Try this:

>       ubyte s[10];
>       ubyte[] t = s[0..0];
>       if (t)
>       {
>           printf("t not empty\n");
>       }

The problem is that expression s[i..j] yeilds a slice which is not well formed. It has a length field of zero, but a pointer field which is non-zero (actually the address of s[i]. Thus, expressions like if (s[i..j]) will NOT test as false if i == j.

The current workaround is to repace "if (t)" with "if (t.length != 0)".

Arcane Jill

Arcane Jill wrote:

> Try this:
> 
>>       ubyte s[10];
>>       ubyte[] t = s[0..0];
>>       if (t)
>>       {
>>           printf("t not empty\n");
>>       }
> 
> The problem is that expression s[i..j] yeilds a slice which is not well formed. It has a length field of zero, but a pointer field which is non-zero (actually the address of s[i]. Thus, expressions like if (s[i..j]) will NOT test as false if i == j.
> 
> The current workaround is to repace "if (t)" with "if (t.length != 0)".

I'm not sure this is a bug or a feature. Of course, a zero-length array reference cannot be indexed without getting an out-of-range exception. But I'm not really sure whether it should be considered identical to null.

In article <cahmcb$92$1@digitaldaemon.com>, Norbert Nemec says...
>
>> The problem is that expression s[i..j] yeilds a slice which is not well formed. It has a length field of zero, but a pointer field which is non-zero (actually the address of s[i]. Thus, expressions like if (s[i..j]) will NOT test as false if i == j.
>> 
>> The current workaround is to repace "if (t)" with "if (t.length != 0)".
>
>I'm not sure this is a bug or a feature. Of course, a zero-length array reference cannot be indexed without getting an out-of-range exception. But I'm not really sure whether it should be considered identical to null.

Except that according to the D website at http://www.digitalmars.com/d/cppstrings.html, it says explicitly, and I quote:

>Checking For Empty Strings
>C++ strings use a function to determine if a string is empty:
>
>    string str;
>    if (str.empty())
>        // string is empty
>
>In D, an empty string is just null:
>
>    char[] str;
>    if (!str)
>        // string is empty

So either the documentation is in error, or it's a bug. Arcane Jill

Arcane Jill wrote:
> In article <cahmcb$92$1@digitaldaemon.com>, Norbert Nemec says...
> 
>>>The problem is that expression s[i..j] yeilds a slice which is not well
>>>formed. It has a length field of zero, but a pointer field which is
>>>non-zero (actually the address of s[i]. Thus, expressions like if
>>>(s[i..j]) will NOT test as false if i == j.
>>>
>>>The current workaround is to repace "if (t)" with "if (t.length != 0)".
>>
>>I'm not sure this is a bug or a feature. Of course, a zero-length array
>>reference cannot be indexed without getting an out-of-range exception. But
>>I'm not really sure whether it should be considered identical to null.
> 
> 
> Except that according to the D website at
> http://www.digitalmars.com/d/cppstrings.html, it says explicitly, and I quote:
> 
> 
>>Checking For Empty Strings
>>C++ strings use a function to determine if a string is empty:
>>
>>   string str;
>>   if (str.empty())
>>       // string is empty
>>
>>In D, an empty string is just null:
>>
>>   char[] str;
>>   if (!str)
>>       // string is empty
> 
> 
> So either the documentation is in error, or it's a bug.
> Arcane Jill

Hmmm. If this is the case then I think it is a mistake. A null string is different from an empty string. One means the absence of a string and the other means that the string has length 0.

This is an important difference. For example, passing null for a string argument usually means "use the default". That kind of semantics would be impossible if empty strings are also valid values.


Hauke

On Sun, 13 Jun 2004 19:39:48 +0200, Hauke Duden <H.NS.Duden@gmx.net> wrote:
> Arcane Jill wrote:
>> In article <cahmcb$92$1@digitaldaemon.com>, Norbert Nemec says...
>>
>>>> The problem is that expression s[i..j] yeilds a slice which is not well
>>>> formed. It has a length field of zero, but a pointer field which is
>>>> non-zero (actually the address of s[i]. Thus, expressions like if
>>>> (s[i..j]) will NOT test as false if i == j.
>>>>
>>>> The current workaround is to repace "if (t)" with "if (t.length != 0)".
>>>
>>> I'm not sure this is a bug or a feature. Of course, a zero-length array
>>> reference cannot be indexed without getting an out-of-range exception. But
>>> I'm not really sure whether it should be considered identical to null.
>>
>>
>> Except that according to the D website at
>> http://www.digitalmars.com/d/cppstrings.html, it says explicitly, and I quote:
>>
>>
>>> Checking For Empty Strings
>>> C++ strings use a function to determine if a string is empty:
>>>
>>>   string str;
>>>   if (str.empty())
>>>       // string is empty
>>>
>>> In D, an empty string is just null:
>>>
>>>   char[] str;
>>>   if (!str)
>>>       // string is empty
>>
>>
>> So either the documentation is in error, or it's a bug.
>> Arcane Jill
>
> Hmmm. If this is the case then I think it is a mistake. A null string is different from an empty string. One means the absence of a string and the other means that the string has length 0.

Agreed, we need both.

> This is an important difference. For example, passing null for a string argument usually means "use the default". That kind of semantics would be impossible if empty strings are also valid values.

I think it is logical that s[0..0] returns an empty string and not null.

I think the docs need an update. The existing example is good as a test for null but it should be noted that empty strings can exist and how to get them.

if (!str)            //tests for null
if (str.length == 0) //tests for null and empty

Regan.

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

> > Hmmm. If this is the case then I think it is a mistake. A null string is different from an empty string. One means the absence of a string and the other means that the string has length 0.
>
> Agreed, we need both.

I always test  s.length  to see if it contains 0 elements. When I want to see if it's null I use  s === null . if(s)  was never reliable for me.