year1998 The notation ab always means call method b on receiver a but year is

Year1998 the notation ab always means call method b

This preview shows page 114 - 116 out of 517 pages.

movie.year=1998 ? The notation a.b always means “call method b on receiver a ”, but @year is the name of an instance variable, whereas year= is the name of an instance method. Self-Check 3.4.2. Suppose we delete line 12 from Figure 3.4 . What would be the result of executing’Inception’,2011).year ? Ruby would complain that the year method is undefined. 3.5 All Programming is Metaprogramming Since defining simple getters and setters for instance variables is so common, we can make the example more Ruby-like by replacing lines 6–11 with the single line attr_accessor :title and lines 12–13 with attr_accessor :year . attr_accessor is not part of the Ruby language—it’s a regular method call that defines the getters and setters on the fly. That is, attr_accessor :foo defines instance methods foo and foo= that get and set the value of instance variable @foo . The related method attr_reader defines only a getter but no setter, and vice versa for attr_writer . This is an example of metaprogramming —creating code at runtime that defines new methods. In fact, in a sense all Ruby programming is metaprogramming, since even a class definition is not a declaration as it is in Java but actually code that is executed at runtime. Given that this is true, you might wonder whether you can modify a class at runtime. In fact you can, by adding or changing instance methods or class methods, even for Ruby’s built-in classes. For example, Figure 3.6 shows a way to do time arithmetic that takes advantage of the now method in the Time class in the standard Ruby library, which returns the number of seconds since 1/1/1970. 1 # Note: returns current time as seconds since epoch 2 class Fixnum 3 def seconds ; self ; end 4 def minutes ; self * 60 ; end 5 def hours ; self * 60 * 60 ; end 6 def ago ; - self ; end 7 def from_now ; + self ; end 8 end 9 10 # => Mon Nov 07 10:18:10 -0800 2011 11 5.minutes.ago 12 # => Mon Nov 07 10:13:15 -0800 2011 13 5.minutes - 4.minutes 14 # => 60 15 3.hours.from_now
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16 # => Mon Nov 07 13:18:15 -0800 2011 Figure 3.6: Doing simple time arithmetic by reopening the Fixnum class. Unix was invented in 1970, so its designers chose to represent time as the number of seconds since midnight (GMT) 1/1/1970, sometimes called the beginning of the epoch . For convenience, a Ruby Time object responds to arithmetic operator methods by operating on this representation if possible, though internally Ruby can represent any time past or future. In this example, we reopened the Fixnum class, a core class that we met earlier, and added six new instance methods to it. Since each of the new methods also returns a fixnum, they can be nicely “chained” to write expressions like 5.minutes.ago . In fact, Rails includes a more complete version of this feature that does comprehensive time calculations. Of course, we cannot write 1.minute.ago since we only defined a method called minutes , not minute . We could define additional methods with singular names that duplicate the functionality of the methods we already have, but that’s not very DRY. Instead, we can take advantage of Ruby’s heavy- duty metaprogramming construct method_missing . If a method call cannot be found in the receiver’s
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  • Spring '19
  • Dr.Marcos

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