Re: Using return type of a predicate function as a template

[Edwin van Leeuwen via Digitalmars-d-learn]

On Thursday, 16 October 2014 at 08:18:02 UTC, Atila Neves wrote:

This works:

import std.range;

auto groupBy(alias func, R)(R values)
if (isInputRange!R)
{

alias K = typeof(func(values.front));
alias V = ElementType!R[];
V[K] grouped;
foreach(value; values) grouped[func(value)] ~= value;
return grouped;
}



Thank you, that is surprisingly straightforward :)

Edwin

Re: Using return type of a predicate function as a template

[Atila Neves via Digitalmars-d-learn]

This works:

import std.range;

auto groupBy(alias func, R)(R values)
if (isInputRange!R)
{

alias K = typeof(func(values.front));
alias V = ElementType!R[];
V[K] grouped;
foreach(value; values) grouped[func(value)] ~= value;
return grouped;
}


unittest {
  struct Test {
string a;
double b;
  }

  auto values = [Test( "a", 1 ), Test( "a", 2 ), Test( "b", 3 )];
  auto grouped = values.groupBy!(a => a.a);
  assert( grouped["a"].length == 2 );
  assert( grouped["a"][1].b == 2 );
  assert( grouped["b"].length == 1 );
  assert( grouped["b"][0].b == 3 );
}

Atila

On Thursday, 16 October 2014 at 08:04:08 UTC, Edwin van Leeuwen 
wrote:
I am trying to implement a groupBy function that groups by the 
return type of a predicate. Currently I have to define the 
returntype of the predicate for it to compile. Is there a way 
to get the return type at compile time and use it.


The code:
V[K] groupBy( alias func, K, V )( V values )
{
  V[K] grouped;
  foreach ( value ; values ) {
grouped[func( value )] ~= value;
  }
  return grouped;
}

unittest {
  struct Test {
string a;
double b;
  }

  auto values = [Test( "a", 1 ), Test( "a", 2 ), Test( "b", 3 
)];

  auto grouped = values.groupBy!( (a) => a.a, string );
  assert( grouped["a"].length == 2 );
  assert( grouped["a"][1].b == 2 );
  assert( grouped["b"].length == 1 );
  assert( grouped["b"][0].b == 3 );
}

So the above works, but I need to call it with:
values.groupBy!( (a) => a.a, string );
Ideally I would call it instead with:
values.groupBy!( (a) => a.a )
and it would infer that the template K needs to be a string, 
since that is the return type of (a) => a.a.


Cheers,

Edwin

Using return type of a predicate function as a template

[Edwin van Leeuwen via Digitalmars-d-learn]

I am trying to implement a groupBy function that groups by the 
return type of a predicate. Currently I have to define the 
returntype of the predicate for it to compile. Is there a way to 
get the return type at compile time and use it.


The code:
V[K] groupBy( alias func, K, V )( V values )
{
  V[K] grouped;
  foreach ( value ; values ) {
grouped[func( value )] ~= value;
  }
  return grouped;
}

unittest {
  struct Test {
string a;
double b;
  }

  auto values = [Test( "a", 1 ), Test( "a", 2 ), Test( "b", 3 )];
  auto grouped = values.groupBy!( (a) => a.a, string );
  assert( grouped["a"].length == 2 );
  assert( grouped["a"][1].b == 2 );
  assert( grouped["b"].length == 1 );
  assert( grouped["b"][0].b == 3 );
}

So the above works, but I need to call it with:
values.groupBy!( (a) => a.a, string );
Ideally I would call it instead with:
values.groupBy!( (a) => a.a )
and it would infer that the template K needs to be a string, 
since that is the return type of (a) => a.a.


Cheers,

Edwin