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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
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 ¾ 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 ...
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, 7,340061,1560,14605,291,1638,10272,45015,16689,3152,446,157,92,20090, 89,572059,268827,3362,3048,3170,3241,3504,3286,3270,3475,3939,3647, ,3044,2928,2913,2769,2752,2933,2703,4056,5501,5217,4969,13555,24995, 16,23726,20721,18802,16523,12318,4345,5810,3423,4690,7105,5739,3260, 7,303232,3329,3057,2935,3429,3326,3456,3257,3754,3192,3523,3336,3276, 89,2838,2824,2624,2807,2871,4941,6588,5625,5563,17177,27475,24377, ,21319,20851,19117,15260,5066,6708,4257,6117,10741,9427,6807,6175, 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, 396,57913,10355,4065,6290,47558,25229,22329,24058,13355,10703,70088, 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, 47,8705,3979,4726,39242,25175,14067,105235,82928,22307,49134,21742, 6,211,11565,9966,1650,86,1564,8316,54,8262,27392,25641,1751,248590, 63,4999,22466,26165,24062,16529,12409,7594,1739,132627,11670,32445, 5,21661,16234,12795,10563,4034,248590,115963,48738,28914,19259,10312, 48,3992,132627,108569,19284,2713,1209,509,218, ...
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 ¾ tip: the -w will cause Ruby to print a bit more if something bad happens
- irb – launch interactive Ruby shell ¾ can type in Ruby programs one line at a time, and watch as each line is executed irb(main):001:0> 3+ => 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
¾ ./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 ¾ Or chmod a+x filename so everyone has exec permission
- Warning: Not very portable ¾ Depends on location /usr/local/bin/ruby
#!/usr/local/bin/ruby -w print("Hello, world!\n")
CMSC 330 19
Yet More Control Statements in Ruby
unless cond then stmt-f else stmt-t end
- Same as “if not cond then stmt-t else stmt-f end”
until cond body end
- Same as “while not cond body end” until i >= n puts message i = i + 1 end
unless grade < 90 then puts "You got an A" else unless grade < 80 then puts "You got a B" end end
CMSC 330 20
Using If and Unless as Modifiers
Can write if and unless after an expression
- puts "You got an A" if grade >= 90
- puts "You got an A" unless grade < 90
Why so many control statements?
- Is this a good idea?
- Advantages? Disadvantages?
CMSC 330 21
Other Useful Control Statements
case x when 1, 3.. when 2, 6.. end
while i>n
break
next
puts message
redo
end
for i in (1..3)
puts i
end
for elt in [1, “math”, 3.4]
puts elt.to_s
end
(1..3).each {
|elt|
puts elt
IO.foreach(filename)
|x|
puts x
generates a string (cf. to_i)
code block does not need 'break' (^) CMSC 330 22
Using Ruby Control Statements
Ruby function to print all even numbers
from 1 to some given value x
def even(x) for i in (1..x) if i % 2 == 0 puts i end end end
def even(x) (1..x).each{ |i| if i % 2 == 0 puts i end } end
CMSC 330 23
Classes and Objects
Class names begin with an uppercase letter
The “new” method creates an object
- s = String.new creates a new String and makes s refer to it
Every class inherits from Object
CMSC 330 24
Everything is an Object
In Ruby, everything is in fact an object
- (-4).abs ¾ integers are instances of Fixnum
- 3 + 4 ¾ infix notation for “invoke the + method of 3 on argument 4”
- "programming".length ¾ strings are instances of String
- String.new ¾ classes are objects with a new method
- (4.13).class ¾ use the class method to get the class for an object ¾ floating point numbers are instances of Float
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
Object Class
String Class
Fixnum Class
String.new String
"CMSC 330" String
-3.30 Float
10 Fixnum
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 ¾ => ["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 ¾ 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 ¾ 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 ¾ 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 /
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 ¾ No expression substitution, fewer escaping characters
Here-documents
s = < 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) ¾ Returns a string
The to_s method returns a String representation
of a class object
CMSC 330 43
Standard Library: String
The String class has many useful methods
- s.length # length of string
- s1 == s2 # structural equality (string contents)
- s = "A line\n"; s.chomp # returns "A line" ¾ Return new string with s's contents except newline at end of line removed
- s = "A line\n"; s.chomp! ¾ Destructively removes newline from s ¾ Convention: methods ending in! modify the object ¾ Another convention: methods ending in? observe the object
- "r1\tr2\t\tr4".each("\t") { |rec| puts rec } ¾ Apply code block to each tab-separated substring CMSC 330 44
Standard Library: String (cont.)
- "hello".index("l", 0) ¾ Return index of the first occurrence of string in s, starting at n
- "hello".sub("h", "j") ¾ Replace first occurrence of "h" by "j" in string ¾ Use gsub ("global" sub) to replace all occurrences
- "r1\tr2\t\tr3".split("\t") ¾ Return array of substrings delimited by tab
Consider these three examples again
- All involve searching in a string for a certain pattern
- What if we want to find more complicated patterns? ¾ Find first occurrence of "a" or "b" ¾ Split string at tabs, spaces, and newlines
Regular Expressions!
CMSC 330 45
Object Copy vs. Reference Copy
Consider the following code
- Assume an object/reference model like Java or Ruby ¾ Or even two pointers pointing to the same structure
Which of these occur?
Object copy Reference copy
x = "groundhog" ; y = x
x "groundhog" (reference) (object)
y "groundhog"
x (reference) "groundhog" (object) y
CMSC 330 46
Object Copy vs. Reference Copy (cont.)
For
- Ruby and Java would both do a reference copy
But for
- Ruby would cause an object copy
- Unnecessary in Java since Strings are final
x = "groundhog" y = String.new(x)
x = "groundhog" ; y = x
CMSC 330 47
Physical vs. Structural Equality
Consider these cases again:
If we compare x and y, what is compared?
- The references, or the contents of the objects they point to?
If references are compared (physical equality)
the first would return false but the second true
If objects are compared both would return true
x "groundhog" (reference) (object)
y "groundhog"
x (reference) "groundhog" (object) y
CMSC 330 48
String Equality
In Java, x == y is physical equality, always
- Compares references, not string contents
In Ruby, x == y for strings uses structural equality
- Compares contents, not references
- == is a method that can be overridden in Ruby!
- To check physical equality, use the equal? method ¾ Inherited from the Object class
It’s always important to know whether you’re
doing a reference or object copy
- And physical or structural comparison
