Randomly insert new variable

My code currently creates a board of size n x n (user input) What I want to do now is to have certain positions on this board be called "kings", and I want roughly 1 king per 20 spaces i.e for a 10 x 10, there are 100 spaces so 5 kings. How do I randomly pick a board position to put a king?

function board = makeboard(size)
  board = cell(size+1,size+1);
   % initialize the board
   for i=1:size+1
       for j=1:size+1
           if i==1
               if j<=size
                   board{i,j+1} = j;
                   board{j+1,i} = j;
               end
           elseif j==1
               % do nothing
           else
               board{i,j}= 'x';
           end
       end
   end
   amount=size*size;
   king=amount/20;
   king=floor(king);

    end

Answer 1

[Walter Roberson]

Sorry, all of the normal ways of doing this involve using the MATLAB function named size(), which is not possible in your code because you named a variable "size". But you could change

else
   board{i,j}= 'x';

to

   elseif rand < 1/5
     board{i,j} = 'k';
   else
     board{i,j} = 'x';