Ruby's yield statement

Ruby provides a yield statement that eases the creation of iterators.

The first thing I noticed was that its behavior is different from the C# yield statement (which I knew from before).

Ruby's yield statement gives control to a user specified block from the method's body. A classic example is the Fibonacci Sequence:

class NumericSequences
   def fibo(limit)
     i = 1
     yield 1 
     yield 1 
     a = 1
     b = 1
     while (i < limit)
         t = a
         a = a + b
         b = t
         yield a
         i = i+1
     end
  end 
  ...
end

The fibo method can be used by specifying a block that will be executed each time the control reaches a yield statement. For example:

irb(main):001:0> g = NumericSequences::new
=> #<NumericSequences:0xb7cd703c>
irb(main):002:0> g.fibo(10) {|x| print "Fibonacci number: #{x}\n"}
Fibonacci number: 1
Fibonacci number: 1
Fibonacci number: 2
Fibonacci number: 3
Fibonacci number: 5
Fibonacci number: 8
Fibonacci number: 13
Fibonacci number: 21
Fibonacci number: 34
Fibonacci number: 55
Fibonacci number: 89

The following example shows the sequence of numbers of a specific row of the Pascal's triangle :

  def pascal_triangle_row(n) 
     for k in 0..n
          yield(fact(n)/(fact(k)*fact(n-k)))
     end
  end

This method can be used like this:

irb(main):001:0> g = NumericSequences::new
=> #<NumericSequences:0xb7c9103c>
irb(main):002:0> (0..8).each {|n| g.pascal_triangle_row(n) {|r| print "#{r} "}; print "\n"}
1 
1 1 
1 2 1 
1 3 3 1 
1 4 6 4 1 
1 5 10 10 5 1 
1 6 15 20 15 6 1 
1 7 21 35 35 21 7 1 
1 8 28 56 70 56 28 8 1 
=> 0..8

Methods using the yield statement can be combined. For example in order to create the sequence of Pascal's triangle read by rows we can write:

  def pascal(limit_row)
     i = 0
     while (i <= limit_row) 
       pascal_triangle_row(i){|x| yield(x)}
       i = i+1
     end 
  end

And use it the same way:

irb(main):005:0> g.pascal(10) {|x| print "#{x} "}
1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1 1 7 21 35 35 21 7 1 1 8 28 56 70 56 28 8 1 1 9 36 84 126 126 84 36 9 1 1 10 45 120 210 252 210 120 45 10 1 => nil