STL set_union and set_intersection on unsorted vectors

Question

Below is my code on set intersection and union test that I did. I don\'t understand why it the output is incorrect when I commented out the sort function. Why is the sort necess

Answer 1

That's required. This is excerpt from documentation of std::union.:-

Constructs a sorted range beginning at d_first consisting of all elements present in one or both sorted ranges [first1, last1) and [first2, last2).
1) Expects both input ranges to be sorted with operator<
2) Expects them to be sorted with the given comparison function comp
If some element is found m times in [first1, last1) and n times in [first2, last2), then all m elements will be copied from [first1, last1) to d_first, preserving order, and then exactly std::max(n-m, 0) elements will be copied from [first2, last2) to d_first, also preserving order.