May 23 '17 at 10:27

Community

1
1

answered Jan 26 '16 at 19:40

NathanOliver

171,901
28
288
402

score 2 · Answer 2 · answered Jan 26 '16 at 19:41

You can do this if and only if your vector is sorted by some strict weak ordering, e.g. sorted by < for numbers. In that case, give the set the appropriate comparison object (the one you used to sort your vector) and the order will fit.

Otherwise, if your vector was not sorted by a fitting criterion, you cannot make the set keep the original order as sorting the elements is one of the invariants of the set.

how to convert std::vector to std::set without losing the order

2 Answers2