Question about iterator

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

Hi –

On Sat, 1 Dec 2007, Paul P. 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

On 01.12.2007 13:51, Paul P. 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

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 Dec 1, 2007 10:08 AM, Paul P. [email protected] wrote:

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

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 P. 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

On Dec 1, 2007, at 11:58 AM, Todd B. 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?

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 J. wrote:

On Dec 1, 2007, at 11:58 AM, Todd B. 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?

Hi –

On Sun, 2 Dec 2007, Paul P. wrote:

class Cursus_applic
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 Dec 1, 2:05 pm, Paul P. [email protected] 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

Hi –

On Sun, 2 Dec 2007, John J. wrote:

puts x
Ruby doesn’t know how to compare Cursus objects. You must tell it how
end

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

On Dec 2, 12:20 am, Paul P. [email protected] 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.

Regards,
Jordan

2007/12/2, MonkeeSage [email protected]:

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

On Dec 3, 1:57 am, Robert K. [email protected] wrote:

Hi Paul,

Kind regards

robert


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

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

THanks Jordan
this was exactly what I was looking for

Paul

Jordan Callicoat wrote:

On Dec 1, 2:05 pm, Paul P. [email protected] 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

Robert K. 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 W.