10-Jun-15 Ruby (Bet you can’t do this in Java!)

“Hello World” in Ruby puts "Hello World!"

Euclid’s algorithm def euclid x, y while x != y if x > y then x -= y end if y > x then y -= x end end x end puts "Enter two numbers: " STDOUT.flush x = Integer(gets) y = Integer(gets) puts "The GCD of #{x} and #{y} is #{euclid x, y}“ Enter two numbers: The GCD of 24 and 108 is 12

General principles Everything is dynamic and may be changed All values are objects (there are no “primitives”) Variables are typeless and need not be declared Ruby avoids unnecessary punctuation A statement on a line by itself needs no semicolon Multiple statements on a line are separated by semicolons A line is continued if it ends in an operator, comma, or backslash Parameters to a method don’t usually need parentheses Conditions (in if, while, etc.) usually don’t need parentheses Correct capitalization is required, not optional Convention: Multiword variables use underscores, not camelCase Convention: Standard indentation is two spaces

Numbers Numbers may be written in decimal, hexadecimal, octal, or binary Decimal: Hex: 0xCAFEBABE or 0XCAFEBABE Octal: or 0o Binary: 0b or 0B etc. Numbers larger than four bytes are automatically treated as Bignum objects For readability, numbers may contain (but not begin or end with) underscores Examples: 3_405_691_582, 0b_111_101_101 Integers may be indexed to retrieve their bits Example: 5.step(0, -1) { |i| print 6[i] } 

Other data types Strings can be singly quoted or doubly quoted Doubly quoted strings can interpolate values for #{ expression } and for variable, and allow the usual escape characters In a singly quoted string, \' is the only recognized escape character Strings are not immutable Arrays are untyped and expandable, for example [1, 2, "hi"] Ruby has hashes: { :banana  'yellow', :cherry => 'red' } Ruby has regular expressions, dates, and times Ruby has ranges, such as Ruby has symbols, which stand for themselves You can think of them as immutable strings Examples are :banana and :cherry Since they are immutable, they make good keys for hashes They are also often used to refer to methods and variables: attr :do_it

Operators Almost all the Java operators, except ++ and -- Ruby uses :: to mean the same as. in Java Ruby uses =~ and !~ for Perl-style regular expressions ** is the exponentiation operator + is used to concatenate strings There is a to_s method for converting other things to strings.. is an inclusive range,... is an exclusive range So 1..4 and both mean 1, 2, 3, 4 0..k is a range object, but [0..k] is an array with one element, a range defined? is a prefix operator that returns a true value or nil Ruby has both !, &&, || and also not, and, or Precedence is ! > && > ||, but not > and == or

Assignment and alias statements Assignment statements use =, +=, *=, etc. You can have multiple assignment: x, y = y, x swaps the values of x and y Ruby does not have the ++ and -- operators You can create a new name for a method, operator, global variable, or regular expression backreference Syntax: alias new_name original_name Example: alias display puts

if statements Multi-line version: if condition then code elsif condition then code else code end The “ then ” is optional at the end of a line Single-line versions: if condition then code elsif condition then code else code end “ then ” is required to separate condition from code statement if condition

unless statements “ unless ” means “if not” Multi-line version: unless condition then code else code end The “ then ” is optional at the end of a line Single-line versions: unless condition then code else code end “ then ” is required to separate condition from code statement unless condition

case and === case expr_1 when expr_2 then code when expr_3,..., expr_n then code else code end Cases do not fall through; no break is needed Comparisons use expr_n === expr_1 and not the other way around expr_n === expr_1 has many meanings Simple equality test expr_1 =~ expr_n expr_n.kindof? expr_1 expr_n.include? expr_1 The === operator has the misleading name “case equality operator”

Loops in Ruby Ruby has several loops while condition do statements end begin statements end while condition until condition statements end begin statements end until condition for variable in range do statements end loop do statements end statement while condition statement until condition loop { statements } However, loops are not used as often in Ruby as in other languages Instead, Ruby programmers use iterator methods

Loop controls break gets you out of loops Within a loop, next jumps to just before the loop test Within a block, next exits the block with nil retry restarts the loop body after reevaluating the condition or getting the next iterated element Within a loop, redo restarts the loop body, but does not reevaluate the condition or get the next iterated element Within a block, redo restarts the yield or call

Exceptions raise message raises a RuntimeError raise exception raises an error of the given type raise exception, message rescue rescue ExceptionType,..., ExceptionType rescue ExceptionType,..., ExceptionType => variable Catches exceptions within a method, or within a begin...end block, or can be used as a statement suffix Example on next slide

