How to create matrix from descending number of vector elements without loop?

Example: I have vector [1 2 5 3 4], and want to create the square matrix [1 2 5 3 4; 1 2 5 3 0; 1 2 5 0 0; 1 2 0 0 0; 1 0 0 0 0], so that the first row has all elements, 2nd row has all but last element, 3rd has all but last 2 elements...all the way to the final row having only the first element.

I am able to do this with a for loop, but can it be done without the loop to save time (actual vector is very large)?

[Andrei Bobrov]

How big? (What size of your vector?)

Answer 1

[Andrei Bobrov]

A = [1 2 5 3 4];

n = numel(A);
out = ones(n,1)*A.*flip(triu(ones(n)),2);

or just ( MATLAB >= R2016b)

out = A.*rot90(triu(ones(n)));

[Matthew Polatas]

I have a second question: What if I were keeping the last elements instead of the first? For example, [1 2 5 3 4] becomes [1 2 5 3 4; 2 5 3 4 0; 5 3 4 0 0; 3 4 0 0 0; 4 0 0 0 0]?

Thanks, and your previous answer works great.

[Andrei Bobrov]

rot90(toeplitz(A(end:-1:1),[A(end),zeros(1,4)]),-1)

Answer 2

[Stephen23]

>> V = [1 2 5 3 4];
>> flipud(tril(repmat(V,5,1)))
ans =
   1   2   5   3   4
   1   2   5   3   0
   1   2   5   0   0
   1   2   0   0   0
   1   0   0   0   0

and

>> hankel([1 2 5 3 4] )
ans =
   1   2   5   3   4
   2   5   3   4   0
   5   3   4   0   0
   3   4   0   0   0
   4   0   0   0   0

[Andrei Bobrov]

+1