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Re: list comprehension

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From:
mimosinnet
Date:
February 11, 2019 18:51
Subject:
Re: list comprehension
Message ID:
20190211185136.GB10445@mimomedia
Dear Brad,

Thanks very much for the answer. I have been playing with your examples in the
code below (and learned a lot!). Based on your insight, I would suggest these
solutions to be added to the wikipedia:
https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_programming_languages

Example1: Set.new: gather { for L { take $_ } };
Example2: Set.new: cross( K, X.grep: P(x) );

Cheers!

<--- Working code
my \L = 1..10; my \K = 1..10; my \X = 5..15;

# Example 1
my $e1 = Set.new: gather { for L { take $_ } };

# Example 2
my $s1 = Set.new: gather {
   for K -> \k {
       for X -> \x {
           if x < 8 {
               take (k,x);
           }
       }
   }
}
my $s2 = Set.new: (K X[,] X).grep: -> ( \k, \x ) { x < 8 };
my $s3 = Set.new: ( -> \x { |(-> \k { (k,x) if x < 8 } for K) } for X );
my $s4 = Set.new: gather { -> \k { -> \x { take (k,x) if x < 8; } for X } for K }
my $s5 = Set.new: cross( K, X.grep: * < 8 );

say $e1; say $s1; say $s2; say $s3; say $s4; say $s5;
<---

El Sunday, 10 de February del 2019 a les 12:05, Brad Gilbert va 
escriure:

>In
>
>     {l for l in L}
>
>The reason it is in `{}` is to create a Set from iterating over `L`.
>
>> In Python, the set-builder's braces are replaced with square brackets, parentheses, or curly braces, giving list, generator, and set objects, respectively.
>
>So in Python:
>
>    [ l for l in L ]     gives a list
>    ( l for l in L )     gives a generator
>    { l for l in L }     gives a set
>
>In Perl6 those would most likely be written as:
>
>    L.List   or   L.Array   or   L.list
>    L.Seq
>    L.Set
>
>---
>
>The way to do that is
>
>    my \L = ((1..10) xx 3).flat.pick(*).list;
>
>    set( L ) # A
>    L.Set # B
>
>    my %set is SetHash;
>    { ++%set{$_} for L }  # C
>
>    # D
>    do {
>        # add the {} syntax to create a Set (lexically)
>        my sub circumfix:«{ }» ( \L ) { L.Set };
>
>        { $_ for L } # <--
>    }
>
>Something that seems similar to me is `unique`
>
>    .say for L.unique;
>
>By that I mean, some places where you would use a Set, it makes sense
>to use `.unique` instead
>
>---
>
>As for `{(k, x) for k in K for x in X if P(x)}`
>
>The easiest one to directly translate appears to be the Scala one
>
>    my \K = 1..10;
>    my \X = 5..15;
>
>    # for (k <- K; x <- X if P(x)) yield (k,x)
>    Set.new: gather {
>        for K -> \k {
>            for X -> \x {
>                if P(x) {
>                    take (k,x);
>                }
>            }
>        }
>    }
>
>Other ways:
>
>    Set.new: (K X[,] X).grep: -> ( \k, \x ) { P(x) }
>
>    Set.new: K X[,] X.grep: &P
>
>    Set.new: K X[,] X.grep: &P
>
>    Set.new: ( -> ( \k, \x ) { (k,x) if P(x) } for K X[,] X )
>
>    Set.new: ( -> \x { |(-> \k { (k,x) if P x } for K) } for X)
>
>On Sun, Feb 10, 2019 at 10:26 AM mimosinnet <mimosinnet@gmail.com> wrote:
>>
>> Hi all,
>>
>> I wonder what would be the Perl notation for 'set-builders', as exposed
>> in this wikipedia article:
>>
>> https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_programming_languages
>>
>> This is the Python notation:
>>
>> Example 1: {l for l in L}
>> Example 2: {(k, x) for k in K for x in X if P(x)}
>>
>> This is another example in Python:
>>
>> s = {v for v in 'ABCDABCD' if v not in 'CB'}
>>
>> https://en.wikipedia.org/wiki/List_comprehension#Similar_constructs
>>
>> I have been playing with the code below. Nevertheless, I am unsure on
>> how to use the code to define a set.
>>
>> Cheers!
>>
>> <--- Code
>> #!/usr/bin/env perl6
>>
>> my @L = 1 .. 10;
>> my @K = 1 .. 10;
>> my @X = 5 .. 15;
>>
>> say "Example 1:";
>> for @L -> $l {
>>   print "$l " if $l ∈ @L;
>> }
>>
>> say "\nExample 2:";
>> for @K -> $k { for @X -> $x {
>>     print "($k, $x), " if ($k ∈ @K and $x ∈ @X and $x < 8);
>> }}
>> <---
>>
>> --
>> (≧∇≦) Mimosinnet (Linux User: #463211)

-- 
(≧∇≦) Mimosinnet (Linux User: #463211)

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