Blocks A block is a set of statements enclosed by begin..end, by do..end, or by {..} begin introduces a new scope; variables seen for the first time inside a begin..end block are local to that scope do..end and {..} are interchangeable Any block or method can include rescue or rescue/ensure begin...rescue...ensure...end is like Java’s try...catch...finally Example: begin code rescue exception code ensure code end

Defining methods Defining an instance method: def method_name ( parameter1,..., parameterN ) statements end Defining a class method: def Class_name. method_name ( parameter1,..., parameterN ) statements end Parentheses are optional A method may contain explicit return and/or return value statements A method may return multiple values: return x, 2*x, x**2 A method which reaches the end returns the value of the last statement executed (often nil ) A method may also have a block as an (invisible) parameter The block is not mentioned in the def line, but is executed when the method evaluates the yield statement

Defining classes class Employee < Person # ‘<‘ means “extends” = 0 # class variable attr_reader :name # creates a getter def initialize(name) # this is the = name += 1 end end secretary = Employee.new("William")

Calling methods Methods may be called as in Java puts("x = #{x}") Parentheses are usually unnecessary puts "x = #{x}" All values are objects, and may have methods x = y.abs n = "abcdefg".length A method may be passed a block as an “invisible” parameter def twice n yield 2 * n # uses the block 99 end p = twice(7) { |y| puts "y = #{y}" } puts "p = #{p}" y = 14 p = 99

Iterators An iterator returns values one at a time The syntax is object. iterator { | value | statement } or object. iterator do | value | statements end The object is typically an array, a range, or a hash, but it can be any object with a coroutine

Iterators In Ruby, loops are considered low-level, to be used only when there is no appropriate iterator collection.each – step through every element n.times – do a block n times n.downto( limit ) – step from n down to and including limit n.upto( limit ) – step from n up to and including limit string.each_line – get each line from a string string.each_char – get each character (as an integer) from a string

Example use of an iterator a = [1, 1, 2, 3, 5, 8, 13] a.each { |i| print " #{i}" } Output: a.each do |i| print " #{i}" end Output: In the above, each is a method A block is a chunk of code enclosed by {...} or by do...end By convention, braces are used for single-line blocks, do...end for multi-line blocks

String#each_char In a previous slide, I had: string.each_char – get each character (as an integer) from a string This method is described in the documentation, but doesn't seem to exist There is a method String#each_byte, however, that does what the above description says each_char does If we really want to get each character, we can do that class String def each_char each_byte { |b| yield b.chr } end end

Blocks again A block is a chunk of code that can be passed as a parameter to a method A block isn’t a statement—it can’t be used alone It’s passed as an “invisible” parameter, and executed with the yield statement

Simplest use of yield def three_times puts " three_times“ yield yield yield end three_times { puts "hello" } three_times hello hello hello

My version of loop def my_loop yield while true end a = [1, 2, 3, 5, 8, 13, "hi"] my_loop do break if a.empty? print a.pop end puts "Done" hi Done

Fibonacci numbers def fibonacci_upto n i1, i2 = 1, 1 while i1 < n yield i1 i1, i2 = i2, i1 + i2 end end fibonacci_upto(100) { |f| print " ", f }

Passing a parameter to the block def count_to n puts " count_to #{n}" for i in 1..n yield end end count_to 3 { puts "hello" } count_to 3 hello hello hello

Returning a value from a coroutine def count_to_3 puts " count_to_3" yield 1 yield 2 yield 3 end count_to_3 { |result| puts result } count_to_

Regular expressions Ruby has regular expressions, almost identical to the way they are done in Perl Example: hamlet = "The slings and arrows of outrageous fortune." hamlet.scan(/w+/) ["The", "slings", "and", "arrows", "of", "outrageous", "fortune"]

Arrays An array literal can be written with brackets and commas a = [1, 1, 2, 3, 5, 8, 13, "hi"] Arrays are zero based: a[2] == 2 Arrays can be expanded a = a + [21, 34] p a [1, 2, 3, 5, 8, 13, "hi", 21, 34] Arrays can be treated as stacks, with a.push(v) and v = a.pop The join( string ) method creates a string of the elements of the array, with the given string between each pair of elements You can take slices of arrays, sort them, find unique elements, perform set operations, transpose 2-dimensional arrays, etc. Before you write methods to manipulate arrays, you should look to see whether the method you want has already been written

