Asp Forum - Delete every other value in an array

Tim Conner

5/9/2008 2:16:00 PM

What is the best way to delete every other value in a ruby array?
e.g.
%w(a b c d e f g h i j k)

becomes => [a c e g i k]

thanks
--
Posted via http://www.ruby-....

17 Answers

yermej

5/9/2008 2:33:00 PM

On May 9, 9:16 am, Tim Conner <crofty_ja...@hotmail.com> wrote:
> What is the best way to delete every other value in a ruby array?
> e.g.
> %w(a b c d e f g h i j k)
>
> becomes => [a c e g i k]
>
> thanks
> --
> Posted viahttp://www.ruby-....

This will work, but I'm not sure if it's the best way.

a = %w(a b c d e f g h i j k)
1.upto(a.size) {|i| a.delete_at i}

Harry Kakueki

5/9/2008 2:35:00 PM

On Fri, May 9, 2008 at 11:16 PM, Tim Conner <crofty_james@hotmail.com> wrote:
> What is the best way to delete every other value in a ruby array?
> e.g.
> %w(a b c d e f g h i j k)
>
> becomes => [a c e g i k]
>
> thanks
> --
>

Here is one way.

arr = %w[a b c d e f g h i j k]
p arr.select{|x| arr.index(x) % 2 == 0}

Harry

--
A Look into Japanese Ruby List in English
http://www.kakueki.com/ruby...

RaúlGutiérrezS.rgs

5/9/2008 2:38:00 PM

On Fri, 2008-05-09 at 23:16 +0900, Tim Conner wrote:
> What is the best way to delete every other value in a ruby array?
> e.g.
> %w(a b c d e f g h i j k)
>
> becomes => [a c e g i k]

v = %w(a b c d e f g h i j k)

cnt = 0
v.each { |i|
v.delete(0) if cnt % 2 != 0
cnt += 1
}

rgs

James Gray

5/9/2008 2:39:00 PM

On May 9, 2008, at 9:16 AM, Tim Conner wrote:

> What is the best way to delete every other value in a ruby array?
> e.g.
> %w(a b c d e f g h i j k)
>
> becomes => [a c e g i k]

>> ary = ("a".."k").to_a
=> ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]
>> require "enumerator"
=> true
>> ary.enum_slice(2).map { |pair| pair.first }
=> ["a", "c", "e", "g", "i", "k"]

Hope that helps.

James Edward Gray II

ara.t.howard

5/9/2008 3:08:00 PM

On May 9, 2008, at 8:35 AM, yermej wrote:
> a = %w(a b c d e f g h i j k)
> 1.upto(a.size) {|i| a.delete_at i}

it does, but accidentally:

cfp:~ > cat a.rb

a = %w(a b c d e f g h i j k)

1.upto(a.size) do |i|
puts '---'
p :i => i
p :before => a
a.delete_at i
p :after => a
end

cfp:~ > ruby a.rb
---
{:i=>1}
{:before=>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=>["a", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=>2}
{:before=>["a", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=>["a", "c", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=>3}
{:before=>["a", "c", "e", "f", "g", "h", "i", "j", "k"]}
{:after=>["a", "c", "e", "g", "h", "i", "j", "k"]}
---
{:i=>4}
{:before=>["a", "c", "e", "g", "h", "i", "j", "k"]}
{:after=>["a", "c", "e", "g", "i", "j", "k"]}
---
{:i=>5}
{:before=>["a", "c", "e", "g", "i", "j", "k"]}
{:after=>["a", "c", "e", "g", "i", "k"]}
---
{:i=>6}
{:before=>["a", "c", "e", "g", "i", "k"]}
{:after=>["a", "c", "e", "g", "i", "k"]}
---
{:i=>7}
{:before=>["a", "c", "e", "g", "i", "k"]}
{:after=>["a", "c", "e", "g", "i", "k"]}
---
{:i=>8}
{:before=>["a", "c", "e", "g", "i", "k"]}
{:after=>["a", "c", "e", "g", "i", "k"]}
---
{:i=>9}
{:before=>["a", "c", "e", "g", "i", "k"]}
{:after=>["a", "c", "e", "g", "i", "k"]}
---
{:i=>10}
{:before=>["a", "c", "e", "g", "i", "k"]}
{:after=>["a", "c", "e", "g", "i", "k"]}
---
{:i=>11}
{:before=>["a", "c", "e", "g", "i", "k"]}
{:after=>["a", "c", "e", "g", "i", "k"]}

look carefully at what's happening for i >= 6.

