Finding permutation and combination in C++ - c++

Question

I want to find a series of numbers of dynamic length in C++. Suppose I have 2 groups of numbers: arr1[3] = {1, 3, 8} and arr2[4] = {2, 9}, then the expected output is:
'1, 2',
'1, 9',
'3, 2',
'3, 9',
'8, 2',
'8, 9'.
However, if there are 3 groups now : arr1[3] = {1, 3, 8}, arr2[2] = {2, 9} and arr3[5] = {1, 3, 9} then the output should be:
'1, 2, 1',
'1, 2, 3',
'1, 2, 9',
'1, 9, 1',
'1, 9, 3',
'1, 9, 9',
'3, 2, 1',
'3, 2, 3',
'3, 2, 9',
'3, 9, 1',
'3, 9, 3',
'3, 9, 9',
and so on...
So there will 3 x 2 x 3 = 18 outcomes. I got the outcome for 2 groups and 3 groups using respective numbers of for loops.
See this code for 2 groups:
for(int i=1;i<=5;i++) {
for (int j=1;j<=5;j++) {
cout << i << "," << j << "," << endl;
}
}
But then I have to use different codes for different value of group number and have to use switch statement or if-else statement to choose that portion of code.
This will be a great help. Thanks in advance!

Answer 1

I used vector instead of arrays, as they are way easier to deal with.
The trick is to enumerate, in lexicographic order, the positions in the arrays, then display the values at those positions:
#include <vector>
#include <iostream>
using std::vector;
void permutate(vector<vector<int>> values)
{
// the positions in each vector
vector<size_t> pos(values.size());
do
{
// display one of each array at current position
for(size_t i = 0; i < values.size(); ++i)
{
std::cout << values[i][pos[i]] << ", ";
}
std::cout << std::endl;
// increment the last array's display position
size_t p = 0;
pos[p]++;
// while we get to the end of current array, return to 0 and carry to next position
while(pos[p] == values[p].size())
{
pos[p] = 0;
p++;
pos[p]++;
// return when the last array's position get to its size
if (p == values.size())
{
return;
}
}
}
while(true);
}
int main()
{
vector<int> arr1 = {1, 3, 8};
vector<int> arr2 = {2, 9};
vector<int> arr3 = {1, 3, 9};
vector<vector<int>> allThree = {arr1, arr2, arr3};
permutate(allThree);
}
A good exercise, next, would be to template it so you accept std::vector<std::vector<T>>