Hashes A hash (hash table) literal can be written with braces, commas, and the => arrow h = {:apple => :red, :banana => :yellow, :cherry => :red} Element access is similar to that for arrays: h[:banana] :yellow h[:apple] = :green p h {:banana=>:yellow, :cherry=>:red, :apple=>:green} You can use any types for keys and values, but the characteristics of symbols make them especially useful as keys

Adding and removing methods def adds a method; undef removes a method The only parameter to undef is the method name To add an instance method, first “open” the class Example: class String; def nchars; length; end; end There are several ways to add a class method to a class def Person.species; 'human'; end Within the Person class, you can use def self.species You can add a method to individual objects def oscar.mood; 'grouchy' ; end

Attributes (instance variables) Attributes (instance variables) of an object are written with @favorite_language By default, attributes are private You can write getters: def end You can write setters: def = new_name end When you define the setter, there is no space before the = in the header When you call the setter, you can use a space: teacher.name = "Dave" Yes, we are calling the method name= !

Shorthand for getters and setters Writing code for routine getters and setters is tedious, so in Ruby we don’t have to do it Shorthand for creating getters: attr_reader :name, :age, :hobbies Note the use of symbols, not variables or strings Shorthand for creating setters: attr_writer :name, :hobbies Shorthand for creating both at once: attr_accessor :name, :favorite_language By the way, these aren’t special Ruby syntax; they are methods that write the getters and setters for you Ruby uses lots of metaprogramming: programs that write programs

Access controls Public methods can be called from anywhere Protected methods can be called only within the class and its subclasses Private methods cannot be called with an explicit receiver, only with an implicit self In Ruby, methods are public by default The functions public, protected, and private can be called with or without arguments With arguments, they set the access of the named methods Example: private :dump, :swear With no arguments, they set the default access for all subsequent methods

eval eval executes a string Example: eval "puts x + 2" Here’s how you don’t want to use this: eval gets This can be a serious security risk Here’s what Ruby does about this: All data that comes from the outside world, and all data derived from that data, can automatically be marked as tainted Ruby has five $SAFE levels, each of which has a long list of things you cannot do with a tainted object

printf and friends printf format_string, value, …, value Formats are % length code for most things, % length. fractional_digits code for floats %d  decimal, %o  octal, %x  hex, %b  binary, %f  float, %s  string Negative lengths mean left justified Various other controls Example: printf "pi = %8.4f",  pi.=… The (equivalent) methods sprintf and format take the same parameters as printf, but return the resultant string rather than printing it

Some File < IO methods gets – get a line of text getc – get a character of text (as ASCII; use.chr ) ungetc – put back a character pos – the current character position in the input stream lineno – the number of times gets has been called pos= – move to the given position in the file rewind – move to the beginning of the file readlines – read the stream as an array of strings write( string ), print( string ), <<( string ) – write at the current position eof? – test if at the end of file closed? – test if the file has been closed

Some File methods rename( oldname, newname ) – rename a file read( filename ) – read the entire file as a single string readlines( filename ) – read the entire file as an array of strings open( filename, mode ) – with no block, a synonym for File.new with a block, the file is passed to the block, and automatically closed when the block finishes Modes: "r" read, "r+" read and write, "w" write, "a" append exists?( filename ) – test if a file with that name exists writable?( filename ) – test if the file can be written directory?( filename ) – test if the file is a directory zero?( filename ) – test if the file is empty size( filename ) – returns the size of the file mtime( filenam e) – returns the modification time of the file

Streams The following constants refer to standard I/O streams: STDIN, STDOUT, STDERR The following variables are initially set to the corresponding constants: $stdin, $stdout, $stderr In addition, $defout (initially equal to $stdout ) is where output with no specified destination is sent

Some String methods ljust( length ), center( length ), rjust( length ) – left justify, center, or right justify the string by padding with spaces downcase, upcase, swap, capitalize – modify capitalization include?( s_or_c ) – tests whether the string includes the given string or character index( s_or_c [, offset ]) – returns the index after offset(or nil ) at which the gives string starts rindex( s_or_c [, limit ]) – returns the last index (before limi t ), or nil, at which the string starts

Some more String methods strip – remove leading and trailing spaces chop – remove the last character (also chop! is destructive) chomp – remove the last character if it is a newline (also chomp! ) tr( chars, replacement ) – replace the characters in chars with the corresponding characters in replacement; accepts ch1-ch2 notation

