Recursive computation without loop
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Hi! Can I write the following code
for j = 2 : N-1
alpha(j+1,:) = A(j,:).*alpha(j,:) + B(j,:);
end
in a form like this:
J = 2:N-1;
alpha(J+1,:) = A(J,:).*alpha(J,:) + B(J,:);
I tried to use this form but the alphas are incorrectly calculated.
6 个评论
"Recursive computation without loop"
Recursive calculations do not require loops: loops are iterative.
Why do you want to get rid of the loop: what is the problem with it?
avenior
2018-4-1
"To speed up these calculations. In vector form they are much faster executed."
How do you know this? It looks to me like premature optimization, which is a classic programming anti-pattern:
Walter Roberson
2018-4-1
You got faster code that calculated the wrong thing.
With the loop the value of B(1,:) affects alpha(2,:), and that has an effect that changes all later output. With the vectorized version you do not get the feedback of earlier B values affecting all later values.
avenior
2018-4-1
采纳的回答
Walter Roberson
2018-4-1
1 个投票
No, values are not stored into the destination until the entire right hand side finishes. Using a vector index on the output does not do an implicit iterative calculation.
If the question is about whether the calculation can be vectorized, the answer is that it can be vectorized for any given length. However the vectorized version is a bit nasty to write out and would be notably slower than the loop.
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