Ruby Programming: A Deep Dive into Scripting Languages with a Focus on Ruby, Study notes of Programming Languages

Download Ruby Programming: A Deep Dive into Scripting Languages with a Focus on Ruby and more Study notes Programming Languages in PDF only on Docsity! 1 CMSC 330: Organization of Programming Languages Introduction to Ruby CMSC 330 2 Last lecture Many types of programming languages • Imperative, functional, logical, OO, scripting Many programming language attributes • Clear, orthogonal, natural… Programming language implementation • Compiled, interpreted CMSC 330 3 Introduction Ruby is an object-oriented, imperative scripting language • “I wanted a scripting language that was more powerful than Perl, and more object-oriented than Python. That's why I decided to design my own language.” • “I believe people want to express themselves when they program. They don't want to fight with the language. Programming languages must feel natural to programmers. I tried to make people enjoy programming and concentrate on the fun and creative part of programming when they use Ruby.” – Yukihiro Matsumoto (“Matz”) CMSC 330 4 Books on Ruby • Earlier version of Thomas book available on web See course web page CMSC 330 5 Applications of Scripting Languages Scripting languages have many uses • Automating system administration • Automating user tasks • Quick-and-dirty development Major application Text processing CMSC 330 6 Output from Command-Line Tool % wc * 271 674 5323 AST.c 100 392 3219 AST.h 117 1459 238788 AST.o 1874 5428 47461 AST_defs.c 1375 6307 53667 AST_defs.h 371 884 9483 AST_parent.c 810 2328 24589 AST_print.c 640 3070 33530 AST_types.h 285 846 7081 AST_utils.c 59 274 2154 AST_utils.h 50 400 28756 AST_utils.o 866 2757 25873 Makefile 270 725 5578 Makefile.am 866 2743 27320 Makefile.in 38 175 1154 alloca.c 2035 4516 47721 aloctypes.c 86 350 3286 aloctypes.h 104 1051 66848 aloctypes.o ... 2 CMSC 330 7 Climate Data for IAD in August, 2005 ================================================================================ 1 2 3 4 5 6A 6B 7 8 9 10 11 12 13 14 15 16 17 18 AVG MX 2MIN DY MAX MIN AVG DEP HDD CDD WTR SNW DPTH SPD SPD DIR MIN PSBL S-S WX SPD DR ================================================================================ 1 87 66 77 1 0 12 0.00 0.0 0 2.5 9 200 M M 7 18 12 210 2 92 67 80 4 0 15 0.00 0.0 0 3.5 10 10 M M 3 18 17 320 3 93 69 81 5 0 16 0.00 0.0 0 4.1 13 360 M M 2 18 17 360 4 95 69 82 6 0 17 0.00 0.0 0 3.6 9 310 M M 3 18 12 290 5 94 73 84 8 0 19 0.00 0.0 0 5.9 18 10 M M 3 18 25 360 6 89 70 80 4 0 15 0.02 0.0 0 5.3 20 200 M M 6 138 23 210 7 89 69 79 3 0 14 0.00 0.0 0 3.6 14 200 M M 7 1 16 210 8 86 70 78 3 0 13 0.74 0.0 0 4.4 17 150 M M 10 18 23 150 9 76 70 73 -2 0 8 0.19 0.0 0 4.1 9 90 M M 9 18 13 90 10 87 71 79 4 0 14 0.00 0.0 0 2.3 8 260 M M 8 1 10 210 ... CMSC 330 8 Raw Census 2000 Data for DC u108_S,DC,000,01,0000001,572059,72264,572059,12.6,572059,572059,572059,0,0, 0,0,572059,175306,343213,2006,14762,383,21728,14661,572059,527044,15861 7,340061,1560,14605,291,1638,10272,45015,16689,3152,446,157,92,20090,43 89,572059,268827,3362,3048,3170,3241,3504,3286,3270,3475,3939,3647,3525 ,3044,2928,2913,2769,2752,2933,2703,4056,5501,5217,4969,13555,24995,242 16,23726,20721,18802,16523,12318,4345,5810,3423,4690,7105,5739,3260,234 7,303232,3329,3057,2935,3429,3326,3456,3257,3754,3192,3523,3336,3276,29 89,2838,2824,2624,2807,2871,4941,6588,5625,5563,17177,27475,24377,22818 ,21319,20851,19117,15260,5066,6708,4257,6117,10741,9427,6807,6175,57205 9,536373,370675,115963,55603,60360,57949,129440,122518,3754,3168,22448, 9967,4638,14110,16160,165698,61049,47694,13355,71578,60875,10703,33071, 35686,7573,28113,248590,108569,47694,60875,140021,115963,58050,21654,36 396,57913,10355,4065,6290,47558,25229,22329,24058,13355,10703,70088,657 37,37112,21742,12267,9475,9723,2573,2314,760,28625,8207,7469,738,19185, 18172,1013,1233,4351,3610,741,248590,199456,94221,46274,21443,24831,479 47,8705,3979,4726,39242,25175,14067,105235,82928,22307,49134,21742,1177 