C++ Set

Unique values, kept in order

A set is a collection that refuses duplicates and keeps its values sorted. Think of it as amap with only keys: membership, not a payload. Include <set>. The type isset<int> or set<string>.

Use it when the question is “have I already seen this?” or “give me these values once each, sorted.” A vector can hold the same number twice. A set will not.

insert

insert adds a value when it is new. size() is how many unique values you have. After two different inserts, size is 2.

#include <iostream>
#include <set>
using namespace std;

int main() {
  set<int> seen;
  seen.insert(4);
  seen.insert(1);
  cout << seen.size() << endl;
  return 0;
}

Output is 2.

Duplicates are ignored

Insert the same value again and the set stays the same. There is no error and no extra element. That silence is the feature: you can insert every number you see in a stream and end with the distinct ones.

#include <iostream>
#include <set>
using namespace std;

int main() {
  set<int> seen;
  seen.insert(4);
  seen.insert(1);
  seen.insert(4);
  seen.insert(1);
  seen.insert(4);
  cout << seen.size() << endl;
  for (int n : seen) {
    cout << n << " ";
  }
  cout << endl;
  return 0;
}

Output is 2 on the first line, then 1 4. Five inserts, two values, printed in sorted order — not the order of the first insert.

count

count(value) returns 1 if the value is in the set and 0 if it is not. Like map, a set stores each value at most once, so the result is never greater than 1. Use it as a yes-or-no test before you rely on the value being there.

#include <iostream>
#include <set>
using namespace std;

int main() {
  set<int> allowed;
  allowed.insert(200);
  allowed.insert(201);
  allowed.insert(204);
  cout << allowed.count(200) << endl;
  cout << allowed.count(404) << endl;
  return 0;
}

Output is 1 then 0. Status 200 is allowed. 404 is not in the set.

Iterate

Range-for walks the set from smallest to largest. For int that is numeric order. Forstring it is lexicographic order. There is no index; you do not write seen[0].

#include <iostream>
#include <set>
#include <string>
using namespace std;

int main() {
  set<string> tags;
  tags.insert("stl");
  tags.insert("map");
  tags.insert("set");
  tags.insert("map");
  cout << tags.size() << endl;
  for (const string& tag : tags) {
    cout << tag << endl;
  }
  return 0;
}

Size is 3 because the second "map" was ignored. The loop printsmap, then set, then stl.

set versus vector

Keep a vector when position matters or when the same value may appear twice on purpose. Switch to a set when uniqueness is the rule you want the type to enforce.

What comes after set

You now have the three containers this track emphasizes: vector, map, andset. The next chapter stays in the STL and looks at more algorithms: sort you already saw, then count and friends on a range.