www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Why do some T.init evaluate to true while others to false?

reply ArturG <var.spool.mail700 gmail.com> writes:
for example:

if(any floatingpoint.init) will be true
if(any char.init) also true
if("") also true

while others are false e.g.

string s;
if(s) will be false
all others are also false or did i miss any?
May 26 2016
next sibling parent reply Basile B. <b2.temp gmx.com> writes:
On Thursday, 26 May 2016 at 14:03:16 UTC, ArturG wrote:

 for example:

 if(any floatingpoint.init) will be true
 if(any char.init) also true
 if("") also true

 while others are false e.g.

 string s;
 if(s) will be false
 all others are also false or did i miss any?


It's a shortcut that works for certain type and that means:

- pointers: if (ptr)  <=> if (ptr != null)
- pointers: if (!ptr) <=> if (ptr == null)
- integral(*): if (i) <=> if (i > 0)
- integral: if (!i)   <=> if (i == 0)
- classes: if (c)     <=> if (c !is null)
- classes: if (!c)    <=> if (c is null)

(*) integral: generally speaking so: byte, ubyte, short, ushort, 
int, uint, long, ulong, char, wchar, dchar and also, very special 
case, structs with an alias this to one of this integral type.

for array this works and this tests the (.ptr) member but most of 
the people here (incl. me) would recommand rather to always do:

     "if (arr.length)"

because in some cases "if (arr)" will yield "true" even if the 
length is equal to 0.
May 26 2016
parent reply Basile B. <b2.temp gmx.com> writes:
On Thursday, 26 May 2016 at 15:11:50 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 14:03:16 UTC, ArturG wrote:

 [...]





 [...]
 - integral(*): if (i) <=> if (i > 0)


I obviously meant:

- integral(*): if (i) <=> if (i <> 0)

and "<=>" stands for "equivalence"
May 26 2016
parent reply Basile B. <b2.temp gmx.com> writes:
On Thursday, 26 May 2016 at 15:14:21 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 15:11:50 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 14:03:16 UTC, ArturG wrote:

 [...]





 [...]
 - integral(*): if (i) <=> if (i > 0)


 I obviously meant:

 - integral(*): if (i) <=> if (i <> 0)

 and "<=>" stands for "equivalence"


I obviously meant:

integral(*): if (i) <=> if (i != 0),

"<>" is the Pascal operator for C's "!=" ....
May 26 2016
parent reply arturg <var.spool.mail700 gmail.com> writes:
On Thursday, 26 May 2016 at 15:15:57 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 15:14:21 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 15:11:50 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 14:03:16 UTC, ArturG wrote:

 [...]





 [...]
 - integral(*): if (i) <=> if (i > 0)


 I obviously meant:

 - integral(*): if (i) <=> if (i <> 0)

 and "<=>" stands for "equivalence"


 I obviously meant:

 integral(*): if (i) <=> if (i != 0),

 "<>" is the Pascal operator for C's "!=" ....


yes i know about most of those shortcuts its just float.init and 
char.init that work different then the other when you do this

if(someType) // will be true for float and char while someType is 
T.init
May 26 2016
parent reply Basile B. <b2.temp gmx.com> writes:
On Thursday, 26 May 2016 at 15:25:03 UTC, arturg wrote:

 On Thursday, 26 May 2016 at 15:15:57 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 15:14:21 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 15:11:50 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 14:03:16 UTC, ArturG wrote:

 [...]





 [...]
 - integral(*): if (i) <=> if (i > 0)


 I obviously meant:

 - integral(*): if (i) <=> if (i <> 0)

 and "<=>" stands for "equivalence"


 I obviously meant:

 integral(*): if (i) <=> if (i != 0),

 "<>" is the Pascal operator for C's "!=" ....


 yes i know about most of those shortcuts its just float.init 
 and char.init that work different then the other when you do 
 this

 if(someType) // will be true for float and char while someType 
 is T.init


float.init is not equal to 0.0f. In D FP points values are 
initialized to nan (not a number).

By the way for strings it works, it's like the array case I 
described in the first answer).
May 26 2016
parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Thursday, 26 May 2016 at 15:29:52 UTC, Basile B. wrote:


 float.init is not equal to 0.0f. In D FP points values are 
 initialized to nan (not a number).

 By the way for strings it works, it's like the array case I 
 described in the first answer).


yes i guess i tested all/most types and know that float.init is 
float.nan but why is nan true and not false?
May 26 2016
next sibling parent reply Basile B. <b2.temp gmx.com> writes:
On Thursday, 26 May 2016 at 15:34:50 UTC, ArturG wrote:

 On Thursday, 26 May 2016 at 15:29:52 UTC, Basile B. wrote:


 float.init is not equal to 0.0f. In D FP points values are 
 initialized to nan (not a number).

