How to vary sort's block?

Ruby
1

When I have an array of objects (Thing’s, say) with two properties (say
name' andamount’) I want to be able to sort that array by either of
these properties, depending on circumstances (like the value of the
variable `order’).

The script below is an example, and it works, but I don’t like it.
I there a better way to do it, without an if…else construction?

For example, would it be possible to put several {|a,b|…} blocks for
the sort in a hash with keys telling what I want to sort on?

#!/usr/bin/ruby

class Thing
attr_reader :name,:amount

def initialize(name,amount)
@name,@amount = name,amount
end

def list
puts [’’,@name,@amount].join("\t")
end
end

class Array
def byamount
self.sort { |a,b| a.amount <=> b.amount }
end
def byname
self.sort { |a,b| a.name <=> b.name }
end
end

arr = [
Thing.new(‘John’,10),
Thing.new(‘Anny’,20)
]

order = :byname

puts “sorted #{order}:”
if order == :byname
arr.sort { |a,b| a.name <=> b.name }.each { |a|
a.list
# much more code may occur here…
}
elsif order == :byamount
arr.sort { |a,b| a.amount <=> b.amount }.each { |a|
a.list
# much more code may occur here…
}
else
raise “illegal order”
end

2

Wybo D. [email protected] wrote:

puts [‘’,@name,@amount].join(“\t”)
end
arr.sort { |a,b| a.name <=> b.name }.each { |a|
end
The easiest is probably to use sort_by.

Thing = Struct.new(:name, :amount)
arr = [
Thing.new(‘John’,10),
Thing.new(‘Anny’,20),
]

p arr.sort_by {|x| x.name}
p arr.sort_by {|x| x.amount}

Btw, I would not put byamount and byname into Array. If you want to
do
something general, then I’d do this:

module Enumerable
def sort_field(*fields)
sort_by {|x| fields.map {|f| x.send f}}
end
end

Then you can do

p arr.sort_field :name
p arr.sort_field :amount

It might even be reasonable to replace the current implementation of
sort_by
with one that accepts either a list of symbols as arguments or a block.
I
think this might have been proposed already. Matz, any comment on this?
Does this sound reasonable?

Kind regards

robert
3

On Sat, 4 Feb 2006, Robert K. wrote:

The easiest is probably to use sort_by.

Thing = Struct.new(:name, :amount)
arr = [
Thing.new(‘John’,10),
Thing.new(‘Anny’,20),
]

p arr.sort_by {|x| x.name}
p arr.sort_by {|x| x.amount}

this does not allow for more complex sorts, like on name and amount,
does it?

p arr.sort_field :name
p arr.sort_field :amount

but this is really great, I can now even say:
p arr.sort_field(:name,:amount)
Thanks!

It might even be reasonable to replace the current implementation of sort_by
with one that accepts either a list of symbols as arguments or a block. I
think this might have been proposed already. Matz, any comment on this? Does
this sound reasonable?

I saw that it is also possible to give sort an argument instead of a
block:

require ‘pp’

Thing = Struct.new(:name, :amount)
arr = [
Thing.new(‘John’,80),
Thing.new(‘John’,10),
Thing.new(‘John’,30),
Thing.new(‘Anny’,20),
Thing.new(‘Anny’,10),
Thing.new(‘Anny’,30),
Thing.new(‘Anny’,15)
]

def byname(a,b) a.name <=> b.name end
def byamount(a,b) a.amount <=> b.amount end
def by_name_amount(a,b)
(a.name <=> b.name)*4 + (a.amount <=> b.amount)
end

[:byname,:byamount,:by_name_amount].each do |order|
puts “sort #{order}:”
pp arr.sort(&method(order))
end

But ri does not tell me that:

-------------------------------------------------------- Enumerable#sort
enum.sort => array
enum.sort {| a, b | block } => array

 Returns an array containing the items in _enum_ sorted, either
 according to their own +<=>+ method, or by using the results of the
 supplied block. The block should return -1, 0, or +1 depending on
 the comparison between _a_ and _b_. As of Ruby 1.8, the method
 +Enumerable#sort_by+ implements a built-in Schwartzian Transform,
 useful when key computation or comparison is expensive..

    %w(rhea kea flea).sort         #=> ["flea", "kea", "rhea"]
    (1..10).sort {|a,b| b <=> a}   #=> [10, 9, 8, 7, 6, 5, 4, 3, 2,

1]

4

On Sat, 2006-02-04 at 19:40 +0900, Wybo D. wrote:

Yes, see below:

puts ['',@name,@amount].join("\t")

end

arr = [
Thing.new(‘John’,10),
Thing.new(‘Anny’,20)
]

Replace rest of the code with:

orders = {
:byname => lambda { |a,b| a.name <=> b.name },
:byamount => lambda { |a,b| a.amount <=> b.amount }
}

order = :byname

if blk = orders[order]
puts “sorted #{order}:”
arr.sort(&blk).each { |a| a.list }
else
raise “illegal order”
end

END

This would be my first implementation I think, I like dispatch table
type stuff. Be careful what you enclose with all those blocks, though.

