Crossword game: read 2d given array and search diagonally c++

This is a crossword game. I wanna read an array diagonally. I should find some word in all over the 2d given array this array read from a given file and it is n*m size; m not always = n

How can I read 2d given diagonally like this:

Example:

m = 4
n = 4
b o o k
z a k o
s l l e
x y z l

ball: found

[b] o o k
z [a] k o
s l [l] e
x y z [l]

foo: not found

Here is the code:

char ReadArray(char* array, int r, int c, int n, int m)
{
   return (r > 0 && r <= n && c > 0 && c <= m) ?
                  array[n * (r - 1) + (c - 1)] : '\0';
}

char readrc(char* array, int r, int c, int n, int m)
{
   return (r > 0 && r <= n && c > 0 && c <= m) ?
                  array[n * (r - 1) + (c - 1)] : '\0';
}

void read_down_right(char* array, int n, int m, vector<string>& list)
{
   for (int sc = 2 - n; sc <= m - 1; sc++)
   {  
      string str = "";
      for (int r = 1, c = sc; r <= n; r++, c++)
      {
         char chr = readrc(array, r, c, n, m);
         if (chr != '\0')
            str += chr;
      }
      list.push_back(str);
   }
}

void read_down_left(char* array, int n, int m, vector<string>& list)
{
   for (int sc = 2; sc <= m + n - 2; sc--)
   { 
      string str = "";
      for (int r = 1, c = sc; r <= n; r++, c--)
      {
         char chr = readrc(array, r, c, n, m);
         if (chr != '\0')
            str += chr;
      }
      list.push_back(str);
   }
}

pass a reference to a blank list each time. list contains all possible strings afterwards, do a linear search.