www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Why double not? (!!)

reply Ryan <clumsycodemonkey gmail.com> writes:
Why do I see double `not` operators sometimes in D code? An 
example it the last post of this thread.

http://forum.dlang.org/thread/ktlpnikvdwgbvfaamrsk forum.dlang.org


 import core.sys.windows.windows : GetConsoleCP;
 bool hasConsole = !!GetConsoleCP();


Thanks.
Nov 18 2016
parent reply Xinok <xinok live.com> writes:
On Saturday, 19 November 2016 at 03:52:02 UTC, Ryan wrote:

 Why do I see double `not` operators sometimes in D code? An 
 example it the last post of this thread.

 http://forum.dlang.org/thread/ktlpnikvdwgbvfaamrsk forum.dlang.org


 import core.sys.windows.windows : GetConsoleCP;
 bool hasConsole = !!GetConsoleCP();


 Thanks.


It's a more concise way of writing:
     GetConsoleCP() != 0

You can do this in C/C++ as well (and presumably some other 
languages).
Nov 18 2016
parent reply Era Scarecrow <rtcvb32 yahoo.com> writes:
On Saturday, 19 November 2016 at 04:54:22 UTC, Xinok wrote:

 On Saturday, 19 November 2016 at 03:52:02 UTC, Ryan wrote:

 Why do I see double `not` operators sometimes in D code? An 
 example it the last post of this thread.

 http://forum.dlang.org/thread/ktlpnikvdwgbvfaamrsk forum.dlang.org


 import core.sys.windows.windows : GetConsoleCP;
 bool hasConsole = !!GetConsoleCP();


 Thanks.


 It's a more concise way of writing:
     GetConsoleCP() != 0

 You can do this in C/C++ as well (and presumably some other 
 languages).


  Hmmm... thinking about it, it does make perfect sense. The first 
! converts it to bool, the other inverts it back to it's 
positive/negative state.

  Although it's a combination of logic I wouldn't have through of 
unless I saw it. But testing the result on any number (float, 
double or real) won't be precise and would take far longer (and 
more complicated) using another method.
Nov 18 2016
parent reply Is it possible to store different generic types? <jj_1337 live.dk> writes:
On Saturday, 19 November 2016 at 06:58:38 UTC, Era Scarecrow 
wrote:

 On Saturday, 19 November 2016 at 04:54:22 UTC, Xinok wrote:

 On Saturday, 19 November 2016 at 03:52:02 UTC, Ryan wrote:

 Why do I see double `not` operators sometimes in D code? An 
 example it the last post of this thread.

 http://forum.dlang.org/thread/ktlpnikvdwgbvfaamrsk forum.dlang.org


 import core.sys.windows.windows : GetConsoleCP;
 bool hasConsole = !!GetConsoleCP();


 Thanks.


 It's a more concise way of writing:
     GetConsoleCP() != 0

 You can do this in C/C++ as well (and presumably some other 
 languages).


  Hmmm... thinking about it, it does make perfect sense. The 
 first ! converts it to bool, the other inverts it back to it's 
 positive/negative state.

  Although it's a combination of logic I wouldn't have through 
 of unless I saw it. But testing the result on any number 
 (float, double or real) won't be precise and would take far 
 longer (and more complicated) using another method.


It's a very common practice in any language that uses 
truthy/falsey, especially seen a lot in Javascript.

Generally it's not necessary unless you want to be explicit about 
checking upon a bool.

Ex.

auto hasModel = !!view.model;

if (hasModel) {
     ...
}

Could very well just be

auto model = view.model;

if (model) {

}
Nov 18 2016
parent reply Is it possible to store different generic types? <jj_1337 live.dk> writes:
On Saturday, 19 November 2016 at 07:51:36 UTC, Is it possible to 
store different generic types? wrote:

 On Saturday, 19 November 2016 at 06:58:38 UTC, Era Scarecrow 
 wrote:

 On Saturday, 19 November 2016 at 04:54:22 UTC, Xinok wrote:

 On Saturday, 19 November 2016 at 03:52:02 UTC, Ryan wrote:

 Why do I see double `not` operators sometimes in D code? An 
 example it the last post of this thread.

 http://forum.dlang.org/thread/ktlpnikvdwgbvfaamrsk forum.dlang.org


 import core.sys.windows.windows : GetConsoleCP;
 bool hasConsole = !!GetConsoleCP();


 Thanks.


 It's a more concise way of writing:
     GetConsoleCP() != 0

 You can do this in C/C++ as well (and presumably some other 
 languages).


  Hmmm... thinking about it, it does make perfect sense. The 
 first ! converts it to bool, the other inverts it back to it's 
 positive/negative state.

  Although it's a combination of logic I wouldn't have through 
 of unless I saw it. But testing the result on any number 
 (float, double or real) won't be precise and would take far 
 longer (and more complicated) using another method.


 It's a very common practice in any language that uses 
 truthy/falsey, especially seen a lot in Javascript.

 Generally it's not necessary unless you want to be explicit 
 about checking upon a bool.

 Ex.

 auto hasModel = !!view.model;

 if (hasModel) {
     ...
 }

 Could very well just be

 auto model = view.model;

 if (model) {

 }


It's especially difficult with numbers like you did point out and 
it completely depends on languages.

Most languages have false when it's 0, null, undefined etc. and 
everything else is true.

Which means -1 would be true, 0 would be false, 1 would be true, 
0.000001 would be true, -0.000000001 would be true.
Nov 18 2016
parent reply Ryan <clumsycodemonkey gmail.com> writes:
 It's a more concise way of writing:
     GetConsoleCP() != 0

 You can do this in C/C++ as well (and presumably some other 
 languages).


  Hmmm... thinking about it, it does make perfect sense. The 
 first ! converts it to bool, the other inverts it back to 
 it's positive/negative state.






Wouldn't this just be the same as
auto hasConsole = cast(bool) GetConsoleCP(); ?

I think the GetConsoleCP() != 0 code is the clearest about your 
intentions.
Nov 19 2016
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Saturday, 19 November 2016 at 15:40:38 UTC, Ryan wrote:

 Wouldn't this just be the same as
 auto hasConsole = cast(bool) GetConsoleCP(); ?


Yes, it is in D, though the habit often comes from C where things 
are different. But people also may prefer !! for just being 
shorter and once you know the pattern, you'll see them as meaning 
the same thing.


 I think the GetConsoleCP() != 0 code is the clearest about your 
 intentions.


yeah that's my preference too. But they still mean the same thing 
so you get used to all the forms.
Nov 19 2016
parent Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Saturday, 19 November 2016 at 15:50:26 UTC, Adam D. Ruppe 
wrote:

 On Saturday, 19 November 2016 at 15:40:38 UTC, Ryan wrote:

 Wouldn't this just be the same as
 auto hasConsole = cast(bool) GetConsoleCP(); ?


 Yes, it is in D, though the habit often comes from C where 
 things are different. But people also may prefer !! for just 
 being shorter and once you know the pattern, you'll see them as 
 meaning the same thing.


Yes it is a C idiom to coerce the boolean value to be 0 or 1. 
Since C99 C has the built in type bool which will also coerce the 
value.

     bool hasConsole = (bool)GetConsoleCP();

will do. If the code is to be compiled on a pre C99 compiler then 
it is problematic as the value depend on the definition of the 
type bool. In that case you have to use the !! or the !=0 idiom. 
That's why C code still often use them because of the risk of 
being compiled on an old compiler (and Microsoft doesn't 
implement C99 anyway in MSVC, so for real portable code it's kind 
of required).
Nov 19 2016