Question about iterator

ruby-talk
1

dear
with the below mentioned script I would like to produce first all of the
Java courses and then the Ruby ones.
however I can't seem to get it to work
I know I can use the partition method but I'm not able to get it to
work.
can you please help me out here

require "collecties/cursus"
class Cursus_applic
  cursussen = [Cursus.new('Ruby - 1','Jan', 18.15, 10),
  Cursus.new('Ruby - 2','Piet', 18.15, 8),
  Cursus.new('Java - 1','Els', 14, 15),
  Cursus.new('Java - 2','Jan', 14, 10),
  Cursus.new('Java - 3','Piet', 18.15, 8)
  ]

  puts '5. First all Java courses then followed by the others: '
  cursus_java = cursussen.partition {|cursus|cursus_java?(true)}
  puts '6. Alle cursussen gesorteerd op cursus naam: '
  puts '7. Alle cursussen voorafgegaan met de index: '

end
and the cursus.rb is mentioned here below
class Cursus
  attr :naam, false
  attr_reader :docent
  attr :tijdstip, false
  attr_reader :aantal_cursisten
  def initialize naam, docent, tijdstip, aantal
    @naam = naam
    @docent = docent
    @tijdstip = tijdstip
    @aantal_cursisten = aantal
  end

  def overdag?
    @tijdstip < 18
  end

  def naam? cursus_naam
    start = @naam.slice(0, cursus_naam.length)
    start == cursus_naam
  end

  def to_s
    tijdstip = overdag? ? "overdag" : "\'s avonds"
    "\tDe cursus \'#{@naam}\' wordt #{tijdstip} gegeven door #{@docent}"
  end
end

thanks for your help

···

--
Posted via http://www.ruby-forum.com/.

2

Hi --

dear
with the below mentioned script I would like to produce first all of the
Java courses and then the Ruby ones.
however I can't seem to get it to work
I know I can use the partition method but I'm not able to get it to
work.
can you please help me out here

require "collecties/cursus"
class Cursus_applic
cursussen = [Cursus.new('Ruby - 1','Jan', 18.15, 10),
Cursus.new('Ruby - 2','Piet', 18.15, 8),
Cursus.new('Java - 1','Els', 14, 15),
Cursus.new('Java - 2','Jan', 14, 10),
Cursus.new('Java - 3','Piet', 18.15, 8)
]

puts '5. First all Java courses then followed by the others: '
cursus_java = cursussen.partition {|cursus|cursus_java?(true)}

You're using a non-existent method, "cursus_java?" You probably want
to do:

   java, non_java = cursussen.partition {|cursus| cursus.naam?("Java") }

or something along those lines.

puts '6. Alle cursussen gesorteerd op cursus naam: '
puts '7. Alle cursussen voorafgegaan met de index: '

end
and the cursus.rb is mentioned here below
class Cursus
attr :naam, false
attr_reader :docent
attr :tijdstip, false
attr_reader :aantal_cursisten

I believe that attr + false is the same as attr_reader -- except more
cryptic :slight_smile: It's best to stick to:

   attr_reader
   attr_writer
   attr_accessor

since the true/false parameter is not self-explanatory.

David

···

On Sat, 1 Dec 2007, Paul Private wrote:

--
Upcoming training by David A. Black/Ruby Power and Light, LLC:
    * Intro to Rails, London, UK, December 3-6 (by Skills Matter)
See http://www.rubypal.com for details and 2008 announcements!

3

I would not use partition but a custom order. It might also be helpful if you introduce an attribute for the course category. You can then use that for ordering. And it works with more than two categories as well - which you cannot say from the standard partition method.

