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digitalmars.D.learn - How to check whether an empty array variable is null?

reply tcak <1ltkrs+3wyh1ow7kzn1k sharklasers.com> writes:
[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.
Oct 10 2015
next sibling parent rumbu <rumbu rumbu.ro> writes:
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/rrrtkfosfnfuybblexow forum.dlang.org
Oct 10 2015
prev sibling next sibling parent reply Meta <jared771 gmail.com> writes:
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
}
Oct 10 2015
parent Ozan <ozan.nurettin.sueel sap.com> writes:
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
Oct 11 2015
prev sibling parent reply Jonathan M Davis via Digitalmars-d-learn writes:
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
Oct 10 2015
next sibling parent reply Meta <jared771 gmail.com> writes:
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.
Oct 10 2015
parent Meta <jared771 gmail.com> writes:
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.
Oct 10 2015
prev sibling parent reply tcak <1ltkrs+3wyh1ow7kzn1k sharklasers.com> writes:
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.
Oct 10 2015
parent Jonathan M Davis via Digitalmars-d-learn writes:
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
Oct 11 2015