ruby-acm

Introducing Ruby

a guide for beginners by

David A. Blackauthor, The Well-Grounded Rubyist

Spring 2011

ACM Learning Center -- Ruby Learning PathCopyright 2011, Ruby Power and Light, LLC

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Topic overviewIdentifiersVariablesConstantsKeywordsOperatorsLiteral object constructorsConditionals and case statementsObject basicsMethods and messagesObject-specific methodsInstance variables and attributesModulesClassesInheritanceself (default object)Built-in classes:

Array, String, Hash,Range, Regexp, File, numerics,times, dates


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Ruby Created by Yukihiro "Matz" Matsumoto Version 1.0: 1996 Object-oriented Interpreted Optimized for programmer productivity Untyped variables Highly dynamic Major influences: Lisp, Smalltalk, CLOS, Perl


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View from outer space

Programs organized as classes and modules Classes and modules contain method

definitions and constants Classes spawn instances Modules allow for flexible composition of

complex behaviors Ruby also supports "top-level" code (outside

of class/module definitions)

Identifiers


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Ruby identifiers

Variables Constants Method names Keywords


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Variables

Local a = 1str = "Hello"

Instance @name = "David" Global $VERBOSE = true Class @@x = 1


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Variable naming rules

Local: start with a-z or _ consist of A-Za-z0-9_

Instance: start with @ consist of A-Za-z0-9_

Global start with $ can include other punctuation ($? etc.)

Class start with @@ consist of A-Za-z0-9_


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Constants

Constants start with an uppercase letter They are defined inside classes and

modules They are not exactly constant!

but you get warned if you redefine them


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Constants

RUBY_VERSION Holds any value

String Name of a class

Math Name of a module

Math::PI Nested in a module

class Automobile Your classes and modules


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Method names

puts "Hello" puts is a methoddef update_info define a method"Hello".reverse call the "reverse"

method on a string

Integer("123") Uppercase method name(unusual, but some exist)

Method names are similar to local variable names, except that they canstart with an uppercase letter.


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Keywords

if, unless, when, case, def, class, module, super, yield, and more!

Can't be used as local variables:

if = 1 error!

Basic constructs


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Literals and expressions

"Good morning, Mr. Phelps." # literal string

["Ruby", "Perl", "C"] # literal array (of strings)

{ "NY" => "New York", "CT" => "Connecticut", "NJ" => "New Jersey" } # literal hash ("dictionary")

/^[a-z]+$/ # literal regular expression

(0...100) # literal range

a = 10 # variable assignmentb = a + 5 # object reference via the variable

"Good morning".upcase # method call

Many Ruby objects can be created with literal constructors,like quotation marks for strings. You'll see and use all of thesefrequently.


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Operators

Assignment (=) Arithmetic (+ - * / % **) Shortcut arithmetic (+= *= etc.) Boolean (|| &&) Short-circuit Boolean (||= &&=) Bitwise (^ | &)


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Operators that are methods

Many "operators" are actually methods!

2 + 3 is really 2.+(3)2 * 3 is really 2.*(3)16 | 2 is really 16.|(2)

Makes for easy "operator" overloading (via method definitions)


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Conditionals

if x == 1 puts "x is 1"elsif x == 2 puts "x is 2"else puts "x is something other than 1 or 2"end

puts "x is not 3" unless x == 3

if/elsif/.../else/end Negative conditionals with unless End-of-line modifier form accepted


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Case statementscase/when/.../else/endFirst match "wins"Cases can be combined

case xwhen 1 puts "x is 1"when 2, 3 puts "x is 2 or 3"else puts "x is not 1, 2, or 3"end

Objects, messages, and methods


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Object overview

Ruby instructions consists mainly of messages being sent to objects.

Objects are programmable constructs with storage and calculation capabilities

Every expression evaluates to an object Every object is an instance of a class

and classes are also objects (but don't worry about that now!)


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Object derivation

Objects are represented by literal constructors, such as "Hi!" for a

string identifiers (variables and constants) that

have been assigned to complex expressions or calculations

every statement or expression reduces to a single object


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Sending objects messages

A message is sent to an object with the dot operator:string = "Hi".upcase # "HI"low = string.downcase # "hi"

or indirectly with send:method_name = "upcase"string = "Hi".send(method_name) # "HI"


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Messages and methods

Objects receive messages Messages are resolved (if possible) into

method names Every method is defined in a class or

module The object looks for the method along a

method search path, consisting of classes and modules


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How does an object know about a method?

An object's methods include: singleton methods (those defined just for

this object) instance methods defined in modules with

which the object has been extended instance methods defined in the object's

class, and in that class's ancestral classes and modules


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Three ways to teach an object

module ShoutCapable def shout self.upcase + "!!!" endend

string = "Hello"string.extend(ShoutCapable)

class String include ShoutCapableend

string = "Hello"

string = "Hello"

def string.shout self.upcase + "!!!"end

puts string.shout HELLO!!!


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Three ways to teach an objectstring = "Hello"

def string.shout self.upcase + "!!!"end


"Singleton" method definition. Applies to this object only.No other string can shout!