Kind regards

  robert

···

On 01.12.2007 13:51, Paul Private wrote:

dear
with the below mentioned script I would like to produce first all of the
Java courses and then the Ruby ones.
however I can't seem to get it to work
I know I can use the partition method but I'm not able to get it to
work.
can you please help me out here

4

david thanks for your help
I have another question here perhaps you can help me out here
normally when you do a sort it's working like this

cursus =["java", "cobalt","php","ruby"]
x=cursus.sort
puts x

this is working like a charm but when I try to implement this in the
code mentioned earlier I get a error message like this
oefeningen/_cursus_applic.rb:20:in `sort': undefined method `<=>' for
#<Cursus:0x28b55f4> (NoMethodError)
  from oefeningen/_cursus_applic.rb:20

how can I solve this one?
thanks for your help

Paul
David A. Black wrote:

···

Hi --

On Sat, 1 Dec 2007, Paul Private wrote:

require "collecties/cursus"
class Cursus_applic
cursussen = [Cursus.new('Ruby - 1','Jan', 18.15, 10),
Cursus.new('Ruby - 2','Piet', 18.15, 8),
Cursus.new('Java - 1','Els', 14, 15),
Cursus.new('Java - 2','Jan', 14, 10),
Cursus.new('Java - 3','Piet', 18.15, 8)
]

puts '5. First all Java courses then followed by the others: '
cursus_java = cursussen.partition {|cursus|cursus_java?(true)}

You're using a non-existent method, "cursus_java?" You probably want
to do:

   java, non_java = cursussen.partition {|cursus| cursus.naam?("Java") }

or something along those lines.

puts '6. Alle cursussen gesorteerd op cursus naam: '
puts '7. Alle cursussen voorafgegaan met de index: '

end
and the cursus.rb is mentioned here below
class Cursus
attr :naam, false
attr_reader :docent
attr :tijdstip, false
attr_reader :aantal_cursisten

I believe that attr + false is the same as attr_reader -- except more
cryptic :slight_smile: It's best to stick to:

   attr_reader
   attr_writer
   attr_accessor

since the true/false parameter is not self-explanatory.

David

--
Posted via http://www.ruby-forum.com/\.

5

Ruby doesn't know how to compare Cursus objects. You must tell it how
by defining a <=> method, or you can use the #sort_by method with
something that understands <=> (like a string).

If you are simply sorting by alphabetical order of one attribute, then
here's a simple example...

class Animal
  attr_reader :name
  def initialize name
    @type = name
  end
end

names = %w| tiger bear monkey zebra giraffe |
zoo =
names.each { |n| zoo << Animal.new(n) }

zoo.each { |a| puts a.name }
puts "\n------------\n"
zoo_in_order = zoo.sort_by{ |a| a.name }
zoo.each { |a| puts a.name }

Todd

···

On Dec 1, 2007 10:08 AM, Paul Private <paulus4605@gmail.com> wrote:

david thanks for your help
I have another question here perhaps you can help me out here
normally when you do a sort it's working like this

cursus =["java", "cobalt","php","ruby"]
x=cursus.sort
puts x

this is working like a charm but when I try to implement this in the
code mentioned earlier I get a error message like this
oefeningen/_cursus_applic.rb:20:in `sort': undefined method `<=>' for
#<Cursus:0x28b55f4> (NoMethodError)
  from oefeningen/_cursus_applic.rb:20

how can I solve this one?
thanks for your help

6

Even consider this, simply have class Cursus inherit from another class that already implements methods you need such as .sort
Enumerable or Array might be convenient, but I didn't read all of your class Cursus closely...
In defining your class it is easy to override any inherited method, and generally a lot less work to inherit than to create everything from nothing.

Is het voor en hogeschool in het Nederlands? Polytechnische school?

···

On Dec 1, 2007, at 11:58 AM, Todd Benson wrote:

On Dec 1, 2007 10:08 AM, Paul Private <paulus4605@gmail.com> wrote:

david thanks for your help
I have another question here perhaps you can help me out here
normally when you do a sort it's working like this

cursus =["java", "cobalt","php","ruby"]
x=cursus.sort
puts x

this is working like a charm but when I try to implement this in the
code mentioned earlier I get a error message like this
oefeningen/_cursus_applic.rb:20:in `sort': undefined method `<=>' for
#<Cursus:0x28b55f4> (NoMethodError)
  from oefeningen/_cursus_applic.rb:20

how can I solve this one?
thanks for your help

Ruby doesn't know how to compare Cursus objects. You must tell it how
by defining a <=> method, or you can use the #sort_by method with
something that understands <=> (like a string).