 By the way for strings it works, it's like the array case I 
 described in the first answer).


 yes i guess i tested all/most types and know that float.init is 
 float.nan but why is nan true and not false?


because nan is not 0 and that the shortcut for float is

if (fpValue) <=> if (fpValue != 0)
if (!fpValue)<=> if (fpValue == 0)

There's no relation between the initializer and the shortcut. 
It's not because for some values the shortcut matches to the 
initializer that it must always be the case...But I admit I don't 
know the exact rationale.

Does anyone know ?
May 26 2016
parent ArturG <var.spool.mail700 gmail.com> writes:
On Thursday, 26 May 2016 at 15:38:55 UTC, Basile B. wrote:


 because nan is not 0 and that the shortcut for float is

 if (fpValue) <=> if (fpValue != 0)
 if (!fpValue)<=> if (fpValue == 0)

 There's no relation between the initializer and the shortcut. 
 It's not because for some values the shortcut matches to the 
 initializer that it must always be the case...But I admit I 
 don't know the exact rationale.

 Does anyone know ?


Ok sorry for the noise and thanks anyway.
May 26 2016
prev sibling parent reply Basile B. <b2.temp gmx.com> writes:
On Thursday, 26 May 2016 at 15:34:50 UTC, ArturG wrote:

 On Thursday, 26 May 2016 at 15:29:52 UTC, Basile B. wrote:


 float.init is not equal to 0.0f. In D FP points values are 
 initialized to nan (not a number).

 By the way for strings it works, it's like the array case I 
 described in the first answer).


 yes i guess i tested all/most types and know that float.init is 
 float.nan but why is nan true and not false?


Oh, I'm so sorry ! I totally missed the point of the Q.

float.nan is not a "unique" value. Several values verify "nan" 
(Look at std.math.isNan). So I suppose it's simpler to test for 
nullity. Though with the sign there's also two possible 0...
May 26 2016
parent reply Basile B. <b2.temp gmx.com> writes:
On Thursday, 26 May 2016 at 15:48:18 UTC, Basile B. wrote:

 On Thursday, 26 May 2016 at 15:34:50 UTC, ArturG wrote:

 On Thursday, 26 May 2016 at 15:29:52 UTC, Basile B. wrote:


 float.init is not equal to 0.0f. In D FP points values are 
 initialized to nan (not a number).

 By the way for strings it works, it's like the array case I 
 described in the first answer).


 yes i guess i tested all/most types and know that float.init 
 is float.nan but why is nan true and not false?


 Oh, I'm so sorry ! I totally missed the point of the Q.

 float.nan is not a "unique" value. Several values verify "nan" 
 (Look at std.math.isNan). So I suppose it's simpler to test for 
 nullity. Though with the sign there's also two possible 0...


void main(string[] args)
{
     writeln(float.nan == float.init); // false
     import std.math: isNaN;
     writeln(isNaN(float.nan));  // true
     writeln(isNaN(float.init)); //true
}

So the shortcut in the compiler might be more simple, there is 
only a single test for "if(myFloat)"...
May 26 2016
parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Thursday, 26 May 2016 at 15:51:39 UTC, Basile B. wrote:


 Oh, I'm so sorry ! I totally missed the point of the Q.

 float.nan is not a "unique" value. Several values verify "nan" 
 (Look at std.math.isNan). So I suppose it's simpler to test 
 for nullity. Though with the sign there's also two possible 
 0...


 void main(string[] args)
 {
     writeln(float.nan == float.init); // false
     import std.math: isNaN;
     writeln(isNaN(float.nan));  // true
     writeln(isNaN(float.init)); //true
 }

 So the shortcut in the compiler might be more simple, there is 
 only a single test for "if(myFloat)"...


im just playing with this template[1] is there anything else i 
missed? (if you dont mind)
it basically treats any T.init as false and skips the 
function/delegate and just returns type.

[1] https://dpaste.dzfl.pl/d159d83e3167
May 26 2016
parent reply Marc =?UTF-8?B?U2Now7x0eg==?= <schuetzm gmx.net> writes:
On Thursday, 26 May 2016 at 16:45:22 UTC, ArturG wrote:

 im just playing with this template[1] is there anything else i 
 missed? (if you dont mind)
 it basically treats any T.init as false and skips the 
 function/delegate and just returns type.

 [1] https://dpaste.dzfl.pl/d159d83e3167


If you just want to check whether something is equal to its 
type's .init value, use the `is` operator, which does a bitwise 
comparison:

     if(value is typeof(value).init) ...
May 27 2016
parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Friday, 27 May 2016 at 09:25:55 UTC, Marc Schütz wrote:

 On Thursday, 26 May 2016 at 16:45:22 UTC, ArturG wrote:

 im just playing with this template[1] is there anything else i 
 missed? (if you dont mind)
 it basically treats any T.init as false and skips the 
 function/delegate and just returns type.

