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digitalmars.D.learn - Does D has C#'s string.Empty?

reply "AsmMan" <jckj33 gmail.com> writes:
Sep 24 2014
next sibling parent reply "SlomoTheBrave" <SlomoTheBrave nowhere.it> writes:
On Thursday, 25 September 2014 at 05:29:37 UTC, AsmMan wrote:




string.init ?

----
     string a;
     a = string.init;
     assert( a == "");
----

does the job for the type string at least.
Sep 24 2014
next sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 9/25/14 2:41 AM, SlomoTheBrave wrote:

 On Thursday, 25 September 2014 at 05:29:37 UTC, AsmMan wrote:




 string.init ?

 ----
      string a;
      a = string.init;
      assert( a == "");
 ----

 does the job for the type string at least.


null also works. In fact, in the above, a is already string.init or null 
before assigning (D always initializes variables unless asked not to).

a = null; // same as a = string.init;

-Steve
Sep 25 2014
parent reply "AsmMan" <jckj33 gmail.com> writes:
On Thursday, 25 September 2014 at 12:43:57 UTC, Steven
Schveighoffer wrote:

 On 9/25/14 2:41 AM, SlomoTheBrave wrote:

 On Thursday, 25 September 2014 at 05:29:37 UTC, AsmMan wrote:




 string.init ?

 ----
     string a;
     a = string.init;
     assert( a == "");
 ----

 does the job for the type string at least.


 null also works. In fact, in the above, a is already 
 string.init or null before assigning (D always initializes 
 variables unless asked not to).

 a = null; // same as a = string.init;

 -Steve


It made me a bit confusing. How is the implementation of string
comparasion in D? (if someone could point to actual code used in
these comparasion would be really great otherwise I'll check out

"" == null is true
Sep 25 2014
next sibling parent reply ketmar via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On Fri, 26 Sep 2014 00:24:27 +0000
AsmMan via Digitalmars-d-learn <digitalmars-d-learn puremagic.com>
wrote:


 It made me a bit confusing. How is the implementation of string
 comparasion in D?


"" has length of 0. null has length of 0. two strings without content
are essentialy the same.
Sep 25 2014
parent reply "AsmMan" <jckj33 gmail.com> writes:
On Friday, 26 September 2014 at 00:53:24 UTC, ketmar via 
Digitalmars-d-learn wrote:

 On Fri, 26 Sep 2014 00:24:27 +0000
 AsmMan via Digitalmars-d-learn 
 <digitalmars-d-learn puremagic.com>
 wrote:


 It made me a bit confusing. How is the implementation of string
 comparasion in D?


 "" has length of 0. null has length of 0. two strings without 
 content
 are essentialy the same.


but null has length? what's null in D?
Sep 25 2014
next sibling parent ketmar via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On Fri, 26 Sep 2014 01:08:59 +0000
AsmMan via Digitalmars-d-learn <digitalmars-d-learn puremagic.com>
wrote:


 but null has length? what's null in D?


no, it's dynamic array that have `.length`. compiler magic. assigning
`null` to dynamic array variable does some magic under the hood.

dynamic array is actually this: `struct { size_t len; void* ptr};`.
when you assign `null` to dynamic array variable, compiler generates
code to clear both `len` and `ptr`. think about dynarray var as kind
of 'fat pointer', which compiler knows how to operate.
Sep 25 2014
prev sibling next sibling parent ketmar via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On Fri, 26 Sep 2014 01:08:59 +0000
AsmMan via Digitalmars-d-learn <digitalmars-d-learn puremagic.com>
wrote:


 "" has length of 0. null has length of 0. two strings without=20
 content
 are essentialy the same.


 but null has length? what's null in D?


i confused you here, sorry. what i mean is 'null string', not 'null' as
a type itself. i.e.
string a =3D ""; // "empty string"
string a =3D null; // "null string"
Sep 25 2014
prev sibling parent "Chris Cain" <zshazz gmail.com> writes:
On Friday, 26 September 2014 at 01:09:01 UTC, AsmMan wrote:

 On Friday, 26 September 2014 at 00:53:24 UTC, ketmar via 
 Digitalmars-d-learn wrote:

 On Fri, 26 Sep 2014 00:24:27 +0000
 AsmMan via Digitalmars-d-learn 
 <digitalmars-d-learn puremagic.com>
 wrote:


 It made me a bit confusing. How is the implementation of 
 string
 comparasion in D?


 "" has length of 0. null has length of 0. two strings without 
 content
 are essentialy the same.


 but null has length? what's null in D?


To reinforce what ketmar is saying:

strings in D are just `immutable(char)[]` ... a simple array of 
immutable chars. All arrays are just (essentially) `struct Arr(T) 

String is actually a class. So, when we talk about `string s = 
null;`, the structure would look like `ptr = null; length = 0`. 
You can still get the length out of a "nulled out string" (or any 

null pointer exception since it's a class.

