How to randomly select values in one matrix and make them equal to values of a second matrix?

I have two separate matrices, X and Y, both containing 400 random numbers all between 1 and 20 (but X does not equal Y). I would like to create a loop that can randomly select values from X and set them equal to the value in the same position in Y, and for this loop to repeat until a certain condition is met. How would I go about setting up this loop?

Answer 1

[the cyclist]

while ... % <---- condition goes here
    idx = randi(400);
    x(idx) = y(idx);
end