# placing values in the form of a triangle in a matrix

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Duncan on 11 Aug 2014
Commented: Duncan on 11 Aug 2014
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?

Hikaru on 11 Aug 2014
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 on 11 Aug 2014
thanks for this

Patrik Ek on 11 Aug 2014
Edited: Patrik Ek on 11 Aug 2014
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 on 11 Aug 2014
thanks. it works

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