Build a string using recursion in c++

You need a separate worker function to recursively generate the series of indices and main function which operates on it.

For example something like

void worker(stringstream& strs, int[] dims, int dims_size, int step) {
  if (step < dims_size) {
    ... // Add dims[step] to stringstream. Another if may be necessary for
    ... // whether include `,` or not
    worker(strs, dims, dims_size, step + 1);
  } else {
    ... // Add cell value to stringstream.
  }
}

string create_matrix_string(int[] dims, int dims_size, int* matrix) {
  ... // Create stringstream, etc.
  strs << ... // Add matrix name etc.
  worker(strs, dims, dims_size, 0);
  strs << ... // Add ending `;` etc.
}

The main problem here is the value, since the dimension is not known during compilation. You can avoid that by encoding matrix in single-dimensional table (well, that's what C++ is doing anyway for static multidimensional tables) and call it using manually computed index, eg. i + i * j (for two-dimensional table). You can do it, again, by passing an accumulated value recursively and using it in final step (which I omitted in example above). And you probably have to pass two of them (running sum of polynomial components, and the i * j * k * ... * x product for indices from steps done so far.

So, the code above is far from completion (and cleanliness), but I hope the idea is clear.

You can solve this, by doing i, j and k in a container of the size of dim[] - sample:

#include <iostream>
#include <vector>

template< typename Itr >
bool increment( std::vector< int >& ijk, Itr idim, int start )
{
    for( auto i = begin(ijk); i != end(ijk); ++i, ++idim )
    {
        if( ++*i <= *idim )
            return true;
        *i = start;
    }
    return false;
}

int main()
{
    using namespace std;
    int dim[] = { 5, 7, 2, 3 };
    const int start = 1;
    vector< int > ijk( sizeof(dim)/sizeof(*dim), start );
    for( bool inc_done = true; inc_done
        ; inc_done = increment( ijk, begin(dim), start ) )
    {
        // .. here make what you want to make with ijk
        cout << "(";
        bool first = true;
        for( auto j = begin(ijk); j != end(ijk); ++j )
        {
            if( !first )
                cout << ",";
            else
                first = false;
            cout << *j;
        }
        cout << ")= tmp[t] " << endl;
    }
    return 0;
}