Remove the need for nested for loop

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Jack Hillyer
Jack Hillyer 2019-1-11
评论： Star Strider 2019-1-12
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';

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采纳的回答

Star Strider
Star Strider 2019-1-11
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
  2 个评论
Jack Hillyer
Jack Hillyer 2019-1-11
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
Star Strider 2019-1-12
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.

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更多回答（1 个）

Kevin Phung
Kevin Phung 2019-1-12
编辑：Kevin Phung 2019-1-12
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

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