How to iterate std::set?

Question

I have this code:

std::set<unsigned long>::iterator it;
for (it = SERVER_IPS.begin(); it != SERVER_IPS.end(); ++it) {
    u_long f = it; // error here
}

There is no ->first value. How I can obtain the value?

score 192 · Accepted Answer · edited Apr 21 '20 at 16:39

192

You must dereference the iterator in order to retrieve the member of your set.

std::set<unsigned long>::iterator it;
for (it = SERVER_IPS.begin(); it != SERVER_IPS.end(); ++it) {
    u_long f = *it; // Note the "*" here
}

If you have C++11 features, you can use a range-based for loop:

for(auto f : SERVER_IPS) {
  // use f here
}

edited Apr 21 '20 at 16:39

mtk

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answered Oct 12 '12 at 16:23

Robᵩ

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@Mr.C64 Not that it matters much with integral types as in this case. – Some programmer dude Oct 12 '12 at 16:29
2

It might be worth noting that you need to use the first one if you intend to modify the container. For the googlers that is. – Legion Daeth Nov 25 '16 at 05:14
3

I thing C++11 solution should be with reference (auto& f). It is better for most cases. For this specific case as well. – Mariusz Jaskółka Dec 10 '16 at 18:18
Hi Rob, what if I want to reference elements in SERVER_IPS other than declare a new u_long variable? Can I use u_long & f = *it; ? And if not, what should I do? – BioCoder Sep 17 '17 at 06:45
2

@BioCoder - You can use a reference variable, but it must be a **const** reference variable, like so: `const u_long& f = *it;`. – Robᵩ Sep 18 '17 at 14:09
@Robᵩ You mean I can use a reference variable but I can't modify the content of it? – BioCoder Sep 19 '17 at 08:30
@BioCoder - that is correct. You cannot modify an element of an `std::set`. – Robᵩ Sep 19 '17 at 15:43

score 21 · Answer 2 · edited Feb 07 '19 at 09:45

21

Another example for the C++11 standard:

set<int> data;
data.insert(4);
data.insert(5);

for (const int &number : data)
  cout << number;

edited Feb 07 '19 at 09:45

Box Box Box Box

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answered Dec 09 '16 at 08:17

vitperov

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score 18 · Answer 3 · edited Mar 25 '19 at 13:55

18

Just use the * before it:

set<unsigned long>::iterator it;
for (it = myset.begin(); it != myset.end(); ++it) {
    cout << *it;
}

This dereferences it and allows you to access the element the iterator is currently on.

edited Mar 25 '19 at 13:55

Guillaume Jacquenot

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answered Oct 12 '12 at 16:25

Rami Jarrar

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10

Just a minor note: it is generally preferred to use ++it instead of it++ in for loops to avoid one extra copy of the iterator. – user2891462 May 28 '15 at 12:04

score 7 · Answer 4 · edited Apr 21 '20 at 16:38

7

How do you iterate std::set?

int main(int argc,char *argv[]) 
{
    std::set<int> mset;
    mset.insert(1); 
    mset.insert(2);
    mset.insert(3);

    for ( auto it = mset.begin(); it != mset.end(); it++ )
        std::cout << *it;
}

edited Apr 21 '20 at 16:38

mtk

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answered May 19 '16 at 18:59

Luis B

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4

Or even `for(auto i : mset) std::cout << i;` – Jack Deeth Dec 06 '16 at 01:17

score 2 · Answer 5 · edited Jul 01 '23 at 19:51

One more thing that might be useful for beginners is since std::set is not allocated with contiguous memory chunks if someone want to iterate till kth element normal way will not work.

Example:

std::vector<int> vec{1, 2, 3, 4, 5};
int k = 3;
for (auto itr = vec.begin(); itr < vec.begin() + k; itr++)
    cout << *itr << " ";

std::unordered_set<int> s{1, 2, 3, 4, 5};
int k = 3;
int index = 0;
auto itr = s.begin();
while (true)
{
    if (index == k)
        break;
    cout << *itr++ << " ";
    index++;
}

How to iterate std::set?

5 Answers5

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