Re: Why do some T.init evaluate to true while others to false?
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
Re: Why do some T.init evaluate to true while others to false?
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`.
Re: Why do some T.init evaluate to true while others to false?
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;
}
Re: Why do some T.init evaluate to true while others to false?
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
Re: Why do some T.init evaluate to true while others to false?
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
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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
Re: Why do some T.init evaluate to true while others to false?
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?
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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 });
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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;
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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) ...
Re: Why do some T.init evaluate to true while others to false?
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
Re: Why do some T.init evaluate to true while others to false?
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)"...
Re: Why do some T.init evaluate to true while others to false?
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...
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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 ?
Re: Why do some T.init evaluate to true while others to false?
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?
Re: Why do some T.init evaluate to true while others to false?
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).
Re: Why do some T.init evaluate to true while others to false?
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.
Re: Why do some T.init evaluate to true while others to false?
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?
Re: Why do some T.init evaluate to true while others to false?
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
Re: Why do some T.init evaluate to true while others to false?
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 "!="
Re: Why do some T.init evaluate to true while others to false?
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"
Re: Why do some T.init evaluate to true while others to false?
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.