An alternative idea might be (replacing the code I show above):

require 'curry'   # Have to download this, see bottom

sort_proc = lambda { |msg,a,b| a.send(msg) <=> b.send(msg) }

by_name = sort_proc.curry(:name)
by_amount = sort_proc.curry(:amount)

puts "sorted name:"
arr.sort(&by_name).each { |a| a.list }

=> sorted name:

=> Anny 20

=> John 10

puts "sorted amount:"
arr.sort(&by_amount).each { |a| a.list }

=> sorted amount:

=> John 10

=> Anny 20

(You could curry on the fly of course depending on the sort criteria,
it’s reasonably low-overhead).

Does it show I’ve been searching for an opportunity to use currying
since I did that quiz entry? ;D

(Curry: http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/177343 )

5

“Wybo D.” [email protected] schrieb im Newsbeitrag
news:[email protected]…

p arr.sort_by {|x| x.name}
p arr.sort_by {|x| x.amount}

this does not allow for more complex sorts, like on name and amount,
does it?

It does: arr.sort_by {|x| [x.name, x.amount]} (depending on the
precedence
you want). I believe, if you use Struct and want to order according to
field order then you might be able to just use sort() without arguments
(alternatively sort_by {|x| x.to_a} - to_a is defined by Struct).

Thing.new(‘Anny’,20),

Returns an array containing the items in _enum_ sorted, either
according to their own +<=>+ method, or by using the results of the
supplied block. The block should return -1, 0, or +1 depending on
the comparison between _a_ and _b_. As of Ruby 1.8, the method
+Enumerable#sort_by+ implements a built-in Schwartzian Transform,
useful when key computation or comparison is expensive..

   %w(rhea kea flea).sort         #=> ["flea", "kea", "rhea"]
   (1..10).sort {|a,b| b <=> a}   #=> [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Actually ri tells you that - because it’s the block form. Your methods
are
implicitely converted to blocks. In that case I’d rather not use a
method
definition but rather do the more direct lambda / proc:

Thing::BYNAME = lambda {|a,b| a.name <=> b.name}
Thing::BYAMOUNT = lambda {|a,b| a.amount <=> b.amount}
…
arr.sort(&Thing::BYNAME)

You can as well define those constants in global scope but I felt they
are
more appropriately placed in Thing’s scope because they refer exactly to
Things.

Kind regards

robert
6

On Feb 4, 2006, at 4:40 AM, Wybo D. wrote:

When I have an array of objects (Thing’s, say) with two properties
(say
name' andamount’) I want to be able to sort that array by either of
these properties, depending on circumstances (like the value of the
variable `order’).

I would use send():

#!/usr/bin/ruby

class Thing
attr_reader :name,:amount

def initialize(name,amount)
@name,@amount = name,amount
end

def list
puts [’’,@name,@amount].join("\t")
end
end

arr = [
Thing.new(‘John’,10),
Thing.new(‘Anny’,20)
]

order = :name

puts “sorted by #{order}:”
arr.sort_by { |thing| thing.send(order) }.each { |a|
a.list
# much more code may occur here…
} rescue puts(“illegal order”)

END

Hope that helps.

James Edward G. II

7

Thanks all for the very valuable explanations and suggestions.
I finally came to the conclusion that, for my problem (to do a sort,
depending on the value of some variable `order’), sort_by is the most
versatile way. I saved an example for future reference, which may be
useful for others on the list:

#!/usr/bin/ruby

class Thing
attr_reader :name,:amount
def initialize(name,amount)
@name,@amount = name,amount
end
def list
puts [’’,@name,@amount].join("\t")
end
end

arr = [
Thing.new(‘Johnny’,20), Thing.new(‘Anny’,20),
Thing.new(‘John’,10), Thing.new(‘Anny’,10),
Thing.new(‘John’,30), Thing.new(‘Anny’,30),
]

for order in [:name, :amount]
puts “by #{order}:”
arr.sort_by{|x| x.send(order)}.each {|a| a.list}
end

order = :name
puts “by #{order}_length, #{order}, amount:”
arr.sort_by{|x| [ x.send(order).length,
x.send(order),
x.amount
]
}.each {|a| a.list}

PS. Good to see in the definition of arr that the extra comma after the
last element is not an error anymore.

8

“Wybo D.” [email protected] wrote in message
news:[email protected]…

PS. Good to see in the definition of arr that the extra comma after the
last element is not an error anymore.

Agreed. C# is the same.