Re: Unidentified subject!

[Paul Kienzle]

On Feb 15, 2004, at 12:20 AM, Avraham Rosenberg wrote:

> Hi,
> Paul Kienzle and Andy Adler mentioned recently the possibility of 
> forcing
> the creation of a column vector by using the symbol x(:).
> Could anyone give an example, please ? I tried x(:)=sort([1,5,8,4,2]) 
> and
> I got a sorted row vector (Octave-2.1.50 on debian stable). Apparently 
> I
> misunderstood those answers.

First let's make sure we are using the same terminology:

        a column vector is N x 1, or a single column matrix
        a row vector is 1 x M, or a single row matrix

Next, understand that the following does not affect the shape
of x:

        x(:) = y;

It instead does an element-wise copy from y to x in order
so long as x and y are the same size, no matter what the
shape of either.  E.g.,

        octave> x=zeros(2,2);
        octave> x(:)=1:4
        x =
          1  3
          2  4

However, as an expression, x(:) converts x into a column vector:

        octave> x(:)
        ans =
          1
          2
          3
          4

and x(:).' converts x into a row vector:

        octave> x(:).'
        ans =
          1  2  3  4

This even works on expressions (at least in Octave):

        octave> sort([1,5,8,4,2])
        ans =
          1  2  4  5  8

        octave> sort([1,5,8,4,2])(:)
        ans =
          1
          2
          4
          5
          8

Paul Kienzle
address@hidden



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