Check std::vector has duplicates

The algorithm you're looking for is std::adjacent_find.

// The container must be sorted!
const std::vector<int> sortedVector = {1,2,3,3,4,5};
const bool hasDuplicates = std::adjacent_find(sortedVector.begin(), sortedVector.end()) != sortedVector.end();

Unlike std::unique, std::adjacent_find doesn't modify the container.

As a bonus, std::adjacent_find returns an iterator to the first element in the duplicate "pair":

const auto duplicate = std::adjacent_find(sortedVector.begin(), sortedVector.end());

if (duplicate != sortedVector.end())
    std::cout << "Duplicate element = " << *duplicate << "\n";

Looking at google for std::unique I found this page std::unique. I looked at what it did:

Eliminates all except the first element from every consecutive group of equivalent elements from the range [first, last)

So it looks like it does what you want - removes the duplicates.

I then looked at what it returns...

... returns a past-the-end iterator for the new logical end of the range

So the result from std::unique is a sequence which is not necessary the same as the whole vector.

If nothing was removed, the return value would be the end of the vector.

So you want:

vector<int>::iterator it = std::unique(a.begin(), a.end());
bool wasUnique = (it == a.end());

Or for C++11:

auto it = std::unique(a.begin(), a.end());
bool wasUnique = (it == a.end());

Finally for the unique function to work, the vector needs to be sorted, so the complete code would be:

sort(a.begin(), a.end());
auto it = std::unique(a.begin(), a.end());
bool wasUnique = (it == a.end());

You should using set

set<int> s(a.begin(), a.end());
return s.size() != a.size();

If someone is forced to write own algorithm:

bool hasDuplicates(const std::vector<int>& arr) {
    for (std::size_t i = 0; i < arr.size(); ++i) {
        for (std::size_t j = i + 1; j < arr.size(); ++j) {
            if (arr[i] == arr[j])
                return true;
        }
    }
    return false;
}

But in real code you should use things that already exist, and in the standard library.

Sort the vector if it's not already sorted, and then use std::unique(), like this:

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> v = {3, 1, 3, 4, 5};
    sort(v.begin(), v.end());
    auto it = std::unique(v.begin(), v.end());
    std::cout << ((it == v.end()) ? "Unique\n" : "Duplicate(s)\n");
    return 0;
}

Output:

Duplicate(s)

So far all these solutions either modify the container or have O(n²) complexity. You can put a std::map to much better use:

#include <algorithm>
#include <iterator>
#include <map>

template <typename Iterator>
bool has_duplicates( Iterator first, Iterator last )
{
  std::map <typename std::iterator_traits <Iterator> ::value_type, std::size_t> histogram;

  while (first != last)
    if (++histogram[ *first++ ] > 1) 
      return true;

  return false;
}

#include <iostream>
#include <vector>

int main()
{
  using std::begin;
  using std::end;

  int a[] = { 2, 3, 5, 7, 11 };
  int b[] = { 2, 3, 5, 5, 7 };

  std::vector <int> c( begin(a), end(a) );
  std::vector <int> d( begin(b), end(b) );

  std::cout << std::boolalpha;
  std::cout << "a has duplicates false : " << has_duplicates( begin(a), end(a) ) << "\n";
  std::cout << "b has duplicates true  : " << has_duplicates( begin(b), end(b) ) << "\n";
  std::cout << "c has duplicates false : " << has_duplicates( begin(c), end(c) ) << "\n";
  std::cout << "d has duplicates true  : " << has_duplicates( begin(d), end(d) ) << "\n";
}

If your vector is small, say < 32 objects, or if copying and sorting the objects is expensive or impossible due to lack of move or copy constructor/assignment then a straight O(n^2) compare everything against everything else algorithm is the way to go.

Here is my solution:

template <typename Iterator>
bool has_duplicates( Iterator first, Iterator end ) {
    for (auto i = first; i != end; ++i) {
        for (auto j = first; i != j; ++j) {
            if (*i == *j) return true;
        }
    }
    return false;
}

template <typename Container>
bool has_duplicates(const Container &v) {
    for (const auto & i : v) {
        for (const auto & j : v) {
            if (&i == &j) break;
            if (i == j) return true;
        }
    }
    return false;
}