Remove the need for nested for loop

Hi, so I have a vector and I am trying to create a matrix containing the difference between the vector elements. I am currently using a nested for loop, but need to speed it up (the vector has 9000 elements). Is there any way of doing this?

I have a vector a

b=length(a);
for i=1:b
    for j=i+1:b
        da=a(i)-a(j);
    end
end
da=da+da';

Answer 1

[Star Strider]

I am not certain of the result you want.

Try this:

a = randi(9, 1, 5);                                     % Create Vector
da = bsxfun(@minus, a(1:end-1), a(2:end)');             % Matris Of Differences

[Jack Hillyer]

Hi, sorry for being vague, maybe I should have given an example.

If a= [1 2 3]'

da=[0 1 2; -1 0 1; -2 -1 0]

Thank you for the help!

[Star Strider]

My pleasure!

The example definitely helps.

A slight revision of my previous code:

a = [1 2 3];
da = bsxfun(@minus, a, a')                  % Matrix Of Differences

produces:

da =
     0     1     2
    -1     0     1
    -2    -1     0
    

that appears to be the desired result.

Answer 2

[Kevin Phung]

a = [1 2 3];

da = zeros(numel(a));
for i = 1:numel(a)
    da(:,i) =a(i) - a
end

You will save the program some time by predefining the size of da before the loop instead of constantly adding columns to it.

Hope this helps