 [1] https://dpaste.dzfl.pl/d159d83e3167


 If you just want to check whether something is equal to its 
 type's .init value, use the `is` operator, which does a bitwise 
 comparison:

     if(value is typeof(value).init) ...


that still requiers a special case for floating points, arrays 
and optionally empty string literals.
May 27 2016
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 27 May 2016 at 14:43:47 UTC, ArturG wrote:

     if(value is typeof(value).init) ...


 that still requiers a special case for floating points, arrays 
 and optionally empty string literals.


Have you tried? That should work in all cases.
May 27 2016
next sibling parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Friday, 27 May 2016 at 14:48:59 UTC, Adam D. Ruppe wrote:

 On Friday, 27 May 2016 at 14:43:47 UTC, ArturG wrote:

     if(value is typeof(value).init) ...


 that still requiers a special case for floating points, arrays 
 and optionally empty string literals.


 Have you tried? That should work in all cases.


    float f;
    if(f is float.init) "float init".writeln;
    f = float.nan;
    if(f is float.init) "float nan".writeln;
May 27 2016
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 27 May 2016 at 14:56:28 UTC, ArturG wrote:

    float f;
    if(f is float.init) "float init".writeln;
    f = float.nan;
    if(f is float.init) "float nan".writeln;


You changed it to a value that isn't float.init, so of course it 
isn't going to match!

float.nan and float.init are NOT the same thing. float.init is a 
kind of NAN, but not the same kind.
May 27 2016
parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Friday, 27 May 2016 at 15:07:50 UTC, Adam D. Ruppe wrote:

 On Friday, 27 May 2016 at 14:56:28 UTC, ArturG wrote:

    float f;
    if(f is float.init) "float init".writeln;
    f = float.nan;
    if(f is float.init) "float nan".writeln;


 You changed it to a value that isn't float.init, so of course 
 it isn't going to match!

 float.nan and float.init are NOT the same thing. float.init is 
 a kind of NAN, but not the same kind.


yes but i have to check for that when some one does

float.nan.checkThen!((f){ this fun should not run });

which should be the same as

float.init.checkThen!((f){ this fun should not run });
May 27 2016
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 27 May 2016 at 15:19:50 UTC, ArturG wrote:

 yes but i have to check for that when some one does


Why? This is no different than if they set any of the other four 
billion possible values.
May 27 2016
parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Friday, 27 May 2016 at 15:24:18 UTC, Adam D. Ruppe wrote:

 On Friday, 27 May 2016 at 15:19:50 UTC, ArturG wrote:

 yes but i have to check for that when some one does


 Why? This is no different than if they set any of the other 
 four billion possible values.


What do you mean?

operation on float.nan gives you a float.nan so why does the 
shortcut evaluate to true and not false wouldnt that make more 
sense?


float f;
if(f) "why does this print".writeln; // it should not

same for char

char c;
if(c) "why does this print".writeln; // it should not

it works for most other types is there any reason why it doesnt 
work or couldnt work with floating points and character types?
May 27 2016
next sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 5/27/16 11:49 AM, ArturG wrote:

 On Friday, 27 May 2016 at 15:24:18 UTC, Adam D. Ruppe wrote:

 On Friday, 27 May 2016 at 15:19:50 UTC, ArturG wrote:

 yes but i have to check for that when some one does


 Why? This is no different than if they set any of the other four
 billion possible values.


 What do you mean?

 operation on float.nan gives you a float.nan so why does the shortcut
 evaluate to true and not false wouldnt that make more sense?


 float f;
 if(f) "why does this print".writeln; // it should not

 same for char

 char c;
 if(c) "why does this print".writeln; // it should not

 it works for most other types is there any reason why it doesnt work or
 couldnt work with floating points and character types?


conversion to bool is not universally (val !is val.init)

Why are you expecting it to be?

Won't work for enums with first elements that are non-zero either:

enum foo : int {
  bar = 1;
}

foo f;

if(f) writeln("this will output too");

-Steve
May 27 2016
parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Friday, 27 May 2016 at 16:56:21 UTC, Steven Schveighoffer 
wrote:

 Why are you expecting it to be?

 Won't work for enums with first elements that are non-zero 
 either:

 enum foo : int {
  bar = 1;
 }

 foo f;

 if(f) writeln("this will output too");

 -Steve



but by default it works you just changed the default so its ok 
that it doesnt work.
May 27 2016
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 5/27/16 1:42 PM, ArturG wrote:

 On Friday, 27 May 2016 at 16:56:21 UTC, Steven Schveighoffer wrote:

 Why are you expecting it to be?

 Won't work for enums with first elements that are non-zero either:

 enum foo : int {
  bar = 1;
 }

 foo f;

 if(f) writeln("this will output too");



 but by default it works you just changed the default so its ok that it
 doesnt work.


