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

Question

Yuji Zhang 2014-8-24

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评论： Yuji Zhang 2014-8-25

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

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Answer 1

Matz Johansson Bergström 2014-8-24

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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.

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Yuji Zhang 2014-8-24

Ahh i see! Thanks Matz!

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Answer 2

Roger Stafford 2014-8-24

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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.

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Roger Stafford 2014-8-24

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 2014-8-25