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digitalmars.D.learn - sort using delegate as predicate

reply "Simon =?UTF-8?B?QsO8cmdlciI=?= <simon.buerger rwth-aachen.de> writes:
The following code does not compile, because the custom predicate 
of std.algorithm.sort is a template parameter, and therefore can 
only be a function, but not a delegate. In C++, there is a 
variant of sort taking a function-object as a second (run-time) 
parameter, but phobos does not seems to have such a thing. It 
seems like a pretty common problem to me, so does anyone have a 
solution?



class Foo
{
   int[] order;

   bool cmp(int a, int b) const
   {
     return order[a] < order[b];
   }
}

void main()
{
   auto f = new Foo;
   int[] data;
   sort!(f.cmp)(data);
}
Sep 16 2014
next sibling parent Justin Whear <justin economicmodeling.com> writes:
On Tue, 16 Sep 2014 16:19:10 +0000, Simon Bürger wrote:


 The following code does not compile, because the custom predicate of
 std.algorithm.sort is a template parameter, and therefore can only be a
 function, but not a delegate. In C++, there is a variant of sort taking
 a function-object as a second (run-time) parameter, but phobos does not
 seems to have such a thing. It seems like a pretty common problem to me,
 so does anyone have a solution?


Bit of a workaround, but this should do it:

   sort!((a,b) => f.cmp(a, b))(data);
Sep 16 2014
prev sibling parent reply "H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:
On Tue, Sep 16, 2014 at 04:19:10PM +0000, via Digitalmars-d-learn wrote:

 The following code does not compile, because the custom predicate of
 std.algorithm.sort is a template parameter, and therefore can only be
 a function, but not a delegate.


What makes you think so? Template parameters certainly can be delegates.
I use that all the time.



 In C++, there is a variant of sort taking a function-object as a
 second (run-time) parameter, but phobos does not seems to have such a
 thing. It seems like a pretty common problem to me, so does anyone
 have a solution?
 
 
 
 class Foo
 {
   int[] order;
 
   bool cmp(int a, int b) const
   {
     return order[a] < order[b];
   }
 }
 
 void main()
 {
   auto f = new Foo;
   int[] data;
   sort!(f.cmp)(data);
 }


Try this:

	sort!(&f.cmp)(data);

In D, writing `f.cmp` (without the &) can in some contexts be
misinterpreted as calling the function and passing the return value to
the template parameter, which is probably why you're getting a compile
error.


T

-- 
Why ask rhetorical questions? -- JC
Sep 16 2014
parent reply "Simon =?UTF-8?B?QsO8cmdlciI=?= <simon.buerger rwth-aachen.de> writes:
On Tuesday, 16 September 2014 at 16:27:46 UTC, H. S. Teoh via 
Digitalmars-d-learn wrote:

 On Tue, Sep 16, 2014 at 04:19:10PM +0000, via 
 Digitalmars-d-learn wrote:

 The following code does not compile, because the custom 
 predicate of
 std.algorithm.sort is a template parameter, and therefore can 
 only be
 a function, but not a delegate.


 What makes you think so? Template parameters certainly can be 
 delegates.
 I use that all the time.


Well because the delegate contains a pointer to the object, which 
can not be known at compile time. In fact


 sort!(&f.cmp)(data);


fails with the error "variable f cannot be read at compile time", 
which seems reasonable to me. On the other hand


 sort!((a,b) => f.cmp(a, b))(data);


does in fact compile, so i guess problem is solved. Thanks guys.
Sep 16 2014
next sibling parent "H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:
On Tue, Sep 16, 2014 at 04:38:04PM +0000, via Digitalmars-d-learn wrote:

 On Tuesday, 16 September 2014 at 16:27:46 UTC, H. S. Teoh via
 Digitalmars-d-learn wrote:

On Tue, Sep 16, 2014 at 04:19:10PM +0000, via Digitalmars-d-learn wrote:

The following code does not compile, because the custom predicate of
std.algorithm.sort is a template parameter, and therefore can only
be a function, but not a delegate.


What makes you think so? Template parameters certainly can be
delegates.  I use that all the time.


 
 Well because the delegate contains a pointer to the object, which can
 not be known at compile time. In fact
 

sort!(&f.cmp)(data);


 
 fails with the error "variable f cannot be read at compile time",
 which seems reasonable to me. On the other hand
 

sort!((a,b) => f.cmp(a, b))(data);


 
 does in fact compile, so i guess problem is solved. Thanks guys.


Mea culpa, I confused lambdas with delegates. The two are not the same
thing. That will teach me to answer emails while rushing to get ready
for work. :-P Sorry for the noise.


T

-- 
There is no gravity. The earth sucks.
Sep 16 2014
prev sibling parent Justin Whear <justin economicmodeling.com> writes:
On Tue, 16 Sep 2014 16:38:04 +0000, Simon Bürger wrote:

 

 sort!((a,b) => f.cmp(a, b))(data);


 
 does in fact compile, so i guess problem is solved. Thanks guys.


Yes, this compiles because the lambda forms a closure over f.  In some 
respects this might even be better than the C++ as there is the 
possibility for inlining because the lambda literal is passed by alias.
Sep 16 2014