Can I insert into a set, all the elements of a vector that matches a condition, in a single line of code

Question

I have a vector of elements. I want to populate a set using the elements of this vector that match a certain condition. Can I do this using one line, or in any way that is more concise than the below?

// given vector<int> v

set<int> s;
for (const int& i : v)
{
    if (/* some condition on i*/)
        s.insert(i);
}

For example, something along the line of:

// given vector<int> v

set<int> s;
s.insert(v.filter(/* lambda here*/));

It goes without saying that the v.filter method should return an iterator, not a separate populated vector, for performance reasons.

NathanOliver · Accepted Answer · 2018-04-11T14:28:25.517

13

You can use std::copy_if with a lambda and std::inserter to insert the values into the set. That looks like

std::copy_if(v.begin(), v.end(), std::inserter(s, s.begin()), [](auto val) { return val == some_condition; });

edited Apr 11 '18 at 14:28

answered Apr 11 '18 at 14:24

NathanOliver

171,901
28
288
402

You are 20 seconds faster, damn – Slava Apr 11 '18 at 14:26
@Slava Sorry, sloth mode was not active ;) – NathanOliver Apr 11 '18 at 14:26
1

@DanielLangr It all depends on the type. If it is not bigger than a pointer then AFAIK there is no advantage. Of course of the object is big then a reference should be used. – NathanOliver Apr 11 '18 at 14:30

score 3 · Answer 2 · answered Apr 11 '18 at 14:39

3

With range-v3, it would be

set<int> s = v | ranges::view::filter([](int e){ return cond(e); });

or simply (if cond already exist)

set<int> s = v | ranges::view::filter(cond);

answered Apr 11 '18 at 14:39

Jarod42

203,559
14
181
302

score 3 · Answer 3 · answered Apr 11 '18 at 15:18

3

+1 for the std::copy_if() solution that, IMHO, is the natural solution for this problem.

Just for fun, I propose a different solution based on std::for_each()

std::set<int> s;

std::for_each(v.cbegin(), v.cend(),
              [&](int i) { if ( /* some condition */ ) s.insert(i); });

answered Apr 11 '18 at 15:18

max66

65,235
10
71
111

I don't see the gain of `for_each` now that we have for range. – Jarod42 Apr 11 '18 at 16:31
1

@Jarod42 - not a gain at all, IMHO; it's just for fun (to show an alternative way to make the same thing) when the OP ask for a solution "in a single line of code" (but I suppose also the for range can be written and considered as a single line of code) – max66 Apr 11 '18 at 16:43

Can I insert into a set, all the elements of a vector that matches a condition, in a single line of code

3 Answers3