Re: [Vala] idiomatic way to pass an array for modification to a function

[Dan Hitt <dan hitt gmail com>]

Thanks Felipe for your reply.

So two questions:

(1) If i do not want to make a copy, is there a better way to write the
change_values() function?  For example, is there some way that
i could or should modify the change_values() function so that
it does not use pointers?

(2) If i modify change_values to be
void change_values(owned int[] w,int loc) { w[loc+0]=1; w[loc+1]=2;
w[loc+2]=3; }
without making changes in the caller, i get some warnings and
errors from the c-compiler (although not from valac).
Is this expected?  I do understand that i wouldn't want to use
'owned', but i was trying it out just to see what would happen.

Note that i'm very naive and literal about this, so there may be some
obvious context that i'm ignoring.

dan





On Sun, Dec 27, 2015 at 3:20 PM, Felipe Lavratti <felipelav gmail com> wrote:
> Hi,
>
> Everything is a reference in Vala, so, there's no buffer copy in your
> example.
>
> The rationale is opposite: Everything is passed as reference, if you want to
> pass as copy, you have to set the parameter `owned`
>
> ```
> void change_values(owned int[] w,int loc) { w[loc+0]=1; w[loc+1]=2;
> w[loc+2]=3; }
> ```
>
> Also, since you created an array with `new int[]` you should have an
> parameter with the same construction, so that you variable `w` inside
> `change_values` will have properties such as `.length`.
>
> On Sun, Dec 27, 2015 at 6:24 PM Dan Hitt <dan hitt gmail com> wrote:
> > I've been struggling to write a function that can modify an array it
> > receives as an argument.
> >
> > Now, as it happens, it looks like you can just write it like it were
> > C, using pointers.
> >
> > So this code works and does what i expect:
> >
> >     void main( ) {
> >             var t = new int[6];
> >             change_values(t, 0);
> >             change_values(t, 3);
> >             for (var i=0; i<t.length; i++)
> >                     stdout.printf("%d -> %d\n",i,t[i]);
> >     }
> >
> >     void change_values(int *w,int loc) {
> >             w[loc+0]=1;
> >             w[loc+1]=2;
> >             w[loc+2]=3;
> >     }
> >
> > However, it doesn't use 'ref' and i would like to write something more
> > idiomatic.
> >
> > The constraint is that i really do want to have reference-type semantics
> > with
> > no unnecessary copying because the actual application will involve a
> > routine
> > called many times on a long input array.
> >
> > Thanks in advance for any clues on how to make this more stylistic, or any
> > good
> > but detailed references.  (This subject seems to come up from time to
> > time, but the
> > answers are never quite detailed enough for me to comprehend.)
> >
> > dan
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