If you are simply sorting by alphabetical order of one attribute, then
here's a simple example...

class Animal
  attr_reader :name
  def initialize name
    @type = name
  end
end

names = %w| tiger bear monkey zebra giraffe |
zoo =
names.each { |n| zoo << Animal.new(n) }

zoo.each { |a| puts a.name }
puts "\n------------\n"
zoo_in_order = zoo.sort_by{ |a| a.name }
zoo.each { |a| puts a.name }

Todd

7

when I do a sort like this it's working correctly
class Test
cursussen = ["ruby","php","c","cobalt","java"]
z=cursussen.sort { |a, b| a <=> b }
puts z

end

I don't understand why I can't get it to work when I use this
require "oefeningen/_cursus"
class Cursus_applic
  cursussen = [Cursus.new("Ruby - 1","Jan", 18.15, 10),
  Cursus.new("Ruby - 2","Piet", 18.15, 8),
  Cursus.new("Java - 1","Els", 14, 15),
  Cursus.new("Java - 2","Jan", 14, 10),
  Cursus.new("Java - 3","Piet", 18.15, 8)
  ]

  puts '5. Alle Java-cursussen, daarna alle andere cursussen: '

  Java, other = cursussen.partition {|cursus| cursus.naam?("Java") }
  puts Java, other

  puts '6. Alle cursussen gesorteerd op cursus naam: '
z=cursussen.sort { |a, b| a <=> b }
puts z
end
can you or someone help me out
thanks for your help

Paul
John Joyce wrote:

···

On Dec 1, 2007, at 11:58 AM, Todd Benson wrote:

code mentioned earlier I get a error message like this

names = %w| tiger bear monkey zebra giraffe |
zoo =
names.each { |n| zoo << Animal.new(n) }

zoo.each { |a| puts a.name }
puts "\n------------\n"
zoo_in_order = zoo.sort_by{ |a| a.name }
zoo.each { |a| puts a.name }

Todd

Even consider this, simply have class Cursus inherit from another
class that already implements methods you need such as .sort
Enumerable or Array might be convenient, but I didn't read all of
your class Cursus closely...
In defining your class it is easy to override any inherited method,
and generally a lot less work to inherit than to create everything
from nothing.

Is het voor en hogeschool in het Nederlands? Polytechnische school?

--
Posted via http://www.ruby-forum.com/\.

8

Hi --

···

On Sun, 2 Dec 2007, John Joyce wrote:

On Dec 1, 2007, at 11:58 AM, Todd Benson wrote:

On Dec 1, 2007 10:08 AM, Paul Private <paulus4605@gmail.com> wrote:

david thanks for your help
I have another question here perhaps you can help me out here
normally when you do a sort it's working like this

cursus =["java", "cobalt","php","ruby"]
x=cursus.sort
puts x

this is working like a charm but when I try to implement this in the
code mentioned earlier I get a error message like this
oefeningen/_cursus_applic.rb:20:in `sort': undefined method `<=>' for
#<Cursus:0x28b55f4> (NoMethodError)
from oefeningen/_cursus_applic.rb:20

how can I solve this one?
thanks for your help

Ruby doesn't know how to compare Cursus objects. You must tell it how
by defining a <=> method, or you can use the #sort_by method with
something that understands <=> (like a string).

If you are simply sorting by alphabetical order of one attribute, then
here's a simple example...

class Animal
attr_reader :name
def initialize name
   @type = name
end
end

names = %w| tiger bear monkey zebra giraffe |
zoo =
names.each { |n| zoo << Animal.new(n) }

zoo.each { |a| puts a.name }
puts "\n------------\n"
zoo_in_order = zoo.sort_by{ |a| a.name }
zoo.each { |a| puts a.name }

Todd

Even consider this, simply have class Cursus inherit from another class that already implements methods you need such as .sort
Enumerable or Array might be convenient, but I didn't read all of your class Cursus closely...
In defining your class it is easy to override any inherited method, and generally a lot less work to inherit than to create everything from nothing.

The problem, though, is that you don't need Cursus objects to know
about sort; you need them to know about <=>. The object that needs to
know about sort is the collection you're sorting, which is (in all
likelihood) already an array. The objects inside the collection need
to have the <=> method.

