what does A(ones(3,1),:)) do?

A=[1 2 3}
B= A(ones(3,1),:)
B=[1 2 3;1 2 3; 1 2 3]

please explain the second line how it works

Answer 1

[Thorsten]

 >> A = [1 2 3]
 A =
     1     2     3

To select the first row of A, all columns (:)

 >> A(1,:)
 ans =
     1     2     3

You can also select the first row twice, using [1 1]:

 >> A([1 1],:)
 ans =
     1     2     3
     1     2     3

or thrice, using [1 1 1]:

 >> A([1 1 1],:)
 ans =
     1     2     3
     1     2     3
     1     2     3

And you can use ones(1,3) to create [1 1 1]:

 >> A(ones(1,3),:)
 ans =
     1     2     3
     1     2     3
     1     2     3

As long as it is a vector, it can be a row or a column vector, so you can also write

A(ones(3,1),:)