4.15.1. Příklady použití iterátorů

4.15.1. Příklady použití iterátorů
	4.15. Bloky

Podle dopisu v ML ruby-talk(51000) od Christiana Szegedy

The Ruby way (via yield, simply linked list): (A complete implemenatation for a change):

Příklad 4.1. A

class List
  def add(obj)
    @first = [@first,obj]
  end

  def each
    e = @first
    while e
      yield e[1]
      e = e[0]
    end
  end
end

l = List.new

l.add("hello")
l.add("world")

l.each do |s|
  puts s
end

The yield in the "each" function calls the block for each object of the list. The example above prints:

hello
world

Drawbacks: none.

Příklad 4.2. Example B): Traversing a data structure recursively:

class Array
  def traverse_rec
    for e in self do
      if e.kind_of? Array
        e.traverse_rec
      else
        yield e
      end
    end
  end
end

[ [1,2] [1,3,[4,[5] ],2,[3,4] ] ].traverse_rec do |el|
   puts el;
end

outputs all elementes of the tree recursively. (The tree is implemented as an array of (possible arrays of (possible ....))... )

Do something with each element of the a data structure (minor variation of example 1):

Příklad 4.3. Example C)

class List
  def add(obj)
    @first = [@first,obj]
  end

  def map
    e = @first
    while e
      e[1] = yield e[1]
    end
  end
end

l = List.new

l.add("hello")
l.add("world")

l.map do |s|
  s.length
end

Each element of the list is replaced by its length.

Do something according to a dynamic criterion (Sum up all elements of a list conditionally)

Příklad 4.4. Example D)

class List
  def add(*objs)
    objs.each do |obj|
       @first = [@first,obj]
    end
  end

  def sum_if
    e = @first
    sum = 0;
    while e
      sum += e[1] if yield e[1]
    end
  end
end

l = List.new
l.add(1,3,4,5,6,2,1,6);

l.add_if do |i|
  (i%3)==0
end

The last function sums all elements of the list which are divisible by 3. I hope it gives you some insides about the power of yield.

Regards, Christian


4.15. Bloky		4.15.2. Použití iterátoru