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Three ways to teach an object

module ShoutCapable def shout self.upcase + "!!!" endend

string = "Hello"string.extend(ShoutCapable)


Put the method in a module and extend the objectwith the module. The object "learns" all the methodsdefined in the module.


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Three ways to teach an objectclass String include ShoutCapableend

string = "Hello"


Class-wide: include the module in the class. Now allstrings will be about to shout! (See previous slides for themodule code.)


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Singleton methods

Defined directly on an objectdef string.shout

Only for this object (with one special case, to be examined later)

Represents Ruby objects at their most dynamic!

Not an everyday technique, but has some important use-cases


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Modules

Defined using the module keyword Bundles of methods Also handy for name-spacing Can also store constants

and nested modules Can be "attached" to individual objects, via extend

Can be "attached" to entire classes, via include


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Classes

A specialization of modules Also a bundle of methods and constants

(and nested classes, modules, constants...) but classes can also create new objects


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Creating an instance of a class

class Person def name "David" # All people are named David for the moment! endend

d = Person.new # Create a new Person object

print "Here's the person's name: "puts d.name

Output:

Here's the person's name: David


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Inheritance

Classes can have subclasses The subclass inherits from the superclass Instances of the subclass can call methods

defined in the superclass Inheritance cascades: subsubclasses, etc. Every class can have only one superclass

i.e., Ruby supports single inheritance


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Inheritance syntax: > string = "Hi there."=> "Hi there.">> string.upcase=> "HI THERE."

>> string.reverse=> ".ereht iH"

>> string.upcase.reverse=> ".EREHT IH"

>> string.split=> ["Hi", "there."]

>> string.delete('e')=> "Hi thr."

>> string = " Hello again. "=> " Hello again. ">> string.strip=> "Hello again."

>> string = "greetings to you."=> "greetings to you.">> string.capitalize=> "Greetings to you."

>> string.swapcase=> "GREETINGS TO YOU."

>> string = "abc"=> "abc">> string.succ=> "abd"

>> string = "Ends with newline\n"=> "Ends with newline\n">> string.chomp=> "Ends with newline"


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Arrays

Ordered collections Literal constructor: [] Indexed by number

zero-originarray = ["apple", "orange", "pear"]puts array[1] # orange


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Some common array methods

colors = ["red", "orange", "yellow"]

colors[2] # "yellow"colors.reverse # ["yellow", "orange", "red"]colors.push("green") # ["red", "orange", "yellow", "green"] colors.pop # returns "green", which is no longer # in the array colors.sort # ["orange", "red", "yellow"]

more_colors = ["green", "blue"]five_colors = colors + more_colors # creates new arraycolors.concat(more_colors) # adds "green" and "blue" to colors


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Hashes

Unordered, keyed collections "dictionary", "associative array" In 1.9: ordered by order of key

insertion Literal constructor: {} Key/value separator: => Values retrieved by key

states = {"New York" => "NY", "New Jersey" => "NJ", "Texas" => "TX" }

puts states["New Jersey"] # "NJ"


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Some common hash methods

states.has_key?("New York") # truestates["Oregon"] # nilstates.fetch("Oregon") # fatal error!

states.delete("New York") # removes pair from hash

more_states = {"Virginia" => "VA" }states.merge(more_states) # new hash, with all pairsstates.update(more_states) # adds VA to states

Iterators


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Fibonacci iterator

def fibit n,m = 1,1 loop do yield n n,m = m,n+m endend

fibit do |num| puts "Next: #{num}" break if num > 100end

Next: 1Next: 1Next: 2Next: 3Next: 5Next: 8Next: 13Next: 21Next: 34Next: 55Next: 89Next: 144

The method The method call


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Iteration mechanics

def fibit n,m = 1,1 loop do yield n n,m = m,n+m endend

fibit do |num| puts "Next: #{num}" break if num > 100end

Next: 1Next: 1Next: 2Next: 3Next: 5Next: 8Next: 13Next: 21Next: 34Next: 55Next: 89Next: 144

1 1 2 3 5 ... 89144

The iterating methodyields one or morevalues.

The method call includesa code block. Each yieldedvalue is bound, in turn, to the block parameter (num)for one execution of theblock.


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Iteration semantics

The iterator (the method that iterates) yields values to the code block

The code block can take arguments, bound to parameters (between vertical pipes)

The code block returns a value, which is the value of the call to yield


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Returning a value from the blockclass Guesser def guess(limit = 10) g = rand(limit) + 1 outcome = yield(g) if outcome puts "#{g} was right!" else puts "No, #{g} was wrong" end endend

guesser = Guesser.newguesser.guess do |g| g == 3end

Get a random number between 1and limit (default limit: 10). Yield that number to the code block,and save the result.

Did the block return a true value?If so, you win; otherwise, you lose.

When you use the Guesser class, you supplywhatever "win/lose" logic you want, inside the block.In this example, you win if the random number yielded from the guess method is 3, because in thatcase, the block returns true.


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Collection iteration

Very common for arrays and hashes Basic iterator: each

[1,2,3,4,5].each do |x| puts "Next number is #{x}!"endNext number is 1!Next number is 2!Next number is 3!Next number is 4!Next number is 5!


