How to check whether an empty array variable is null?
[code]
int[] list;
list = new int[0];
std.stdio.writeln("Is Null ? ", (list is null));
[/code]
Result is "Is Null? true".
Is this the correct behaviour? I would expect compiler to point
to an address in the heap, but set the length as 0. So, it
wouldn't return null, but the length would be 0 only.
Re: How to check whether an empty array variable is null?
On Saturday, 10 October 2015 at 15:20:04 UTC, tcak wrote:
[code]
int[] list;
list = new int[0];
std.stdio.writeln("Is Null ? ", (list is null));
[/code]
Result is "Is Null? true".
Is this the correct behaviour? I would expect compiler to point
to an address in the heap, but set the length as 0. So, it
wouldn't return null, but the length would be 0 only.
Long discussion:
http://forum.dlang.org/thread/rrrtkfosfnfuybble...@forum.dlang.org
Re: How to check whether an empty array variable is null?
On Saturday, 10 October 2015 at 15:20:04 UTC, tcak wrote:
[code]
int[] list;
list = new int[0];
std.stdio.writeln("Is Null ? ", (list is null));
[/code]
Result is "Is Null? true".
Is this the correct behaviour? I would expect compiler to point
to an address in the heap, but set the length as 0. So, it
wouldn't return null, but the length would be 0 only.
Yes, it's correct behaviour. `array is null` checks whether
array.ptr is null, which is the case for a 0-length array.
void main()
{
auto a = new int[0];
writeln(a.ptr); //a.ptr is null
auto a2 = new int[1];
writeln(a2.ptr); //a2.ptr is not null
a2 = a[0..$];//Slice off the only element of a2
writeln(a2.ptr); //Now a2.ptr is null
}
Re: How to check whether an empty array variable is null?
On Saturday, October 10, 2015 15:20:02 tcak via Digitalmars-d-learn wrote:
> [code]
> int[] list;
>
> list = new int[0];
>
> std.stdio.writeln("Is Null ? ", (list is null));
> [/code]
>
> Result is "Is Null? true".
>
> Is this the correct behaviour? I would expect compiler to point
> to an address in the heap, but set the length as 0. So, it
> wouldn't return null, but the length would be 0 only.
It basically didn't bother to allocate an array on the heap, because you
asked for one with a length of zero. Efficiency-wise, it makes no sense to
allocate anything. You wouldn't be doing anything with the memory anyway.
The only way that you're going to get an array of length 0 which doesn't
have a null ptr is to slice an array down to a length of 0.
- Jonathan M Davis
Re: How to check whether an empty array variable is null?
On Sunday, 11 October 2015 at 00:18:54 UTC, Meta wrote: On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis wrote: It basically didn't bother to allocate an array on the heap, because you asked for one with a length of zero. Efficiency-wise, it makes no sense to allocate anything. You wouldn't be doing anything with the memory anyway. The only way that you're going to get an array of length 0 which doesn't have a null ptr is to slice an array down to a length of 0. - Jonathan M Davis Look at my second example. Sorry, I thought you were responding to me.
Re: How to check whether an empty array variable is null?
On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis wrote: It basically didn't bother to allocate an array on the heap, because you asked for one with a length of zero. Efficiency-wise, it makes no sense to allocate anything. You wouldn't be doing anything with the memory anyway. The only way that you're going to get an array of length 0 which doesn't have a null ptr is to slice an array down to a length of 0. - Jonathan M Davis Look at my second example.
Re: How to check whether an empty array variable is null?
On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis
wrote:
On Saturday, October 10, 2015 15:20:02 tcak via
Digitalmars-d-learn wrote:
[code]
int[] list;
list = new int[0];
std.stdio.writeln("Is Null ? ", (list is null));
[/code]
Result is "Is Null? true".
Is this the correct behaviour? I would expect compiler to
point to an address in the heap, but set the length as 0. So,
it wouldn't return null, but the length would be 0 only.
It basically didn't bother to allocate an array on the heap,
because you asked for one with a length of zero.
Efficiency-wise, it makes no sense to allocate anything. You
wouldn't be doing anything with the memory anyway. The only way
that you're going to get an array of length 0 which doesn't
have a null ptr is to slice an array down to a length of 0.
- Jonathan M Davis
The situation is that the "length" parameter comes from user.
Also the item values come from user as well. I create the array
with "length" parameter. At another part of code, I check firstly
whether the array is created [code] if( array is null ) [/code],
then the items are checked for validation.
Re: How to check whether an empty array variable is null?
On Saturday, 10 October 2015 at 15:46:51 UTC, Meta wrote:
On Saturday, 10 October 2015 at 15:20:04 UTC, tcak wrote:
[code]
int[] list;
list = new int[0];
std.stdio.writeln("Is Null ? ", (list is null));
[/code]
Result is "Is Null? true".
}
Do I miss the point?
Shouldn't have "new int[0]" , "[]" , ".length = 0" and "null" the
same meaning like "array is empty"? I never understood why [] and
null have different meanings in other languages.
And following the discussion...In my mind, aren't pointer
operations dangerous?...yes,sometimes necessary...but always like
an operation on a open heart.
Regards, Ozan
Re: How to check whether an empty array variable is null?
On Sunday, October 11, 2015 05:10:34 tcak via Digitalmars-d-learn wrote:
> On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis
> wrote:
> > On Saturday, October 10, 2015 15:20:02 tcak via
> > Digitalmars-d-learn wrote:
> >> [code]
> >> int[] list;
> >>
> >> list = new int[0];
> >>
> >> std.stdio.writeln("Is Null ? ", (list is null));
> >> [/code]
> >>
> >> Result is "Is Null? true".
> >>
> >> Is this the correct behaviour? I would expect compiler to
> >> point to an address in the heap, but set the length as 0. So,
> >> it wouldn't return null, but the length would be 0 only.
> >
> > It basically didn't bother to allocate an array on the heap,
> > because you asked for one with a length of zero.
> > Efficiency-wise, it makes no sense to allocate anything. You
> > wouldn't be doing anything with the memory anyway. The only way
> > that you're going to get an array of length 0 which doesn't
> > have a null ptr is to slice an array down to a length of 0.
> >
> > - Jonathan M Davis
>
> The situation is that the "length" parameter comes from user.
> Also the item values come from user as well. I create the array
> with "length" parameter. At another part of code, I check firstly
> whether the array is created [code] if( array is null ) [/code],
> then the items are checked for validation.
In general, because of how arrays tend to conflate null and empty, it's a
bad idea to differentiate between null and empty with arrays. I don't know
exactly what you're doing, but there's no reason to check for null before
iterating over an array, because null == "" and null == []. You'll never get
a segfault from operating on a null array unless you try and do something
with its ptr property explicitly. Almost everything treats a null array the
same as an empty array. If you really need to have a null value for arrays,
consider using std.typecons.Nullable to wrap the array.
- Jonathan M Davis
