convert vector to matrix

i have a vector

a=[1,2,3,4,5,6];

i want to convert it to matrix such that the diagonals are all 0 and the rest of the elements are:

M=[0 1 2 3;1 0 4 5;2 4 0 6;3 5 6 0];

     0     1     2     3
     1     0     4     5
     2     4     0     6
     3     5     6     0

Answer 1

[Stephen23]

>> a = [1,2,3,4,5,6];
>> M = tril(ones(4),-1);
>> M(M>0) = a;
>> M = M+M.'
M =
   0   1   2   3
   1   0   4   5
   2   4   0   6
   3   5   6   0

And if you want to automatically adjust the size you can basically invert the binomial coefficient:

>> N = (sqrt(1+numel(a)*8)-1)/2;
>> M = tril(ones(N+1),-1);

[Rik]

If you need to automatically find the correct size:

a = [1,2,3,4,5,6];
M = tril(ones(.5+0.5*sqrt(8*numel(a)+1)),-1);
M(M>0) = a;
M = M+M.';

>> a = 1:3;
>> M = tril(ones(.5+0.5*sqrt(8*numel(a)+1)),-1);
>> M(M>0) = a;
>> M = M+M.'
M =
   0   1   2
   1   0   3
   2   3   0