6,211,11565,9966,1650,86,1564,8316,54,8262,27392,25641,1751,248590,1159 63,4999,22466,26165,24062,16529,12409,7594,1739,132627,11670,32445,2322 5,21661,16234,12795,10563,4034,248590,115963,48738,28914,19259,10312,47 48,3992,132627,108569,19284,2713,1209,509,218,125 ... CMSC 330 9 A Simple Example Let’s start with a simple Ruby program # This is a ruby program x = 37 y = x + 5 print(y) print("\n") ruby1.rb: % ruby -w ruby1.rb 42 % CMSC 330 10 Language Basics # This is a ruby program x = 37 y = x + 5 print(y) print("\n") comments begin with #, go to end of line variables need not be declared line break separates expressions (can also use “;” to be safe) no special main() function or method CMSC 330 11 Run Ruby, Run There are three ways to run a Ruby program • ruby -w filename – execute script in filename tip: the -w will cause Ruby to print a bit more if something bad happens • irb – launch interactive Ruby shell can type in Ruby programs one line at a time, and watch as each line is executed irb(main):001:0> 3+4 => 7 irb(main):002:0> print("hello\n") hello => nil CMSC 330 12 Run Ruby, Run (cont.) Suppose you want to run a Ruby script as if it were an executable ./filename # run program • The first line (“shebang”) tells the system where to find the program to interpret this text file • Must chmod u+x filename first Or chmod a+x filename so everyone has exec permission • Warning: Not very portable Depends on location /usr/local/bin/ruby #!/usr/local/bin/ruby -w print("Hello, world!\n") 5 CMSC 330 25 Objects and Classes Objects are data Classes are types (the kind of data which things are) But in Ruby, classes themselves are objects! Fixnum, Float, String, etc., (including Class), are objects of type Class ClassObject ClassString ClassFixnum StringString.new String"CMSC 330" Float-3.30 Fixnum10 CMSC 330 26 Two Cool Things to Do with Classes Since classes are objects, you can manipulate them however you like • if p then x = String else x = Time end # Time is … # another class y = x.new # creates a String or a Time, # depending upon p You can get names of all the methods of a class • Object.methods => ["send", "name", "class_eval", "object_id", "new", "autoload?", "singleton_methods", ... ] CMSC 330 27 The nil Object Ruby uses a special object nil • All uninitialized fields set to nil (@ refers to a class field) irb(main):004:0> @x => nil • Like NULL or 0 in C/C++ and null in Java nil is an object of class NilClass • It’s a singleton object – there is only one instance of it NilClass does not have a new method • nil has methods like to_s, but not other methods that don’t make sense irb(main):006:0> @x + 2 NoMethodError: undefined method `+' for nil:NilClass CMSC 330 28 What is a Program? In C/C++, a program is... • A collection of declarations and definitions • With a distinguished function definition int main(int argc, char *argv[]) { ... } • When you run a C/C++ program, it’s like the OS calls main(...) In Java, a program is... • A collection of class definitions • With a class MyClass that contains a method public static void main(String[] args) • When you run java MyClass, the main method of class MyClass is invoked CMSC 330 29 A Ruby Program is... The class Object • When the class is loaded, any expressions not in method bodies are executed def sayN(message, n) i = 0 while i < n puts message i = i + 1 end return i end x = sayN("hello", 3) puts(x) defines a method of Object invokes self.sayN invokes self.puts (part of Object) CMSC 330 30 Ruby is Dynamically Typed Recall we don’t declare types of variables • But Ruby does keep track of types at run time x = 3; x.foo NoMethodError: undefined method 'foo' for 3:Fixnum We say that Ruby is dynamically typed • Types are determined and checked at run time Compare to C, which is statically typed # Ruby x = 3 x = "foo" # gives x a # new type /* C */ int x; x = 3; x = "foo"; /* not allowed */ 6 CMSC 330 31 Types in Java and C++ Are Java and C++ statically or dynamically