Getting all possible combinations of an integrer array in C++ - c++

Question

I have an array of integers, for example: a[1,2,3]. I would like to get all possible combinations of these numbers where they don't repeat, possibly with recursion.
I saw something like this done with strings here: Get all combinations without duplicates but don't know how to adapt it to integers without using any standard algorithms.
So I would like something like this as an output: {1},{2},{3},{1,2},{2,3},{1,3},{1,2,3}
Thanks in advance!

Answer 1

You can achieve all permutations of a list with comparable elements using std::next_permutation from <algorithms> library.
The cppreference has a nice article about this: https://en.cppreference.com/w/cpp/algorithm/next_permutation
We can use the code example to apply to your question.
#include <algorithm>
#include <string>
#include <iostream>
#include <vector>
void print_vector(const std::vector<int>& array) {
for (const int item : array)
{
std::cout << item << " ";
}
std::cout << std::endl;
}
int main()
{
std::vector<int> a({ 5,1,8,7,2 });
std::sort(a.begin(), a.end());
do {
print_vector(a);
} while(std::next_permutation(a.begin(), a.end()));
}

Answer 2

You can use std::next_permuation to achieve your goal.
Keep in mind you need to sort the array before starting to use this algorithm.
The loop will exit the first time std::next_permuation returns false.
If the array isn't sorted by the time you start std::next_permuation loop, you will miss all the arrays that are lexicographically lower than the current one when entering the loop.
int main()
{
std::vector<int> a = { 5,1,8,7,2 };
std::sort(a.begin(), a.end());
std::cout << "Possible permutations :\n";
do {
for (auto o : a)
std::cout << o;
std::cout << std::endl;
} while (std::next_permutation(a.begin(), a.end()));
return 0;
}