Some Array methods min, max – return the smallest or largest element uniq – return an array with no duplicate elements compact – return an array with no nil elements sort – return a sorted array & – perform an intersection (only elements in both) | – perform a union (elements in either) grep( regexp) – return elements matching the pattern push( element) – add the element to the end of the array pop – remove and return the last element shift – remove and return the first element

Chaining Nondestructive methods can usually be chained Example: x = gets.chomp.strip.downcase Many destructive methods return nil if they make no changes in the receiver, hence cannot be chained Example: x = gets.chomp!.strip!.downcase! will result in a runtime error

Context def do_it a = [1, 2, 3, 5, 8, 13, "hi"] x = 4 # local variable a.my_each { |v| print v * x, "; " } end do_it 4; 8; 12; 20; 32; 52; hihihihi; Notice that the print v*x statement is being executed in the my_each method, not in the do_it method However, x is local to the do_it method How can this be? Answer: The block carries its context along with it A block is a closure

More iterators collection.each_index – iterate over the indices of a collection collection.each_with_index – iterate over the values in a collection, along with their indices Example: lineup.each_with_index { |man, pos| print pos, man } hash.each_key – iterate over keys hash.each_value – iterate over values hash.each_pair – iterate over key-value pairs collection.select { | v | condition } – choose only items that meet the condition collection.map { | v | transformation } – create a new collection with the transformation applied to each item

Procs A proc is a procedure that is stored in a variable Call a proc by using the variable’s call method p = Proc.new { |x, y, z| puts 100 * x + 10 * y + z } p.call 14, 9, max = Proc.new do |a, b| if a > b then a else b end end puts max.call(0.8, 0.12) 0.8

Procs are closures, too def scoper p x = 3 p.call end x = 77 p = Proc.new { puts x } x = 19 scoper p 19

Procs as parameters A block passed as a parameter to a function becomes a Proc The formal parameter must be last, and is prefixed with an ampersand def foo &b b.call end foo { puts "Hi!" } Hi!

Reflection The class method returns the class of an object The superclass method returns the superclass of an object The name method returns the name of a class The new method creates a new object of a class The methods method returns the methods than an object knows how to respond to The instance_variables method returns the attributes of an object There are many other methods to examine (and modify!) a Ruby program while it is executing

Undefined methods If an undefined method is called, Ruby raises a NoMethodError If you supply a method_missing method for your object, Ruby will call this instead Example: def method_missing(name, *args) puts "Call of missing method" + " #{name}(#{args.join ', ' })" end fribble 2, :ace Call of missing method fribble(2, ace)

Adding methods to a class To add (or replace) a method to a class, just open up the class definition again and define the method You can even do this with Ruby’s built-in classes class Array def every_other i = 0 while i < self.length yield self[i] i += 2 end end end [1, 2, 3, 5, 8, 13].every_other {|v| print v, "; "} 1; 3; 8;

Modules Classes in Ruby, as in Java, may extend only one other class Example: class Enterprise < Starship Everything in the superclass is available in the subclass A module is defined just like a class (using the word module instead of class ) Modules cannot be instantiated, but they may be included in a class Including a module is like copying the code into the class Example: class Enterprise < Starship include Location...

Metaprogramming Metaprogramming is using programs to write programs Example uses: You want to add “boilerplate” code to every method You want to “wrap” functions so that they print to a log when they are called and when they return You want to examine a database and automatically create methods to access that database You can do metaprogramming in any language, but... In Ruby the program can make the changes to itself, as it runs

The command line irb starts an interactive Ruby interpreter ruby starts Ruby, with input from the command line End with an end-of-file character ( ^D or F6 ) Not actually very useful ruby filename.rb executes the given file ruby -e quotedString executes the quoted string Example: ruby -e 'puts "The time is #{Time.now}" ' ruby -v tells you Ruby’s version number On Unix, if the first line of a Ruby file is #!/usr/bin/ruby (or wherever ruby is located), the file can be executed by just entering the file name

Looking ahead Ruby on Rails (or just Rails) is the “killer app” that catapulted Ruby to prominence Web applications are extremely complex server-side programs that communicate with the user’s browser using HTML, XML, and CSS, do session management, and handle a server-side database Rails uses metaprogramming to write your web application for you It’s hard to convey just how much work this saves The downside? You still have to understand the programs that it writes

Tutorials Your textbook has an introduction to Ruby There are many Ruby tutorials on the web--here are some of special interest Very similar in style to the w3schools tutorials Short enough to read through Pretty complete A favorite free online book For something just a “little bit different” :)

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