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so that you can double-click Ruby files to execute them.
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On Unix-related platforms, it s possible to add a shebang line as the first line of a Ruby script so that it can be executed without having to invoke the Ruby interpreter explicitly. For example:
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#!/usr/bin/ruby puts "Hello, world!"
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You can take this script, give it a simple filename such as hello (no RB extension needed), make the file executable (using chmod), and run it directly using its filename rather than having to invoke the Ruby interpreter explicitly. 10 covers this technique in more depth. More information about the shebang line specifically is available at http://en.wikipedia.org/wiki/Shebang_(Unix).
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APPENDIX A RUBY PRIMER AND REVIEW FOR DEVELOPERS
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With the normal Ruby interpreter also comes an interactive Ruby interpreter called irb. This allows you to write Ruby code in an immediate, interactive environment where the results of your code are given as soon as you type it. Here s an example irb session:
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# irb irb(main):001:0> test => nil irb(main):002:0> => 20 irb(main):003:0> => false irb(main):004:0>
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puts "test"
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10 + 10 10 == 20 exit
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irb gives you the results of methods and expressions immediately. This makes it an ideal tool for debugging or putting together quick snippets of code, and for testing concepts.
Expressions, Logic, and Flow Control
Expressions, logic, and flow control make up a significant part of any developer s tools in any programming language. This section looks at how Ruby implements them.
Basic Expressions
Ruby supports expressions in a style familiar to almost any programmer:
"a" + "b" + "c"
10 + 20 + 30
("a" * 5) + ("c" * 6)
APPENDIX A RUBY PRIMER AND REVIEW FOR DEVELOPERS
aaaaacccccc
a = 10 b = 20 a * b
You can assign the results of expressions to variables, which you can then use in other expressions. Method calls, variables, literals, brackets, and operators can all combine so long as subexpressions always feed values of the correct type into their parent expressions or provide methods that allow them to be coerced into the right types. The next section covers this topic in more depth. (Expressions are covered in depth in 3.)
Class Mismatches
Ruby is a dynamic language, but unlike Perl, objects aren t converted between different classes automatically. For example, this expression is valid in Perl:
"20" + 10
However, in Ruby, you get an error response with the same expression:
TypeError: can't convert Fixnum into String from (irb):1:in `+' from (irb):1
In Ruby, you can only use objects that are of the same class or that support automatic translation between classes (coercion) in operations with one another.
APPENDIX A RUBY PRIMER AND REVIEW FOR DEVELOPERS
However, Ruby comes with a set of methods that exist on many types of objects, which make conversion easy. For example:
"20" + 10.to_s
In this example, the number 10 is converted to a string "10" in situ with the to_s method. Consider this inverse example, where you convert the string "20" into an integer object using the to_i method before adding 10 to it:
"20".to_i + 10
Note Methods are covered in depth in s 2, 3, and 6, as well as later in this appendix.
The to_s method provided by all number classes in Ruby results in a number being converted into a String object. C and C++ programmers might recognize this concept as similar to casting. Other conversions that can take place are converting integers to floats using to_f, and vice versa with to_i. You can convert strings and numbers using to_s, to_i, and to_f. Many other classes support to_s for converting their structure and other data into a string (the Time class provides a good demonstration of this). This topic is covered in 3 in the section Converting Between Classes.
Comparison Expressions
Comparison expressions in Ruby, as in most other languages, return true or false, except that in some situations comparisons might return nil, Ruby s concept of null or nonexistence. For example:
2 == 1
false
APPENDIX A RUBY PRIMER AND REVIEW FOR DEVELOPERS
2 == 2
true
(2 == 2) && (1 == 1)
true
x = 12 x * 2 == x + 1
false
x * x == x ** 2
true
In each of the preceding examples, you test whether variables, literals, or other expressions are equal to one another using == (symbolizing is equal to ). You can check that multiple expressions result in true (logical and if x and y are true) using && (symbolizing and ). As in other languages, the concept of a logical or is symbolized by ||:
(2 == 5) || (1 == 1)
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