Delete every other value in an array

Tim Conner

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May 9, 2008, 10:16:21 AM5/9/08

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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-forum.com/.

yermej

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May 9, 2008, 10:33:29 AM5/9/08

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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

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May 9, 2008, 10:34:53 AM5/9/08

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On Fri, May 9, 2008 at 11:16 PM, Tim Conner <crofty...@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

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

Raúl Gutiérrez S.

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May 9, 2008, 10:37:45 AM5/9/08

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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

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May 9, 2008, 10:39:01 AM5/9/08

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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

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May 9, 2008, 11:08:21 AM5/9/08

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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://codeforpeople.com/
--
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

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May 9, 2008, 11:11:06 AM5/9/08

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On May 9, 2008, at 8:37 AM, Raúl Gutiérrez S. wrote:
> 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
> }

are you sure? ;-)

cfp:~ > cat a.rb

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

cnt = 0
v.each do |i|

puts '---'
p :i => i

p :before => v

v.delete(0) if cnt % 2 != 0
cnt += 1

p :after => v
end

puts '==='
p v

cfp:~ > ruby a.rb
---

{:i=>"a"}