Remove duplicates from a vector and keep the order

Description: Remove the duplicates from {4, 7, 4, 1, 7, 9, 1} and keep the order of the first appearance.

Hint:

• Keep an std::unordered_set of the values you have seen.

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Solution

#include <iostream>
#include <unordered_set>
#include <vector>

int main() {
    std::vector<int> numbers = {4, 7, 4, 1, 7, 9, 1};
    std::unordered_set<int> seen;
    std::vector<int> unique;
    for (int n : numbers) {
        if (seen.insert(n).second) {          // insert returns true when it is new
            unique.push_back(n);
        }
    }
    for (int n : unique) std::cout << n << ' ';
    std::cout << '\n';
}

How it works

• insert returns a pair, and the second part is false when the value was already there.

Read STL Containers.

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