how can i vectorize this loop?

2017 11 12
2 个回答
回答已采纳
更新时间：2017 11 13
15 次查看（30 天）

0 个投票

% x is a vector given in main program
function y = userfunction2(x,k)
x=y;
for i=i:k
y=userfunction(y);
end

5 个评论
显示 3更早的评论 隐藏 3更早的评论

Akira Agata
Akira Agata 2017-11-12
I don't think this function works.
What is the purpose of the second line ('x=y;')? Why input variable x is replaced by y? What is y??
What is the userfunction(y) in the third line?
William Brannon
William Brannon 2017-11-12
编辑：Walter Roberson 2017-11-12
function y = LeftShift(x)
% x is a row n-vector and y is a row n-vector that is the left shift of x.
n=length(x);
y=zeros(n,1);
for k=1:n
if k<n;
y(k)=x(k+1);
else
y(n)=x(1);
end
end
William Brannon
William Brannon 2017-11-12
It is a fuction to shift a given vector. the first value in hte vector goes to the nth place, the other values shift 1 position to the right.
William Brannon
William Brannon 2017-11-12
the original function works, but I am supposed to do it without a loop. LeftShift is okay with a loop,but the other one is supposed to shift it k times, which is entered by another user.
William Brannon
William Brannon 2017-11-12
What I really want is afunction that does this without a loop "k" times:
ShiftLeft(...(ShiftLeft(y)...)

请先登录，再进行评论。

请先登录，再回答此问题。

 采纳的回答

Walter Roberson
Walter Roberson 2017-11-12

1 个投票

See fold()

2 个评论
显示 无 隐藏 无

William Brannon
William Brannon 2017-11-12
I'm sorry, but I don't understand how this works. What would the code actually look like?
I was thinking of vector operations to complete it, but I am not that familiar with how to use it in this case.
Walter Roberson
Walter Roberson 2017-11-12
ShiftLeftAux = @(V) {ShiftLeft(V)}
t = cell(1,k); t{1} = {y});
t2 = fold(ShiftLeftAux, t);
result = t2{1};
Not nearly as clean as I would like.
This does not really vectorize the loop, though: it merely hides the loop into fold(), the same as if you had just written a routine that did the operation multiple times.
If you want actual vectorization you should instead be using circshift().
There is no way in MATLAB to say "vectorize this arbitrary linear code": there are only ways to get it to apply the linear code without you writing a visible for loop. The results are typically slower than using a for loop.

请先登录，再进行评论。

更多回答（1 个）

Akira Agata
Akira Agata 2017-11-13

0 个投票

Seems that you would like to do circshift ?
Your 'y = ShiftLeft(x)' seems equivalent to 'y = circshift(x,1)', like:
x = 1:10;
y = circshift(x,1);
Then, y becomes
>> y
ans =
10 1 2 3 4 5 6 7 8 9
If you want to shift k times, you should simply do 'y = circshift(x,k)'. Please see more detail on circshift documentation page .

类别

在 帮助中心 和 File Exchange 中查找有关 Loops and Conditional Statements 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by