typed? • A little of both • Many things are checked statically Object x = new Object(); x.println(“hello”); // No such method error at compile time • But other things are checked dynamically Object o = new Object(); String s = (String) o; // No compiler warning, fails at run time // (Some Java compilers may be smart enough to warn about above cast) CMSC 330 32 Tradeoffs? More programs type check Fewer programs type check Can use objects incorrectly and not realize until execution Helps prevent some subtle errors Less work when writing code More work to do when writing code Dynamic typesStatic types CMSC 330 33 Classes and Objects in Ruby class Point def initialize(x, y) @x = x @y = y end def addX(x) @x += x end def to_s return "(" + @x.to_s + "," + @y.to_s + ")" end end p = Point.new(3, 4) p.addX(4) puts(p.to_s) constructor definition class contains method/ constructor definitions instance variables prefixed with “@” method with no arguments instantiation invoking no-arg method CMSC 330 34 Notes For Java Programmers Ruby does not support method overloading • A typical Java class might have two or more constructors • Since Ruby does not support method overloading there can only be one initialize method Ruby does issue an exception or warning if classes defines more than one initialize method • But last initialize method defined is the valid one CMSC 330 35 Classes and Objects in Ruby (cont’d) Recall classes begin with an uppercase letter inspect converts any instance to a string irb(main):033:0> p.inspect => "#<Point:0x54574 @y=4, @x=7>" Instance variables are prefixed with @ • Compare to local variables with no prefix • Cannot be accessed outside of class The to_s method can be invoked implicitly • Could have written puts(p) Like Java’s toString() methods CMSC 330 36 Inheritance Recall that every class inherits from Object class A def add(x) return x + 1 end end class B < A def add(y) return (super(y) + 1) end end b = B.new puts(b.add(3)) extend superclass invoke add method of parent 7 CMSC 330 37 super( ) in Ruby Within the body of a method • Call to super( ) acts just like a call to that original method • Except that search for method body starts in the superclass of the object that was found to contain the original method CMSC 330 38 Global Variables in Ruby Ruby has two kinds of global variables • Class variables beginning with @@ (static in Java) • Global variables across classes beginning with $ class Global @@x = 0 def Global.inc @@x = @@x + 1; $x = $x + 1 end def Global.get return @@x end end $x = 0 Global.inc $x = $x + 1 Global.inc puts(Global.get) puts($x) define a class (“singleton”) method CMSC 330 39 Special Global Variables Ruby has a bunch of global variables that are implicitly set by methods The most insidious one: $_ • Default method return, argument in many cases Example Using $_ leads to shorter programs • And confusion • It's suggested you avoid using it gets # implicitly reads input into $_ print # implicitly writes $_ CMSC 330 40 Creating Strings in Ruby Substitution in double-quoted strings with #{ } • course = "330"; msg = "Welcome to #{course}" • "It is now #{Time.new}" • The contents of #{ } may be an arbitrary expression • Can also use single-quote as delimiter No expression substitution, fewer escaping characters Here-documents s = <<END This is a text message on multiple lines and typing \\n is annoying END CMSC 330 41 Substitution in Ruby Strings Writing elt as #{elt} makes it clear that it is a variable to be evaluated, not a literal word to be printed. This is a cleaner way to express output; it builds a single string and presents it as a single argument to puts. ruby> for elt in [100,-9.6,"pickle"] | puts "#{elt}\t(#{elt.class})" | end 100 (Fixnum) -9.6 (Float) pickle (String) CMSC 330 42 Creating Strings in Ruby (cont.) Ruby also has printf and sprintf • printf("Hello, %s\n", name); • sprintf("%d: %s", count, Time.now) Returns a string The to_s method returns a String representation of a class object