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x = 1 until x > 99 puts x x = x * 2 end
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It s also possible to use while and until in a single line setting, as with if and unless:
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i = 1 i = i * 2 until i > 1000 puts i
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CHAPTER 3 RUBY S BUILDING BLOCKS: DATA, EXPRESSIONS, AND FLOW CONTROL
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The value of i is doubled over and over until the result is over 1,000, at which point the loop ends.
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Code Blocks
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Code blocks have been used in several code examples in this chapter. For example:
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x = [1, 2, 3] x.each { |y| puts y }
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1 2 3
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The each method accepts a single code block as a parameter. The code block is defined within the { and } symbols, or within do and end delimiters:
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x = [1, 2, 3] x.each do |y| puts y end
The code between { and } or do and end is a code block, essentially an anonymous, nameless method or function. This code is passed to the each method that then runs the code block for each element of the array. You can write methods of your own to handle code blocks. For example:
def each_vowel(&code_block) %w{a e i o u}.each { |vowel| code_block.call(vowel) } end each_vowel { |vowel| puts vowel }
CHAPTER 3 RUBY S BUILDING BLOCKS: DATA, EXPRESSIONS, AND FLOW CONTROL
a e i o u
each_vowel accepts a code block, as designated by the ampersand (&) before the variable name code_block in the method definition. It then iterates over each vowel in the literal array %w{a e i o u} and uses the call method on code_block to execute the code block once for each vowel, passing in the vowel variable as a parameter each time.
Note Code blocks passed in this way result in objects that have many methods of their own, such as
call. Remember, almost everything in Ruby is an object!
An alternate technique is to use the yield method, which automatically detects any passed code block and passes control to it:
def each_vowel %w{a e i o u}.each { |vowel| yield vowel } end each_vowel { |vowel| puts vowel }
This example is functionally equivalent to the last, although it s less obvious what it does because you see no code block being accepted in the function definition. Which technique you choose to use is up to you.
Note Only one code block can be passed at any one time. It s not possible to accept two or more code
blocks as parameters to a method. However, code blocks may accept none, one, or more parameters themselves.
It s also possible to store code blocks within variables, using the lambda method:
print_parameter_to_screen = lambda { |x| puts x } print_parameter_to_screen.call(100)
CHAPTER 3 RUBY S BUILDING BLOCKS: DATA, EXPRESSIONS, AND FLOW CONTROL
As with accepting a code block into a method, you use the lambda object s call method to execute it, as well as to pass any parameters in.
Note The term lambda is used due to its popularity elsewhere and in other programming languages. You
can certainly continue to call them code blocks, and they are sometimes referred to as Procs.
Other Useful Building Blocks
So far in this chapter, we ve covered the primary built-in data classes of numbers, strings, arrays, and hashes. These few types of objects can get you a long way and will be used in all your programs. You ll be looking at objects more in depth in 6, but before you get that far there are a few other important points you need to look at first.
Dates and Times
A concept that s useful to represent within many computer programs is time, in the form of dates and times. Ruby provides a class called Time to handle these concepts. Internally, Time stores times as a number of microseconds since the Unix time epoch: January 1st, 1970 00:00:00 Greenwich Mean Time (GMT)/Coordinated Universal Time (UTC). This makes it easy to compare times using the standard comparison operators, such as < and >. Let s look at how to use the Time class:
puts Time.now
Tue Mar 27 00:00:00 +0100 2007
Time.now creates an instance of class Time that s set to the current time. However, because you re trying to print it to the screen, it s converted into the preceding string.
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