Circular Matrix computation

Commodity function for building a circulant matrix given a vector

Description

circulant Computes the circulant matrix of a vector.

  circulant(vec) is a max(size(vec))-by-max(size(vec)) circulant
  matrix built with the elements of vec

  Note: The parameter may be either a column or row vector

  Example:
     v = [ 1 2 3 4 ];
     circulant( v )

     ans =

     1 2 3 4
     2 3 4 1
     3 4 1 2
     4 1 2 3

• Acknowledgements: This submission has inspired the following:; Circulant matrix
• MATLAB release: MATLAB 7.7 (R2008b)
• Other requirements: Any version of Matlab with the circshift function

[Jos (10584)]

Please add a proper help section including a H1 line, syntax and example. At present this submission is completely useless for anyone but the author.

[Massimo Fierro]

I will gladly do, but I thought that the description "The function accepts both column and row vectors and produces the circulant matrix associated." would suffice.

[Duane Hanselman]

Authors should not pad the star rating of their submissions.

[Jos (10584)]

Some indexing will give you the same result
V = IN(:) ;
idx = hankel(1:numel(V),0:numel(V)-1)
% idx = hankel(1:numel(V)) + hankel(zeros(1,numl(V)),0:numel(V)-1)
OUT = V(ix) ;

[Massimo Fierro]

Thanks Jos for the comment, I didn't know about the hankel function.
Had I known, I would have definitely done it that way (as indexing always works better in Matlab).

I am quite surprised, though, that there is no function in the standard library for producing such a common matrix.

[Husam Aldahiyat]

Don't listen to these elitists.
Good work and nice effort.

[Jos (10584)]

The author should be commended for adding a good help section. Although faster and better algorithms exist, I concur with Husam A. that this is indeed a nice effort, and therefor happily raise my rating.