placing values in the form of a triangle in a matrix

I want to write a code which does the following:

say my input is '5' , the code will run and the output will be in a matrix in the form

1  6  10  13  15
2  7  11  14
3  8  12
4  9 
5

or if my input is '3'

1  4  6
2  5
3

any ideas on how I should start?

Answer 1

[Hikaru]

If you want it to be a matrix, then I don't think there is a way to do it without adding 0's in the lower part of the triangle. The code below will work.

n = 3; %input
A = hankel(1:n);
c = A(n,1);
for i = 2:n
    for j = 1:n-1
        A(j,i) = c+1;
        c = c + 1;
    end
    n = n-1;
end

[Duncan]

thanks for this

Answer 2

[Patrik Ek]

Try this,

n = 5;
A0 = fliplr( triu(ones(n)) );
A = cumsum( A0(:) );
A = reshape(A,n,n);
A(A0<=0) = 0

The idea is to create an upper diagonal matrix of ones which you flip, then accumulate the sum of all elements (after reshaping the elements to a column matrix with (:) ). That means that the sum increase for every element, represented in the final matrix, but not for any other element. Then reshape the matrix again and set all elements which are not the flipped upper diagonal to zero to zeros.

[Duncan]

thanks. it works