I can use filter algorithm with my types easily.

struct A
{
	string value;
	int count;
}


void main(  string[] args )
{
	A[] aArr;
	aArr  ~= A("HTTP", 3);
	aArr  ~= A("HTTPS", 2);
	aArr  ~= A("UNKNOWN_TCP", 4);
	aArr.filter!( a => a.count == 2);

But I couldn't compile when I want to use reduce algorithm. I simply want to get the sum of "count" variables inside of A[]. 	

	auto sum = aArr.reduce!((a,b) => a.count + b.count);

The line above gives

C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(770): Error: cannot implicitly convert expression (__lambda3(result, front(_param_1))) of type int to A
C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(791): Error: template instance app.main.reduce!((a, b) => a.count + b.count).reduce!(A, A[]) error instantiating
source\app.d(363):        instantiated from here: reduce!(A[])

How can I achieve summing count variables inside A[]?

Best Regards
Kadir Erdem Demir

Ps: The problem caused by my lack of D basics I admit, the reason I can't look up references more  before ask question I am in a bit tight schedule. Sorry for my dummy questions.

On 8/02/2015 1:47 a.m., Kadir Erdem Demir wrote:
> I can use filter algorithm with my types easily.
>
> struct A
> {
>      string value;
>      int count;
> }
>
>
> void main(  string[] args )
> {
>      A[] aArr;
>      aArr  ~= A("HTTP", 3);
>      aArr  ~= A("HTTPS", 2);
>      aArr  ~= A("UNKNOWN_TCP", 4);
>      aArr.filter!( a => a.count == 2);
>
> But I couldn't compile when I want to use reduce algorithm. I simply
> want to get the sum of "count" variables inside of A[].
>
>      auto sum = aArr.reduce!((a,b) => a.count + b.count);
>
> The line above gives
>
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(770): Error:
> cannot implicitly convert expression (__lambda3(result,
> front(_param_1))) of type int to A
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(791): Error:
> template instance app.main.reduce!((a, b) => a.count +
> b.count).reduce!(A, A[]) error instantiating
> source\app.d(363):        instantiated from here: reduce!(A[])
>
> How can I achieve summing count variables inside A[]?
>
> Best Regards
> Kadir Erdem Demir
>
> Ps: The problem caused by my lack of D basics I admit, the reason I
> can't look up references more  before ask question I am in a bit tight
> schedule. Sorry for my dummy questions.

auto sum = aArr.map!`a.count`.reduce!((a,b) => a + b);

Not much difference.
I tried sum instead of reduce, but it didn't work. Wouldn't matter much as it is the same thing pretty much anyway.

> auto sum = aArr.map!`a.count`.reduce!((a,b) => a + b);

Rikki Thanks a lot. It works.

Function map!"a.count"(aArr) surprises me a little.
Because when I read std.algorithm reference: `Implements the homonym function (also known as transform)`.

Which reminds me C++ transform and it will never used for returning a element of the struct.  I expect transform to modify the elements of the range but in D it seem to me it also used traversing the elements.

How can I imagine what "map" does in my mind, because it doesn't matches with the transform concept in my mind?

Regards
Kadir Erdem

On 2015-02-07 at 13:47, Kadir Erdem Demir wrote:
>
>      auto sum = aArr.reduce!((a,b) => a.count + b.count);
>
> The line above gives
>
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(770): Error: cannot implicitly convert expression (__lambda3(result, front(_param_1))) of type int to A
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(791): Error: template instance app.main.reduce!((a, b) => a.count + b.count).reduce!(A, A[]) error instantiating
> source\app.d(363):        instantiated from here: reduce!(A[])


    // auto sum = aArr.reduce!((a,b) => a.count + b.count); // Wrong

    auto sum = reduce!((a, b) => a + b.count)(0, aArr);     // Good


See here: http://dlang.org/phobos/std_algorithm.html#.reduce

On Saturday, 7 February 2015 at 13:38:00 UTC, Kadir Erdem Demir wrote:
> How can I imagine what "map" does in my mind, because it doesn't matches with the transform concept in my mind?

You can think of map as taking a range of something (in this case, an array of A), and calling a user-supplied function on each element in that range. The user-supplied function is a function that describes how to "map" each value in the range to a result. In your case, the function defines how to map from an A to its `count` member variable (it is a function of type A->int).

All "aArr.map!`a.count`" means is that for each A in aArr, return its `count` member. map!`a.count` is some syntax sugar D has to make function calls shorter; It expands to the following:

aArr.map!((A a) {
    return a.count;
})

The main difference between `map` in D and `transform` in C++ is, I believe, twofold. First off, `transform` is eager, meaning it does as much work as possible as son as possible. On the other hand, `map` does as little work as possible as late as possible. For the following code:

iota(10).map!(n => writeln(n)).take(5).array

Only "0 1 2 3 4" will be printed, as map is lazy and will not do work it doesn't have to.

Second of all, map returns a range that is the result of applying the supplied function to each element of aArr. C++'s tranform copies the result to another user-supplied range. If you wanted the equivalent of transform in C++, you could do this:

auto result = new int[](10);
iota(10).map!(n => n + 1).copy(result)

And result will be filled with the results of map.