How can you randomly pick a position in a matrix and make it count down?

I'm stuck on trying to make a matrix with numbre that are counted down one by one randomly. I'm trying to make a matrix with all 20. Next up I want Matlab to choose a position in this matrix randomly.

Next up I want to make the chosen position in the matrix go down with 1. I also want that when a certain position reached 0, that this position isn't chosen anymore.

So every time it choses a matrixposition, the number on that position goes down with 1 until all positions are 0.

One problem that I stumble upon is that the count down stops when one position is zero, while I want to make it stop when all the positions are zero.

Until now I have this code;

countmatrix = ones(3,4)*20 % makes matrix with all 20

while (position ~= 0)

for i =1:1

a = randi(3); % Chooses random row

b = randi(4); % Chooses random column

position = countmatrix(a,b) % Picks position in matrix with a and b

position2 = position - 1; % Substracts position with one, and adds in new variable

countmatrix(a,b) = position2; % New variable is put in the matrix

end

end

Answer 1

[Mathieu NOE]

hello

simple code

pause to let you observe the evolution of the matrix in the command window

n = 3;
m = 5;

A = rand(n,m);

rand_vector = randperm(n*m); % generate a random vector that will "pick" one random cell of the matrix and give it a new value

for ci = 1:n*m
    ind2pick = rand_vector(ci);
    A(ind2pick) = 1 % NB : linear indexing method
    pause(1)
    
end