I didn't change the default. The default is to pick the first member and 
use that as the init value. I may not have even considered what foo.init 
might be when I was creating my enum.

-Steve
May 27 2016
parent ArturG <var.spool.mail700 gmail.com> writes:
On Friday, 27 May 2016 at 18:03:23 UTC, Steven Schveighoffer 
wrote:


 I didn't change the default. The default is to pick the first 
 member and use that as the init value. I may not have even 
 considered what foo.init might be when I was creating my enum.

 -Steve


by default i ment this

enum foo
{
     bar
}
foo f;
if(f) "dosnt print".writeln;

but i understand what you mean which adds a problem to my 
checkThen template, as the return type of the template depends on 
the return type of the callable which right now returns the init 
value of the callable return type if the type you pass into the 
template evaluates to false.

an example:

class Foo { int x; }
Foo foo(){ return null; }

foo.checkThen!( f => f.x = 5; ).writeln; // writes f.x.init 
because i kinda need a common return type if foo wouldnt return 
null
May 27 2016
prev sibling parent Alex Parrill <initrd.gz gmail.com> writes:
On Friday, 27 May 2016 at 15:49:16 UTC, ArturG wrote:

 On Friday, 27 May 2016 at 15:24:18 UTC, Adam D. Ruppe wrote:

 On Friday, 27 May 2016 at 15:19:50 UTC, ArturG wrote:

 yes but i have to check for that when some one does


 Why? This is no different than if they set any of the other 
 four billion possible values.


 What do you mean?

 operation on float.nan gives you a float.nan so why does the 
 shortcut evaluate to true and not false wouldnt that make more 
 sense?


NaN in IEEE 754 floating-point numbers (the floating-point number 
system most languages and processors use) is defined as a number 
with all exponent bits set and a non-zero mantissa. The mantissa 
value is the "NaN payload", and can be any value.

`is` does a binary comparison on floating-point numbers, so NaNs 
with different payloads will not be considered equal, as you have 
found out with `float.init !is float.nan`.
May 30 2016
prev sibling parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Friday, 27 May 2016 at 14:48:59 UTC, Adam D. Ruppe wrote:

 On Friday, 27 May 2016 at 14:43:47 UTC, ArturG wrote:

     if(value is typeof(value).init) ...


 that still requiers a special case for floating points, arrays 
 and optionally empty string literals.


 Have you tried? That should work in all cases.


does this count?

struct Foo
{
     int x;
     float f;
}

void main()
{
     Foo foo;
     if(foo is typeof(foo).init) "A: does'nt work".writeln;
     foo = Foo();
     if(foo is typeof(foo).init) "B: works".writeln;
}


if you remove the float from the struct both cases work or if you 
define the float inside the struct like this:

struct Foo
{
     int x;
     // float f = float.init; // does'nt work
     float f = float.nan;
}
May 30 2016
parent Marc =?UTF-8?B?U2Now7x0eg==?= <schuetzm gmx.net> writes:
On Monday, 30 May 2016 at 19:06:53 UTC, ArturG wrote:

 does this count?

 struct Foo
 {
     int x;
     float f;
 }

 void main()
 {
     Foo foo;
     if(foo is typeof(foo).init) "A: does'nt work".writeln;
     foo = Foo();
     if(foo is typeof(foo).init) "B: works".writeln;
 }


This one is a bug in DMD. It works correctly with LDC.

`Foo()` is supposed to be identical to `Foo.init`.

File here:
https://issues.dlang.org/show_bug.cgi?id=16105
May 31 2016
prev sibling parent reply ag0aep6g <anonymous example.com> writes:
On 05/26/2016 04:03 PM, ArturG wrote:

 for example:

 if(any floatingpoint.init) will be true
 if(any char.init) also true
 if("") also true

 while others are false e.g.

 string s;
 if(s) will be false
 all others are also false or did i miss any?


What does it matter?
May 26 2016
parent reply ArturG <var.spool.mail700 gmail.com> writes:
On Thursday, 26 May 2016 at 15:25:26 UTC, ag0aep6g wrote:

 On 05/26/2016 04:03 PM, ArturG wrote:

 for example:

 if(any floatingpoint.init) will be true
 if(any char.init) also true
 if("") also true

 while others are false e.g.

 string s;
 if(s) will be false
 all others are also false or did i miss any?


 What does it matter?


You would have to create special cases for them.
May 26 2016
parent ag0aep6g <anonymous example.com> writes:
On 05/26/2016 05:28 PM, ArturG wrote:

 On Thursday, 26 May 2016 at 15:25:26 UTC, ag0aep6g wrote:


[...]

 What does it matter?


 You would have to create special cases for them.


When? If you want to check if something is the .init value, compare 
against .init.
May 26 2016