Question

The picking itself is easy, but I’m having trouble making sure I can’t place the points on the same spot. I could of course re-random, but is there a better way? Thanks a lot!

Answer 1

choosing K indices from N possible locations randomly is equivalent to first random shuffling the array and then take the first K indices. If you program in c++ (which is what you are giving as tag), it can be as easy as:

first, make your 2D array flat, say its stored in a vector: vector elems;

#include <vector>
#include <algorithm>
using namespace std;

.... initialize elems to whatever you want, here all zeros ......

vector<int> elems (N*M, 0);

// assigning your indices from 0 to elems.size()-1
vector<int> index (elems.size());
for (int i=0; i<index.size(); i++)
{
    index[i] = i;
}

// now random permute
random_shuffle (index.begin(), index.end());

// now assign the elements to whatever you want, here assign them to 1
elems[index[0]] = 1;
elems[index[1]] = 1;