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

Because it is a requirement for std::set_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

(emphasis mine)

you shouldn't call these algorithms on unsorted ranges.