Using reduce with user types
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.
Re: Using reduce with user types
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.
Re: Using reduce with user types
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
Re: Using reduce with user types
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
Re: Using reduce with user types
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.
