Re: a{:} = x

[John W. Eaton]

On 11-Jan-2009, Jaroslav Hajek wrote:

| On Fri, Jan 9, 2009 at 5:50 PM, John W. Eaton <address@hidden> wrote:
| > On  9-Jan-2009, Jaroslav Hajek wrote:
| >
| > | I just noticed that the following code works in Octave:
| > | a = {1,2,3}
| > | a{:} = 1
| > | and ends up with a = {1,1,1}. Matlab gives an error? Extension or bug?
| >
| > I think the correct way to place a particular value in each cell would
| > be
| >
| >  a(:) = {1}
| >
| 
| Agreed.
| 
| 
| > | What's more weird,
| > | a{:} = a{:} works as well and likewise gives a = {1,1,1}.
| >
| > IF a{:} = 1 is accepted, then I think this sort of makes sense because
| > a{:} produces a comma-separated list, so a{:} = a{:} would be
| > equivalent to
| >
| >  a{:} = 1, 2, 3
| >
| > But then, you should also see the 2, 3 results printed to the screen.
| 
| Why? Isn't it better to give an error message?
| In Matlab 2007, the following code:
| 
| x = 1,2,3
| 
| produces
| 
| x = 1
| ans = 2
| ans = 3
| 
| whereas
| 
| y = {1,2,3}; x = {:}
| 
| produces just
| 
| x = 1
| 
| Which behaviour is correct? I think y{:} is supposed to be equivalent
| to y{1},y{2},y{3}, but apparently it is not.

Yeah, although normaly a comma-separated list works like a list of
values separated by commas, these commas must be different.  :-/

I suppose it makes sense if you think about the comma that is a
statement separator as being different from the comma that is to
separate the elements of an argument list.  But hmm.  That just makes
things more confusing in a way.

| behavior a little more, but I'd apreciate even more a message like
| this:
| error: the number of rhs values does not match lhs

I think that would be reasonable.  Does any real code actually depend
on this "feature"?

| I've uploaded a patch fixing the a{x:y} = z case. Note that the
| incorrect idiom was used in a number of places within Octave's
| m-sources, so I guess this may reveal bugs in OctaveForge as well.
| 
| http://hg.savannah.gnu.org/hgweb/octave/rev/fd11a08a9b31

Thanks.

| The other stuff like the "a = 1,2,3" case is left intact, but I'd like
| if someone explained the logic of the current behaviour to me.

It is just three separate expressions, separated by commas.  So it is
equivalent to

  a = 1
  2
  3

jwe