the indexes map only by accident since each delete modifies the
mapping in the array (size reduced by one after each delete)

a @ http://codeforp...
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

ara.t.howard

5/9/2008 3:11:00 PM

On May 9, 2008, at 8:37 AM, Ra=FAl Guti=E9rrez S. wrote:
> v =3D %w(a b c d e f g h i j k)
>
> cnt =3D 0
> v.each { |i|
> v.delete(0) if cnt % 2 !=3D 0
> cnt +=3D 1
> }

are you sure? ;-)

cfp:~ > cat a.rb
v =3D %w(a b c d e f g h i j k)

cnt =3D 0
v.each do |i|
puts '---'
p :i =3D> i
p :before =3D> v
v.delete(0) if cnt % 2 !=3D 0
cnt +=3D 1
p :after =3D> v
end

puts '=3D=3D=3D'
p v

cfp:~ > ruby a.rb
---
{:i=3D>"a"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"b"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"c"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"d"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"e"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"f"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"g"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"h"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"i"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"j"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
---
{:i=3D>"k"}
{:before=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
{:after=3D>["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]}
=3D=3D=3D
["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]

you cannot simultaneously iterate and delete from and enumerable in =20
ruby.

a @ http://codeforp...
--
we can deny everything, except that we have the possibility of being =20
better. simply reflect on that.
h.h. the 14th dalai lama

ara.t.howard

5/9/2008 3:17:00 PM

On May 9, 2008, at 8:16 AM, Tim Conner wrote:
> What is the best way to delete every other value in a ruby array?
> e.g.
> %w(a b c d e f g h i j k)
>
> becomes => [a c e g i k]
>
> thanks
> --
> Posted via http://www.ruby-....
>

cfp:~ > cat a.rb
# the list
a = %w(a b c d e f g h i j k)

# build your index up all at once
evens = Array.new(a.size / 2){|i| i * 2}
ods = Array.new(a.size / 2){|i| i * 2 + 1}

# apply it
p a.values_at(*evens)
p a.values_at(*ods)

# apply it destructively
a.replace a.values_at(*evens)
p a

cfp:~ > ruby a.rb
["a", "c", "e", "g", "i"]
["b", "d", "f", "h", "j"]
["a", "c", "e", "g", "i"]

a @ http://codeforp...
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

Robert Dober

5/9/2008 4:27:00 PM

On Fri, May 9, 2008 at 4:16 PM, Tim Conner <crofty_james@hotmail.com> wrote:
> What is the best way to delete every other value in a ruby array?
> e.g.
> %w(a b c d e f g h i j k)
>
> becomes => [a c e g i k]

what about #values_at
x.values_at( *(0...x.size).map{|i| (i%2).zero? && i || nil}.compact) )

too bad compact does not do what *I* want ;).

HTH
Robert
>
> thanks
> --
> Posted via http://www.ruby-....
>
>

--
http://ruby-smalltalk.blo...

---
Whereof one cannot speak, thereof one must be silent.
Ludwig Wittgenstein

Robert Dober

5/9/2008 4:29:00 PM

oh I see this was part of Ara's paper on the theme ;)
BPBYT!!!

Joel VanderWerf

5/10/2008 4:46:00 PM

Tim Conner wrote:
> What is the best way to delete every other value in a ruby array?
> e.g.
> %w(a b c d e f g h i j k)
>
> becomes => [a c e g i k]
>
> thanks

This solution is less elegant and rubylike than some of the others, but
at least it is fairly portable...

a = %w(a b c d e f g h i j k)

n = (a.size/2.0).ceil
n.times do |i|
a[i] = a[2*i]
end
a.slice!(n..-1)

p a # ==> ["a", "c", "e", "g", "i", "k"]

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

comp.lang.ruby

Delete every other value in an array

Tim Conner

yermej

Harry Kakueki

RaúlGutiérrezS.rgs

James Gray

ara.t.howard

ara.t.howard

ara.t.howard

Robert Dober

Robert Dober

Joel VanderWerf

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