creating matrix with colon

Question

Aditya 2015-4-22

编辑： James Tursa 2015-4-22

Hi

I am trying to create a matrix such that

a = [2 3 4];
b =zeros(3,length(-4:1:4));
b = (-a(1:3):1:a(1:3));

but this gives me an error

i know that a matrix of different row lengths cannot be created therefore i define the size at the start

what i intend to get is

b = [-2 -1 0 1 2 0 0 0 0;
     -3 -2 -1 0 1 2 3 0 0;
     -4 -3 -2 -1 0 1 2 3 4;]

i know this can be easily done via for loop but is there any way of doing this in single command?

Answer 1

Andrei Bobrov 2015-4-22

编辑：Andrei Bobrov 2015-4-22

k = ones(3,1)*(-4:4);
[m,n] = size(k);
k2 = k.*rot90(triu(ones(size(k))),2);
b = full(spdiags(k2,1-m:n-1,m,n));

or

a = (2:4)';
b = bsxfun(@plus,-a,0:max(a));
b(bsxfun(@gt,b,a)) = 0;

or

bcell = arrayfun(@(x)-x:x,a,'un',0);

Aditya 2015-4-22

Many thanks