#include <iostream>
// In a combination, the subset elements can be listed in any order.
void create_2_letters_combination(char letters[], int size) {
for (int i = 0; i < size; i++) {
for (int j = i + 1; j < size; j++) {
std::cout << letters[i] << letters[j] << "\n";
}
}
}
int main() {
char letters[] = {'A', 'B', 'C', 'D'};
int size = sizeof(letters) / sizeof(letters[0]);
create_2_letters_combination(letters, size);
}
/*
run:
AB
AC
AD
BC
BD
CD
*/