what does c(:).' mean? c should be a vector

Hi I saw this in Matlab document:

function obj = DocPolynom(c)
   % Construct a DocPolynom object using the coefficients supplied
   if isa(c,'DocPolynom')
      obj.coef = c.coef;
   else
      obj.coef = c(:).';
   end
end

What does c(:).' mean? Is .' the transpose of each element? But I think c should be a vector here. Any help's appreciated. Thanks!

Source of the above code: http://www.mathworks.com/help/matlab/matlab_oop/a-polynomial-class-1.html

Answer 1

[Matz Johansson Bergström]

By writing c(:)' you are making sure that the vector has the dimension 1xn, where n is the length of the vector, a row vector. However, there is a very important difference between c(:).' and c(:)', because they are not the same.

The difference is when you are working with complex vectors, for instance

tmp = (1:5) + 1i; 
tmp(:).'

which gives you each complex number in a row vector, compared to

tmp(:)'

which is giving you a row vector of the complex conjugate of each element.

[Yuji Zhang]

Ahh i see! Thanks Matz!

Answer 2

[Roger Stafford]

This is a short way to reshape c, whether it is a vector or array, into a row vector. You are then guaranteed that obj.coef will be a vector with just one row and however many columns as there are elements in c.

reshape(c,[],1)

will do the same thing.

[Yuji Zhang]

*Thanks Roger. Isn't reshape c(:)?

c(:) turns any matrix c into one column.

c(:)' is then one row.*

Why c(:).' ? Let me know. Thanks a lot!

[Image Analyst]

Correct, no matter what c is to start with c(:) turns it into a column vector. Then ' transposes it into a row vector. The dot means "element by element" which doesn't really have any meaning here (since c is a 1D vector and it's not operating on any other variable) so c(:).' is the same as c(:)'.

[Roger Stafford]

No, the dot prevents matlab from taking the complex conjugate of elements along with the transposition. It has nothing to do with element-by-element operation. Their documentation says: "b = a.' computes the non-conjugate transpose of matrix a and returns the result in b" and "b = a' computes the complex conjugate transpose of matrix a and returns the result in b."

If c is entirely real-valued, then c(:).' and c(:)' are the same.

[Yuji Zhang]

Ahh I see that! So the dot has two meanings! Thanks a lot for your help Roger!