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From: Ruben on 29 Jun 2010 05:42
Dear all,

I would like to know if there's a function that could help me to determine if there is a loop given a sequence of numbers in a vector. And also return the values creating the loop. For instance:

A = [7 6 2 1 3 4 8 1 2 4 8 1 ....]
B = [1 2 1 2 1 2 ...]
C = [1 2 3 4 5 6 7 8 ...]

The function should return something like this:

is_loop(A) => loop = [1 2 4 8]
is_loop(B) => loop = [1 2]
is_loop(C) => loop = []

I thougth of using strings to do this but I didn't find any suitable function.
Any suggestions?
Thank you :-)
From: Jan Simon on 29 Jun 2010 07:48
Dear Ruben,

> I would like to know if there's a function that could help me to determine if there is a loop given a sequence of numbers in a vector. And also return the values creating the loop. For instance:
>
> A = [7 6 2 1 3 4 8 1 2 4 8 1 ....]
> The function should return something like this:
> is_loop(A) => loop = [1 2 4 8]

Please explain, why and how A leads to [1,2,4,8].
How do you define a "loop"?
If you define the problem exactly, the way to program it should be done to 90% already.

Good luck, Jan
From: Ruben on 29 Jun 2010 08:07
Sorry, my bad! I should have written

A = [7 6 2 1 3 4 8 1 3 4 8 1 ....]

is_loop(A) => loop = [1 3 4 8]



"Jan Simon" <matlab.THIS_YEAR(a)nMINUSsimon.de> wrote in message <i0cmhk$1hc$1(a)fred.mathworks.com>...
> Dear Ruben,
>
> > I would like to know if there's a function that could help me to determine if there is a loop given a sequence of numbers in a vector. And also return the values creating the loop. For instance:
> >
> > A = [7 6 2 1 3 4 8 1 2 4 8 1 ....]
> > The function should return something like this:
> > is_loop(A) => loop = [1 2 4 8]
>
> Please explain, why and how A leads to [1,2,4,8].
> How do you define a "loop"?
> If you define the problem exactly, the way to program it should be done to 90% already.
>
> Good luck, Jan
From: Ruben on 29 Jun 2010 08:11
In fact, I just tried to simplify the problem when I posted it. What I really need to do is to find loops given a source point and a set of senders and receivers (like in network loops).

An example is:

[path, loop] = is_loop(2, [2,1,3,4,8,7,9], [1,3,4,8,7,9,4])

path =

2 1 3 4 8 7 9


loop =

4 8 7 9


The code is the following, but I was wondering if I could write this code in a more efficient way:

function [path, loop] = is_loop(source, senders, receivers)

next = source;
path = [];
pos = [];
while (~isempty(next) && isempty(pos))
%- Check if next element is already in the path
pos = find(path == next, 1, 'first');

%- Update
path = [path, next];
next = receivers(senders == next);
end

if (~isempty(pos))
path = path(1:end-1);
loop = path(pos:end);
end
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