Vectorisation of a simple for loop

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David Schranz
David Schranz il 28 Set 2016
Commentato: David Schranz il 29 Set 2016
Will it be possible to write this code vectorised?
for i = 1:size(x,1)-1
a(i+1) = a(i)+x(i)+b(i);
b(i+1) = b(i)+a(i)+c(i);
c(i+1) = c(i)+b(i)+c(i);
Any help would be greatly appreciated.

Risposta accettata

Thorsten il 28 Set 2016
A = [1 1 0 0 ; 1 1 1 0 ; 0 1 2 0; 1 0 0 1];
Q = cell2mat(arrayfun(@(i) A^i, 1:9, 'Uni', false));
Q = [zeros(1, 4); reshape(Q(end,:), 4, [])'];
a = Q(:,1); b = Q(:,2); c = Q(:,3);
  1 Commento
David Schranz
David Schranz il 29 Set 2016
Thank you very much for your solution. I would have never figured that out.

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Più risposte (2)

Teja Muppirala
Teja Muppirala il 28 Set 2016
Just wondering, but what is your purpose in vectorizing that code? If it's to make it faster, I don't think you can do much better than what you've already got, especially if you have a recent version of MATLAB.
If it's to make it more readable, or more general (flexibly deal with more variables), so you don't have to keep writing every letter like this
d(i+1)= ...
then you could use matrix equations.
x=ones(10,1); % External input
A = [1 1 0; 1 1 1; 0 1 2]; % State matrix
B = [1;0;0]; % Input matrix
v = zeros(length(B),length(x)); % v contains [a,b,c]
for i = 1:size(x,1)-1
v(:,i+1) = A*v(:,i) + B*x(i);
For many applications, matrix representation is usually how this type of calculation is expressed.
  1 Commento
David Schranz
David Schranz il 29 Set 2016
Yes, I try to make things faster. And yes it looks like the vectorised version is not faster. Nevertheless your code helped me to understand the solution provided by Thorsten. Also I consider using your code sample to flexibly deal with more variables. Thank you very much for your answer.

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KSSV il 28 Set 2016
It is very much possible:
  2 Commenti
Stephen23 il 28 Set 2016
Modificato: Stephen23 il 28 Set 2016
@Dr. Siva Srinivas Kolukula: the solution presented in that answer, and indeed the filter command itself, is a fundamentally 1D solution, even if applied to an array. The question above has dependencies between the variables a, b, and c. How do you propose to represent these using a 1D filter?
KSSV il 28 Set 2016
If I am not mistaken, a similar question appeared a week ago which was vectorised using filter...I searched for the question, but could not get it.

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