Vectorizing a loop that depens on past information

[Alexandre Velloso]

Hello,

Can anybody think of a solution to the problem below?

x is a vector containing real numbers being x(1) <> 0;

I want to vectorize the following loop:

for i = 2: length( x )

if x(i) ==0

x(i) = x( i - 1);

end
end

thanks,

Alexandre

[Siyi]

One solution:

y = find(x);
x1 = x(y(cumsum(x ~= 0)))

[Bruno Luong]

"Alexandre Velloso" <aleve...@gmail.com> wrote in message <h13if6$6hr$1...@fred.mathworks.com>...

Not sure it will be faster, but here is vectorized

z = x==0;
nz=find(~z);
z=find(z);
[trash loc]=histc(z,[nz length(x)+1]);
x(z) = x(nz(loc))

% Bruno

[Bruno Luong]

Siyi <Mr.Siy...@gmail.com> wrote in message <bd9d00c3-a148-4d2a-

>
> One solution:
>
> y = find(x);
> x1 = x(y(cumsum(x ~= 0)))

Neat!

Bruno

[Bruno Luong]

I have just timed it, for-loop is about 4-5 time faster. I would stick with for-loop solution.

Bruno

[Jos]

"Alexandre Velloso" <aleve...@gmail.com> wrote in message <h13if6$6hr$1...@fred.mathworks.com>...

Use FIND and CUMSUM. For a ND-vectorized solution that can handle NaN's as well , see:
http://www.mathworks.com/matlabcentral/fileexchange/19906

Jos

[Alexandre Velloso]

Thank you, guys.

I agree, Bruno.

Siyi, your solution was very good and fast. It seems exactly what I was looking for.

[Alexandre Velloso]

Thank u, Jos.