www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Specify variable type for range of associative arrays.

reply "Christopher Davies" <ferociousturtle gmail.com> writes:

IEnumerable (Range) of some type that is then conditionally 
filtered. It looks like this:

IEnumerable<Dictionary<string, string>> foo = bar;

if (baz)
{
     foo = foo.Where(d => d["key"] == value);
}

I'm trying to do the same in D. Here's what I want to do:

Range!(string[string]) records = 
csvReader!(string[string])(input, null);

if (where != "")
{
     records = filter!(r => r[where] == val)(records);
}

But Range!(string[string]) isn't right, even though that's what 
the csvReader and filter statements produce. How do I declare 
that type?

I do have a working example, but it's way too verbose (and not 
following D's indentation/bracket style... sorry about that!):

   auto gen = new Generator!(string[string])({
     auto records = csvReader!(string[string])(input, null);

     foreach (record; records) {
       yield(record);
     }
   });

   if (where != "") {
     auto prevGen = gen;
     gen = new Generator!(string[string])({
       auto records = filter!(r => r[where] == val)(prevGen);

       foreach (record; records) {
         yield(record);
       }
     });
   }

Thanks in advance!
Aug 08 2015
parent reply "Nicholas Wilson" <iamthewilsonator hotmail.com> writes:
On Sunday, 9 August 2015 at 01:29:16 UTC, Christopher Davies 
wrote:


 IEnumerable (Range) of some type that is then conditionally 
 filtered. It looks like this:

 IEnumerable<Dictionary<string, string>> foo = bar;

 if (baz)
 {
     foo = foo.Where(d => d["key"] == value);
 }

 I'm trying to do the same in D. Here's what I want to do:

 Range!(string[string]) records = 
 csvReader!(string[string])(input, null);

 if (where != "")
 {
     records = filter!(r => r[where] == val)(records);
 }


using UFCS (universal function call syntax) you would normally 
write that as:
records =records.filter!(r => r[where] == val)();
and then leveraging D's optional parentheses as:
records =records.filter!(r => r[where] == val)
This allows you to chain them along with map, filter, reduce etc. 
with ease
e.g.
     auto result = someRange.filter!(e =>e.isFooCompatible).map!(e 
=> foo(e)).map!(e => e.toBar).array;

do you care about the type of result? Not really. It's a range. 
meaning you can pass iterate over it, pass it to other algorithms.


 But Range!(string[string]) isn't right, even though that's what 
 the csvReader and filter statements produce. How do I declare 
 that type?


Type inference is your friend.
       auto foo = bar;
will work for any type that does not disallow copying ( disable 
this(this); )

To answer your question what you probably want is not
       auto records = csvReader!(string[string])(input, null);

       if (where != "")
       {
              records = records.filter!(r => r[where] == val);
       }
but:
       auto records = csvReader!(string[string])(input, null);

       if (where != "")
       {
              auto filteredRecords = records.filter!(r => r[where] 
== val);
              //do something with filteredRecords ...
       }
or just
       if (where != "")
       {
               // if you need the result exclude comment below
               // or if your operation is for side effects only
               // leave it.
              /*auto result =*/ csvReader!(string[string])(input, 
null)
                                          .filter(e => somePred(e))
                                          
.continueChainingRanges(withSomeArgs)
                                          .untilYoureDone;

       }

If you just want a copy of the filtered results

if (where != "")
       {

              auto result = csvReader!(string[string])(input, null)
                                          .filter(e => e[where] == 
val).array;
              // .array causes a separate copy of the values of 
the result of csvReader

       }

Nic
Aug 09 2015
parent reply "Chris Davies" <ferociousturtle gmail.com> writes:
On Sunday, 9 August 2015 at 12:54:39 UTC, Nicholas Wilson wrote:

 On Sunday, 9 August 2015 at 01:29:16 UTC, Christopher Davies 
 wrote:

 [...]


 using UFCS (universal function call syntax) you would normally 
 write that as:
 records =records.filter!(r => r[where] == val)();
 and then leveraging D's optional parentheses as:
 records =records.filter!(r => r[where] == val)
 This allows you to chain them along with map, filter, reduce 
 etc. with ease
 e.g.
     auto result = someRange.filter!(e 
 =>e.isFooCompatible).map!(e => foo(e)).map!(e => e.toBar).array;

 do you care about the type of result? Not really. It's a range. 
 meaning you can pass iterate over it, pass it to other 
 algorithms.


[...]


 Type inference is your friend.
       auto foo = bar;
 will work for any type that does not disallow copying ( disable 
 this(this); )

 To answer your question what you probably want is not
       auto records = csvReader!(string[string])(input, null);

       if (where != "")
       {
              records = records.filter!(r => r[where] == val);
       }
 but:
       auto records = csvReader!(string[string])(input, null);

       if (where != "")
       {
              auto filteredRecords = records.filter!(r => 
 r[where] == val);
              //do something with filteredRecords ...
       }
 or just
       if (where != "")
       {
               // if you need the result exclude comment below
               // or if your operation is for side effects only
               // leave it.
              /*auto result =*/ 
 csvReader!(string[string])(input, null)
                                          .filter(e => 
 somePred(e))
                                          
 .continueChainingRanges(withSomeArgs)
                                          .untilYoureDone;

       }

 If you just want a copy of the filtered results

 if (where != "")
       {

              auto result = csvReader!(string[string])(input, 
 null)
                                          .filter(e => e[where] 
 == val).array;
              // .array causes a separate copy of the values of 
 the result of csvReader

       }

 Nic



Thanks so much for the reply. Good to know about UFCS.

The problem is, based on user input, I am optionally filtering a 
list, possibly passing it through 0, 1, 2 or more filters based 
on their input. Each successive filter runs on either the 
original range or the result of the previous filter, if there was 
one. Then I want to run a ussr-specified computation on the final 
range... So it would be very nice to be able to reassign the 
variable after each filter. Is there no good way to do that other 
than with Generator?
Aug 09 2015
parent reply "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm gmx.net> writes:
On Sunday, 9 August 2015 at 14:23:33 UTC, Chris Davies wrote:

 The problem is, based on user input, I am optionally filtering 
 a list, possibly passing it through 0, 1, 2 or more filters 
 based on their input. Each successive filter runs on either the 
 original range or the result of the previous filter, if there 
 was one. Then I want to run a ussr-specified computation on the 
 final range... So it would be very nice to be able to reassign 
 the variable after each filter. Is there no good way to do that 
 other than with Generator?


You can use InputRange:
http://dlang.org/phobos/std_range_interfaces.html#InputRange

e.g.

     auto input = yourOriginalData.map!someTransformation;
     InputRange!string range;
     if(where != "")
         range = inputRangeObject(input.filter!(s => s == where));
     else
         range = inputRange(input);
Aug 09 2015
parent "Christopher Davies" <ferociousturtle gmail.com> writes:
On Sunday, 9 August 2015 at 14:35:23 UTC, Marc Schütz wrote:

 You can use InputRange:
 http://dlang.org/phobos/std_range_interfaces.html#InputRange

 e.g.

     auto input = yourOriginalData.map!someTransformation;
     InputRange!string range;
     if(where != "")
         range = inputRangeObject(input.filter!(s => s == 
 where));
     else
         range = inputRange(input);


This is exactly it. Thank you!
Aug 09 2015