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ssl def threeTimes   yield   yield   yield end threeTimes { puts "Hello" } produces: Hello Hello Hello The block (the code between the braces) is associated with the call to the method threeTimes. Within this method, yield is called three times in a row. Each time, it invokes the code in the block, and a cheery greeting is printed. What makes blocks interesting, however, is that you can pass parameters to them and receive values back from them. For example, we could write a simple function that returns members of the Fibonacci series up to a certain value.

ssl [The basic Fibonacci series is a sequence of integers, starting with two 1's, in which each subsequent term is the sum of the two preceding terms. The series is sometimes used in sorting algorithms and in analyzing natural phenomena.] def fibUpTo(max)   i1, i2 = 1, 1        # parallel assignment   while i1 <= max    yield i1     i1, i2 = i2, i1+i2   end end fibUpTo(1000) { |f| print f, " " } produces: 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 In this example, the yield statement has a parameter. This value is passed to the associated block. In the definition of the block, the argument list appears between vertical bars.

ssl In this instance, the variable f receives the value passed to the yield, so the block prints successive members of the series. (This example also shows parallel assignment in action. We'll come back to this on page 75.) Although it is common to pass just one value to a block, this is not a requirement; a block may have any number of arguments. What happens if a block has a different number of parameters than are given to the yield? By a staggering coincidence, the rules we discuss under parallel assignment come into play (with a slight twist: multiple parameters passed to a yieldare converted to an array if the block has just one argument).

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