When you compare equality between two different arrays in D, 
you're checking for if they have all of the same elements (thus, 
ptr being different between two arrays is irrelevant). Trivially 
any two strings with length 0 are equal, so there are 4 billion 
equivalent empty strings in D (in 32-bit mode, and 2^64 in 64 bit 
mode, of course).

So, if you want an empty string, you can do any one of:

`s = null;`
`s.length = 0;`
`s = "";`

and you'll be fine.
Sep 25 2014
prev sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 9/25/14 8:24 PM, AsmMan wrote:

 On Thursday, 25 September 2014 at 12:43:57 UTC, Steven
 Schveighoffer wrote:

 On 9/25/14 2:41 AM, SlomoTheBrave wrote:

 On Thursday, 25 September 2014 at 05:29:37 UTC, AsmMan wrote:




 string.init ?

 ----
     string a;
     a = string.init;
     assert( a == "");
 ----

 does the job for the type string at least.


 null also works. In fact, in the above, a is already string.init or
 null before assigning (D always initializes variables unless asked not
 to).

 a = null; // same as a = string.init;





 It made me a bit confusing. How is the implementation of string
 comparasion in D? (if someone could point to actual code used in
 these comparasion would be really great otherwise I'll check out

 "" == null is true


A string is simply an array of char in D, similar to how it is in C. It 


The code to compare all arrays is somewhere in the runtime, not sure 
exactly where, but the compiler has some tricks it can use when 
generating code, so it may not call those functions.

Essentially, comparison is a foreach over the length of the arrays. If 
they are null, then their lengths are 0. But the actual pointers may be 
different, it doesn't matter.

Interestingly:

string s = "";
string t = [];

Note that both are valid, because both are arrays. but...

assert(s.ptr !is null);
assert(t.ptr is null);

Why? Because string literals are special in that they actually point to 
null terminated memory. So "hello" is actually an array of 5 chars + a 
6th '\0' char. However, the length of "hello" is set to 5. The reason 
for this is so you can just pass a string literal into a C function, and 
it works. But due to this rule, "" cannot point at null, it must point 
to valid memory for the terminating '\0'.

An array does not need to conform to C rules, so it can safely set the 
pointer to null.

If we compare s and t:

assert(s == t);
assert(s !is t);

this is because == checks to see if the arrays are *equivalent*, meaning 
they have the same length, and all the elements are equivalent. 'is' 
checks to see if the arrays are *identical*, meaning they point at the 
same memory.

-Steve
Sep 26 2014
parent "H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:
On Fri, Sep 26, 2014 at 02:01:22PM -0400, Steven Schveighoffer via
Digitalmars-d-learn wrote:

 On 9/25/14 8:24 PM, AsmMan wrote:


[...]

It made me a bit confusing. How is the implementation of string
comparasion in D? (if someone could point to actual code used in
these comparasion would be really great otherwise I'll check out

"" == null is true




[...]

In D, it's generally a bad idea to compare arrays with null, unless
you're prepared to handle a lot of implementation-specific details.
Instead, you should check .length, if it's 0, the string is empty,
otherwise, it's not.

If you need to distinguish between an empty string and null, you might
want to consider using std.typecons.Nullable instead. It's slightly more
verbose, but makes the intent of the code clearer and may lead to more
maintainable code.


T

-- 
One disk to rule them all, One disk to find them. One disk to bring them all
and in the darkness grind them. In the Land of Redmond where the shadows lie.
-- The Silicon Valley Tarot
Sep 26 2014
prev sibling parent "AsmMan" <jckj33 gmail.com> writes:
On Thursday, 25 September 2014 at 06:41:03 UTC, SlomoTheBrave 
wrote:

 On Thursday, 25 September 2014 at 05:29:37 UTC, AsmMan wrote:




 string.init ?

 ----
     string a;
     a = string.init;
     assert( a == "");
 ----

 does the job for the type string at least.


Thanks.:)
Sep 25 2014
prev sibling parent reply Daniel Kozak via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
V Thu, 25 Sep 2014 05:29:36 +0000
AsmMan via Digitalmars-d-learn <digitalmars-d-learn puremagic.com>
napsáno:





string.init
Sep 24 2014
parent "SlomoTheBrave" <SlomoTheBrave nowhere.it> writes:
On Thursday, 25 September 2014 at 06:41:55 UTC, Daniel Kozak via 
Digitalmars-d-learn wrote:

 V Thu, 25 Sep 2014 05:29:36 +0000
 AsmMan via Digitalmars-d-learn 
 <digitalmars-d-learn puremagic.com>
 napsáno:





 string.init


first ^^
Sep 24 2014