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digitalmars.D.learn - A little help with Ranges

reply Merlin Diavova <md mdiavov.com> writes:
Hi all,

I'm Merlin, I'm just starting out in D and super excited.

My questions are:-

1. In a range pipeline how does one handle the event of a filter 
range returning empty?

2. How does one unwrap a single result from a range operation?


Look forward to your assistance!

Merlin
Aug 26 2021
parent reply Stefan Koch <uplink.coder googlemail.com> writes:
On Friday, 27 August 2021 at 01:51:42 UTC, Merlin Diavova wrote:

 Hi all,

 I'm Merlin, I'm just starting out in D and super excited.

 My questions are:-

 1. In a range pipeline how does one handle the event of a 
 filter range returning empty?

 2. How does one unwrap a single result from a range operation?


 Look forward to your assistance!

 Merlin


1. you don't have to handle it.
it just won't go.

2. `takeOne` put it into the search on the dlang site
Aug 26 2021
parent reply Merlin Diavova <md mdiavov.com> writes:
On Friday, 27 August 2021 at 02:10:48 UTC, Stefan Koch wrote:

 On Friday, 27 August 2021 at 01:51:42 UTC, Merlin Diavova wrote:

 Hi all,

 I'm Merlin, I'm just starting out in D and super excited.

 My questions are:-

 1. In a range pipeline how does one handle the event of a 
 filter range returning empty?

 2. How does one unwrap a single result from a range operation?


 Look forward to your assistance!

 Merlin


 1. you don't have to handle it.
 it just won't go.

 2. `takeOne` put it into the search on the dlang site


Thanks for the quick response!

Took a look at takeOne and yep that's the ticket.

What I meant about the handling an empty filter is, what if I 
want to take an alternative route if the filter returns empty?
Aug 26 2021
next sibling parent Stefan Koch <uplink.coder googlemail.com> writes:
On Friday, 27 August 2021 at 02:17:21 UTC, Merlin Diavova wrote:

 On Friday, 27 August 2021 at 02:10:48 UTC, Stefan Koch wrote:

 On Friday, 27 August 2021 at 01:51:42 UTC, Merlin Diavova 
 wrote:

 Hi all,

 I'm Merlin, I'm just starting out in D and super excited.

 My questions are:-

 1. In a range pipeline how does one handle the event of a 
 filter range returning empty?

 2. How does one unwrap a single result from a range operation?


 Look forward to your assistance!

 Merlin


 1. you don't have to handle it.
 it just won't go.

 2. `takeOne` put it into the search on the dlang site


 Thanks for the quick response!

 Took a look at takeOne and yep that's the ticket.

 What I meant about the handling an empty filter is, what if I 
 want to take an alternative route if the filter returns empty?


You check if it's by calling empty.
or do you want a default value? I am sure there is a convince 
function for that somewhere in phobos.
Aug 26 2021
prev sibling parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 8/26/21 7:17 PM, Merlin Diavova wrote:


 What I meant about the handling an empty filter is, what if I want to 
 take an alternative route if the filter returns empty?


Then the operations downstream will not produce any results. For 
example, the array will be empty below:

import std.stdio;
import std.range;
import std.algorithm;
import std.string;
import std.functional;

void main() {
   auto significantLines = stdin
                           .byLineCopy
                           .map!strip
                           .filter!(not!empty)

                           .array;

   if (significantLines.empty) {
     writeln("There were no significant lines.");

   } else {
     writefln!"The lines: %-(\n%s%)"(significantLines);
   }
}

Ali
Aug 26 2021
parent reply Merlin Diavova <md mdiavov.com> writes:
On Friday, 27 August 2021 at 04:01:19 UTC, Ali Çehreli wrote:

 On 8/26/21 7:17 PM, Merlin Diavova wrote:


 [...]


 Then the operations downstream will not produce any results. 
 For example, the array will be empty below:

 import std.stdio;
 import std.range;
 import std.algorithm;
 import std.string;
 import std.functional;

 void main() {
   auto significantLines = stdin
                           .byLineCopy
                           .map!strip
                           .filter!(not!empty)

                           .array;

   if (significantLines.empty) {
     writeln("There were no significant lines.");

   } else {
     writefln!"The lines: %-(\n%s%)"(significantLines);
   }
 }

 Ali


And there it is!

I was missing

```d
.filter!(not!empty)
```

My code now works exactly how I wanted. Thanks!
Aug 26 2021
parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 8/27/21 12:41 AM, Merlin Diavova wrote:

 On Friday, 27 August 2021 at 04:01:19 UTC, Ali Çehreli wrote:

 On 8/26/21 7:17 PM, Merlin Diavova wrote:


 [...]


 Then the operations downstream will not produce any results. For 
 example, the array will be empty below:

 import std.stdio;
 import std.range;
 import std.algorithm;
 import std.string;
 import std.functional;

 void main() {
   auto significantLines = stdin
                           .byLineCopy
                           .map!strip
                           .filter!(not!empty)
                           .filter!(line => line.front

                           .array;

   if (significantLines.empty) {
     writeln("There were no significant lines.");

   } else {
     writefln!"The lines: %-(\n%s%)"(significantLines);
   }
 }

 Ali


 
 And there it is!
 
 I was missing
 
 ```d
 .filter!(not!empty)
 ```
 
 My code now works exactly how I wanted. Thanks!


Be careful with this! `not!empty` is *only* working because you are 
using arrays (where `empty` is a UFCS function defined in std.range). 
Other ranges this will not work on. Instead, I would recommend a lambda 
(which will work with arrays too):

```d
.filter!(r => !r.empty)
```

-Steve
Aug 27 2021