• Andre Pany (28/28) Jul 05 2018 Hi,
• Timoses (28/47) Jul 05 2018 I always find this kind of confusing as well. What kind of ranges
• Alex (19/22) Jul 05 2018 Well, no. It depends on how you define the formatting string.
• vit (12/43) Jul 05 2018 roundRobin doesn't return RandomAccessRange => chunks doesn't
• Andre Pany (4/17) Jul 05 2018 Thanks a lot for all the answers.

Andre Pany <andre s-e-a-p.de> writes:

Hi,

the purpose of this code is to generate CSV based on 3 double 
arrays.
I wonder why map cannot directly use the result of the chunks 
function.

import std.experimental.all;

void main()
{
     double[] timestamps = [1.1];
     double[] temperatures = [2.2];
     double[] pressures = [3.3];

     string content = roundRobin(timestamps, temperatures, 
pressures)
         .chunks(3)
         //.map!(c => c.array)
         .map!(c => "%.10g,%.10g,%.10g".format(c[0],c[1],c[2]))
         .join("\n");

     writeln(content);
}

The exact error message is (this line is 3 times repeated):
app1.d(12): Error: no [] operator overload for type Take!(Result)
app1.d(12): Error: no [] operator overload for type Take!(Result)
app1.d(12): Error: no [] operator overload for type Take!(Result)
C:\SAPDevelop\D\dmd2\windows\bin\..\..\src\phobos\std\algorit
m\iteration.d(499):        instantiated from here: MapResult!(__lambda1,
Chunks!(Result))
app1.d(12):        instantiated from here: map!(Chunks!(Result))

Is it correct that I need to call ".map!(c => c.array)"?

Kind regards
André

Timoses <timosesu gmail.com> writes:

On Thursday, 5 July 2018 at 09:47:32 UTC, Andre Pany wrote:
 Hi,

 the purpose of this code is to generate CSV based on 3 double 
 arrays.
 I wonder why map cannot directly use the result of the chunks 
 function.

 import std.experimental.all;

 void main()
 {
     double[] timestamps = [1.1];
     double[] temperatures = [2.2];
     double[] pressures = [3.3];

     string content = roundRobin(timestamps, temperatures, 
 pressures)
         .chunks(3)
         //.map!(c => c.array)
         .map!(c => "%.10g,%.10g,%.10g".format(c[0],c[1],c[2]))
         .join("\n");

     writeln(content);
 }

I always find this kind of confusing as well. What kind of ranges 
are returned by functions handling ranges and returning another 
range?

It is clear what type of range can be passed by looking at the 
constraint for chunks[1]:
Chunks!Source chunks(Source)(Source source, size_t chunkSize)
if (isInputRange!Source);

It would be awesome to have something like and output constraint:
out(Output; isForwardRange!Output && 
isInputRange!(ElementType!Output))

or something similar to know what the function will return..

In your case it seems like the typeof(c.front) is not a random 
access range:
     auto c = roundRobin(timestamps, temperatures, pressures)
         .chunks(3);
     pragma(msg, isRandomAccessRange!(typeof(c.front)));
     // prints false

So you could do

     auto content = roundRobin(timestamps, temperatures, pressures)
         .evenChunks(timestamps.length)
         .map!(c =>
               c.map!(a => format("%.10g", a))
                .join(",")
               )
         .join("\n");

although there might be more elegant solutions.

[1]: https://dlang.org/phobos/std_range.html#chunks

Alex <sascha.orlov gmail.com> writes:

On Thursday, 5 July 2018 at 09:47:32 UTC, Andre Pany wrote:
 Is it correct that I need to call ".map!(c => c.array)"?

 Kind regards
 André

Well, no. It depends on how you define the formatting string. 
This would also work:

´´´
import std.experimental.all;

void main()
{
     double[] timestamps = [1.1];
     double[] temperatures = [2.2];
     double[] pressures = [3.3];

     string content = roundRobin(timestamps, temperatures, 
pressures)
         .chunks(3)
         //.map!(c => c.array)
         .map!(c => "%(%g,%)".format(c))
         .join("\n");

     writeln(content);
}
´´´

vit <vit vit.vit> writes:

On Thursday, 5 July 2018 at 09:47:32 UTC, Andre Pany wrote:
 Hi,

 the purpose of this code is to generate CSV based on 3 double 
 arrays.
 I wonder why map cannot directly use the result of the chunks 
 function.

 import std.experimental.all;

 void main()
 {
     double[] timestamps = [1.1];
     double[] temperatures = [2.2];
     double[] pressures = [3.3];

     string content = roundRobin(timestamps, temperatures, 
 pressures)
         .chunks(3)
         //.map!(c => c.array)
         .map!(c => "%.10g,%.10g,%.10g".format(c[0],c[1],c[2]))
         .join("\n");

     writeln(content);
 }

 The exact error message is (this line is 3 times repeated):
 app1.d(12): Error: no [] operator overload for type 
 Take!(Result)
 app1.d(12): Error: no [] operator overload for type 
 Take!(Result)
 app1.d(12): Error: no [] operator overload for type 
 Take!(Result)
 C:\SAPDevelop\D\dmd2\windows\bin\..\..\src\phobos\std\algorit
m\iteration.d(499):        instantiated from here: MapResult!(__lambda1,
Chunks!(Result))
 app1.d(12):        instantiated from here: map!(Chunks!(Result))

 Is it correct that I need to call ".map!(c => c.array)"?

 Kind regards
 André

roundRobin doesn't return RandomAccessRange => chunks doesn't 
return range of RandomAccessRange => Error: no [] operator 
overload

try this:

     string content = roundRobin(timestamps, temperatures, 
pressures)
         .array
         .chunks(3)
         //.map!(c => c.array)
         .map!(c => "%.10g,%.10g,%.10g".format(c[0],c[1],c[2]))
         .join("\n");

Andre Pany <andre s-e-a-p.de> writes:

On Thursday, 5 July 2018 at 12:00:03 UTC, vit wrote:
 On Thursday, 5 July 2018 at 09:47:32 UTC, Andre Pany wrote:
 [...]

 roundRobin doesn't return RandomAccessRange => chunks doesn't 
 return range of RandomAccessRange => Error: no [] operator 
 overload

 try this:

     string content = roundRobin(timestamps, temperatures, 
 pressures)
         .array
         .chunks(3)
         //.map!(c => c.array)
         .map!(c => "%.10g,%.10g,%.10g".format(c[0],c[1],c[2]))
         .join("\n");

Thanks a lot for all the answers.

Kind regards
Andre