Why does next_permutation skip some permutations?

Question

Why doesn't this simple function output all permutations of the inputted 5 letter string? I think there should be 120 and it only outputs 90.

#include <iostream> #include <vector> #include <algorithm> #include <string> using namespace std;  // Creates permutation lists for strings vector<string> createdcombos2(string letters) {      vector<string> lettercombos;          cout << "Letters are: " << letters << endl; //input string      do          lettercombos.push_back(letters);             while(next_permutation(letters.begin(), letters.end()));          cout <<"Letter combos: " << endl;  //print out permutations      for (auto i : lettercombos)         cout << i << endl;     cout << endl << lettercombos.size() << endl; //number of permutations      return lettercombos; }   int main()  {     string letters = "gnary";      vector<string> lettercombos;      lettercombos = createdcombos2(letters); } 

Answer 1

To return all permutations in a loop until next_permutation returns false, the vector must be sorted before the start of the loop. next_permutation returns the permutations in ascending order. So if you start off with an unsorted vector, it will begin part way through the series of permutations.

std::sort(letters.begin(), letters.end()); do      lettercombos.push_back(letters);         while(next_permutation(letters.begin(), letters.end()));