David

--
Upcoming training by David A. Black/Ruby Power and Light, LLC:
    * Intro to Rails, London, UK, December 3-6 (by Skills Matter)
See http://www.rubypal.com for details and 2008 announcements!

9

Hi --

when I do a sort like this it's working correctly
class Test
cursussen = ["ruby","php","c","cobalt","java"]
z=cursussen.sort { |a, b| a <=> b }
puts z

end

I don't understand why I can't get it to work when I use this
require "oefeningen/_cursus"
class Cursus_applic
cursussen = [Cursus.new("Ruby - 1","Jan", 18.15, 10),
Cursus.new("Ruby - 2","Piet", 18.15, 8),
Cursus.new("Java - 1","Els", 14, 15),
Cursus.new("Java - 2","Jan", 14, 10),
Cursus.new("Java - 3","Piet", 18.15, 8)
]

puts '5. Alle Java-cursussen, daarna alle andere cursussen: '

Java, other = cursussen.partition {|cursus| cursus.naam?("Java") }
puts Java, other

puts '6. Alle cursussen gesorteerd op cursus naam: '
z=cursussen.sort { |a, b| a <=> b }
puts z
end
can you or someone help me out
thanks for your help

When you do:

   a <=> b

you are actually calling a method on a, with b as argument:

   a.<=>(b)

If a doesn't have a <=> method, then that's an error. You have to
define a <=> method for the Cursus class. Typically, that method would
delegate the comparison to some property of the objects:

   def <=>(other)
     self.name <=> other.name
   end

or something like that. Then comparing two courses would be
accomplished by comparing their names.

David

···

On Sun, 2 Dec 2007, Paul Private wrote:

--
Upcoming training by David A. Black/Ruby Power and Light, LLC:
    * Intro to Rails, London, UK, December 3-6 (by Skills Matter)
See http://www.rubypal.com for details and 2008 announcements!

10

Like others have mentioned, you're calling <=> on an instance of class
Cursus, but haven't defined it for that class. I think what you mean
is...

cursussen.sort { |a, b| a.to_s <=> b.to_s }

...Cursus#to_s is only implicitly called when a string object is
required (like for puts, or when it is coerced by "string +", &c).
Otherwise you have to explicitly call it yourself.

Regards,
Jordan

···

On Dec 1, 2:05 pm, Paul Private <paulus4...@gmail.com> wrote:

when I do a sort like this it's working correctly
class Test
cursussen = ["ruby","php","c","cobalt","java"]
z=cursussen.sort { |a, b| a <=> b }
puts z

end

I don't understand why I can't get it to work when I use this
require "oefeningen/_cursus"
class Cursus_applic
  cursussen = [Cursus.new("Ruby - 1","Jan", 18.15, 10),
  Cursus.new("Ruby - 2","Piet", 18.15, 8),
  Cursus.new("Java - 1","Els", 14, 15),
  Cursus.new("Java - 2","Jan", 14, 10),
  Cursus.new("Java - 3","Piet", 18.15, 8)
  ]

  puts '5. Alle Java-cursussen, daarna alle andere cursussen: '

  Java, other = cursussen.partition {|cursus| cursus.naam?("Java") }
  puts Java, other

  puts '6. Alle cursussen gesorteerd op cursus naam: '
z=cursussen.sort { |a, b| a <=> b }
puts z
end
can you or someone help me out
thanks for your help

Paul

John Joyce wrote:
> On Dec 1, 2007, at 11:58 AM, Todd Benson wrote:

>>> code mentioned earlier I get a error message like this

>> names = %w| tiger bear monkey zebra giraffe |
>> zoo =
>> names.each { |n| zoo << Animal.new(n) }

>> zoo.each { |a| puts a.name }
>> puts "\n------------\n"
>> zoo_in_order = zoo.sort_by{ |a| a.name }
>> zoo.each { |a| puts a.name }

>> Todd

> Even consider this, simply have class Cursus inherit from another
> class that already implements methods you need such as .sort
> Enumerable or Array might be convenient, but I didn't read all of
> your class Cursus closely...
> In defining your class it is easy to override any inherited method,
> and generally a lot less work to inherit than to create everything
> from nothing.

> Is het voor en hogeschool in het Nederlands? Polytechnische school?

