How to insert different size matrices into each row of a zero matrix

I am using a for loop to solve multiple itterations of an equation. for each output, i wan it to store in a new row of a zero matrix, while keeping the output from the previous itteration as shown below

M = [ x1 0 0 0 ;

x1 x2 0 0 ;

x1 x2 x3 0;

x1 x2 x3 x4]

Answer 1

[KSSV]

M = zeros(4) ;
for i = 1:4
    M(i,1:i) = rand(1,i) ;
end
M

M = 4×4

0.3674 0 0 0 0.1426 0.9721 0 0 0.8203 0.1518 0.5443 0 0.9805 0.8734 0.5388 0.7684

Answer 2

[Walter Roberson]

newvalues = as appropriate
M(end+1, 1:length(newvalues)) = newvalues;

Answer 3

[VBBV]

syms x [1 10]
n = 10;

for k = 1:n
     M(k,1:k) = x(1:k);
end
M

M =