Return iterating object and index while iterating

array = [1,2,3,{:name => "Peter"}, "hello"]
array.each do |element| # it can be "inject", "map" or other iterators
  # How to return object "array" and position of "element"
  # also next and priviouse "element"
end

of course I can return index by array.index[element] but I am searching for more natural solution. Like proxy_owner in Rails associations

Ruby 1.8.7

What I want to output? I want to return object wich I iterating (array in my case), also number of iteration (index in case of each_with_index)next and priviouse开发者_JAVA技巧 element of iteration.

As input I have got an Array and iterator (each, map, inject etc)

---

Use Enumerable#each_cons. The following is a copy from ruby1.8.7 rdoc. It should work on ruby 1.8.7.

---

• each_cons(n) {...}
• each_cons(n)

Iterates the given block for each array of consecutive elements. If no block is given, returns an enumerator.

---

With this , you can give an array:

['a', 'b', 'c', 'd', 'e'].each_cons(3).to_a

# => ["a", "b", "c"], ["b", "c", "d"], ["c", "d", "e"]]

or do:

['a', 'b', 'c', 'd', 'e'].each_cons(3) {|previous, current, nekst|
    puts "#{previous} - #{current} - #{nekst}"
}

# => a - b - c
# => b - c - d
# => c - d - e

If you want indice,

['a', 'b', 'c', 'd', 'e'].each_cons(3).to_a.each_with_index {|(previous, current, nekst), i|
    puts "#{i + 1}. #{previous} - #{current} - #{nekst}"
}

# => 1. a - b - c
# => 2. b - c - d
# => 3. c - d - e

You can pass the array to other enumerators quite generally, for example, with inject:

['a', 'b', 'c', 'd', 'e'].each_cons(3).to_a.inject(''){|str, (previous, current, nekst)|
    str << previous+current+nekst
}

# => "abcbcdcde"

---

In ruby1.8, there is each_with_index.

If you want that on other iterators like inject, map, ..., and if you are using ruby1.9, there is Enumerator#with_index method that you can attach to various iterators.

Enumerator#with_index

---

Ruby's each_with_index functionality can be recreated easily:

ary = %w[zero one two three]
ary.zip((0 .. (ary.size - 1)).to_a).to_a # => [["zero", 0], ["one", 1], ["two", 2], ["three", 3]]

ary.zip((0 .. (ary.size - 1)).to_a).each do |a, i|
  puts "this element: #{a}"
  puts "previous element: #{ary[i - 1]}" if (i > 0)
  puts "next element: #{ary[i + 1]}" if (i < (ary.size - 1))
  puts
end
# >> this element: zero
# >> next element: one
# >> 
# >> this element: one
# >> previous element: zero
# >> next element: two
# >> 
# >> this element: two
# >> previous element: one
# >> next element: three
# >> 
# >> this element: three
# >> previous element: two
# >> 

Once you know the index for the current object you can peek into the array you're iterating over and get the previous and next values easily.

So, you could do:

module Enumerable
  def my_each_with_index
    self.zip((0 .. (self.size - 1)).to_a).each do |a, i|
      yield a, i
    end
  end
end

ary.my_each_with_index { |a,i| puts "index: #{i} element: #{a}" }
# >> index: 0 element: zero
# >> index: 1 element: one
# >> index: 2 element: two
# >> index: 3 element: three

---

Can't test, but if remember correctly:

a = [4, 3, 3, 1, 6, 6,1]
p a.enum_for(:each_with_index).inject([]){ |m,args| m<<args }
#=> [[4, 0], [3, 1], [3, 2], [1, 3], [6, 4], [6, 5], [1, 6]]

Replace inject with select, reject or whatever. With Ruby 1.9:

p a.each_with_index.inject([]){ |m,args| m<<args }

---

Edit: This is for 1.9, I saw now that your question explicitly mentions 1.8.7. I'll leave it here for reference, but if it disturbs anyone I can delete it.

sawa already pointed out Enumerator#with_index:

http://www.ruby-doc.org/core/classes/Enumerator.html#M000303

Examples:

>> [1,2,3].map.with_index { |x, i| [x,i] }
=> [[1, 0], [2, 1], [3, 2]]
>> [1,2,3].each_cons(2).with_index { |(x, y), i| p [x,y,i] }
[1, 2, 0]
[2, 3, 1]
=> nil

Also related to this question are Enumerator#next and Enumerator#peek, which will return the next object in the enumerator, with respectively without moving the internal position forward.

http://www.ruby-doc.org/core/classes/Enumerator.html#M000307

http://www.ruby-doc.org/core/classes/Enumerator.html#M000308