--
Posted viahttp://www.ruby-forum.com/.

11

THanks Jordan
this was exactly what I was looking for

Paul

Jordan Callicoat wrote:

···

On Dec 1, 2:05 pm, Paul Private <paulus4...@gmail.com> wrote:

class Cursus_applic
  puts Java, other

>> zoo.each { |a| puts a.name }
> In defining your class it is easy to override any inherited method,
> and generally a lot less work to inherit than to create everything
> from nothing.

> Is het voor en hogeschool in het Nederlands? Polytechnische school?

--
Posted viahttp://www.ruby-forum.com/.

Like others have mentioned, you're calling <=> on an instance of class
Cursus, but haven't defined it for that class. I think what you mean
is...

cursussen.sort { |a, b| a.to_s <=> b.to_s }

...Cursus#to_s is only implicitly called when a string object is
required (like for puts, or when it is coerced by "string +", &c).
Otherwise you have to explicitly call it yourself.

Regards,
Jordan

--
Posted via http://www.ruby-forum.com/\.

12

Hi Paul,

Glad to help. One more thing: it would probably look nicer to hide
that behind <=>

class Cursus
  ...
  def <=>(other)
    self.to_s <=> other.to_s
  end
end

...then you can call cursussen.sort w/o the block.

Regards,
Jordan

···

On Dec 2, 12:20 am, Paul Private <paulus4...@gmail.com> wrote:

THanks Jordan
this was exactly what I was looking for

Paul

13

Just a few picky remarks. :slight_smile: Sorting on the string representation,
while it may work, is unstable. It's better to sort based on members
because with your approach you create a dependency between to_s and
sorting which is neither necessary nor obvious.

Also, I would only implement this in the class if it is the /default/
order for instances of Cursus.

Kind regards

robert

···

2007/12/2, MonkeeSage <MonkeeSage@gmail.com>:

On Dec 2, 12:20 am, Paul Private <paulus4...@gmail.com> wrote:
> THanks Jordan
> this was exactly what I was looking for
>
> Paul

Hi Paul,

Glad to help. One more thing: it would probably look nicer to hide
that behind <=>

class Cursus
  ...
  def <=>(other)
    self.to_s <=> other.to_s
  end
end

...then you can call cursussen.sort w/o the block.

--
use.inject do |as, often| as.you_can - without end

14

I agree with you. In fact, I would have probably implemented the
courses (Cursus) as ostructs or hashes in the backend. But given to
current implementation, sorting on to_s amounts sorting on @naam,
which is, I think, what the OP was asking for. So, while it is
unstable, generally speaking; here, it amounts to a stable sort on a
(default) instance variable.

Regards,
Jordan

···

On Dec 3, 1:57 am, Robert Klemme <shortcut...@googlemail.com> wrote:

2007/12/2, MonkeeSage <MonkeeS...@gmail.com>:

> On Dec 2, 12:20 am, Paul Private <paulus4...@gmail.com> wrote:
> > THanks Jordan
> > this was exactly what I was looking for

> > Paul

> Hi Paul,

> Glad to help. One more thing: it would probably look nicer to hide
> that behind <=>

> class Cursus
> ...
> def <=>(other)
> self.to_s <=> other.to_s
> end
> end

> ...then you can call cursussen.sort w/o the block.

Just a few picky remarks. :slight_smile: Sorting on the string representation,
while it may work, is unstable. It's better to sort based on members
because with your approach you create a dependency between to_s and
sorting which is neither necessary nor obvious.

Also, I would only implement this in the class if it is the /default/
order for instances of Cursus.

Kind regards

robert

--
use.inject do |as, often| as.you_can - without end

15

Robert Klemme wrote:

Just a few picky remarks. :slight_smile: Sorting on the string representation,
while it may work, is unstable. It's better to sort based on members
because with your approach you create a dependency between to_s and
sorting which is neither necessary nor obvious.

Also, I would only implement this in the class if it is the /default/
order for instances of Cursus.

And how about performance on using to_s when sorting? Each time an object is compared for the sort, 2 string conversions has to be done and put in the GC, etc? Has anyone made benchmarks on this?

Best regards,

Jari Williamsson