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Methods based on each

each is the generic enumerator call the block once for each element

The Enumerable module defines iterator methods based on each select, find, inject, reject, map, etc. These are all defined in terms of each.


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map

array = [1,2,3,4,5]new_array = array.map {|x| x * 10 }p new_array [10,20,30,40,50]

hash = { "NY" => "New York", "CT" => "Connecticut", "NJ" => "New Jersey" }state_list = hash.map do |abbr, state| "#{state} is abbreviated #{abbr}"endp state_list

["New York is abbreviated NY", "New Jersey is abbreviated NJ", "Connecticut is abbreviated CT"]

map performs atransformation,calling the codeblock once for eachelement in the originalcollection, and returningan array consisting ofthe results of all of thecalls to the block.


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select

array = [1,2,3,4,5]big = array.select {|x| x > 3 }p big

[4,5]

hash = { "NY" => "New York", "CT" => "Connecticut", "NJ" => "New Jersey" }big_names = hash.select {|abbr, state| state.split.size > 1 }p big_names

[["NY", "New York"], ["NJ", "New Jersey"]]

select returns anarray of all theelements of thecollection forwhich calling theblock returns atrue value.

(reject does theopposite.)


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Ranges

Ranges test for inclusion Ranges can behave like collections

if they are made up of discrete, countable units

Ranges can involve numbers, strings, anything that has a "succ" method


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Range instantiation

r = 1..10 # inclusive range, 1 through 10r = 1...10 # exclusive range, 1 through


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Range inclusion tests

(1..10).include?(3) # true(1..10).include?(11) # false(1...10).include?(3) # true(1...10).include?(10) # false(1..10).include?(3.141) # true

('a'..'z').include?('g') # true('a'..'z').include?('word') # true

Inclusion tests look for >= and


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Ranges as collections

(1..10).to_a # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10](1..10).map {|e| e * 10 } # [10,20,30,40,50,60,70,80,90,100]array = [*1..10] # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

(1.1..10.0).to_a # Error: can't iterate from float(1..10.1).to_a # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

('a'..'e').to_a # ["a", "b", "c", "d", "e"]('a'..'e').include?('black') # true

Even though the range includes 'black', 'black' is not oneof the members of its collection. (This changes in 1.9, wherethere's a difference between include? and cover? 'a'..'e' is saidto cover, but not include, 'black'.)


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Regular expressions

Literals constructed with // Anchors:

Beginning of string: \A End of string: \z End of string, minus newline: \Z Beginning of line: ^ End of line: $

Modifiers: Include \n in . : /m Ignore unescaped whitespace: /x No /g (global modifier): use gsub or scan


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Using regexes: match>> s = "Good morning. I am a string."=> "Good morning. I am a string.">> m = /mor/.match(s)=> # # match returns a MatchData object>> m.string # which knows its string=> "Good morning. I am a string.">> m[0] # and its match=> mor">> m.pre_match # and other data=> "Good ">> m.post_match=> "ning. I am a string.">> m = /Good (morning). (I)/.match(s) # parenthetical captures=> #>> m.captures # available via #captures, or=> ["morning", "I"]>> m.to_a=> ["Good morning. I", "morning", "I"]>> m[1] # array-style, or=> "morning">> $1=> morning" # magic global variables


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Using regexes: =~

>> str = "I am a string"=> "I am a string">> str =~ /am/ # returns offset of match=> 2>> str =~ /hello/ # or nil if no match=> nil

Faster than Regexp#match Sets global MatchData object $~ Sets global captures variables ($1, $2, etc.)


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Using regexes: scan

Use scan for global matching>> string = "This is his wish."=> "This is his wish.">> array = string.scan(/is/)=> ["is", "is", "is", "is"]>> array = string.scan(/(.)(is)/)=> [["h", "is"], [" ", "is"], ["h", "is"], ["w", "is"]]>> string.scan(/is/) {|match| puts "Found: #{match}" }Found: isFound: isFound: isFound: is>> string.scan(/(.)(is)/) {|captures| puts "Captures: #{captures.inspect}" }Captures: ["h", "is"]Captures: [" ", "is"]Captures: ["h", "is"]Captures: ["w", "is"]

With captures, one sub-array per scan

Block called once per scan


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Using regexes: sub and gsub>> string = "The rain in Spain"=> "The rain in Spain">> string.sub(/in/, "IN")=> "The raIN in Spain">> string.gsub(/in/, "IN")=> "The raIN IN SpaIN">> string.sub!(/in/, "IN")=> "The raIN in Spain">> string=> "The raIN in Spain">> string.gsub!(/in/, "IN")=> "The raIN IN SpaIN">> string=> "The raIN IN SpaIN">> string.gsub(/IN/) {|match| match.reverse }=> "The raNI NI SpaNI">> string.gsub(/(a)I/) {|e| $1.upcase }=> "The rAN IN SpAN">> string.gsub(/(a)I/, '\1!')=> "The ra!N IN Spa!N"

sub does one replacementgsub does all

sub! and gsub! permanentlychange the string

The code block receivesthe matched substring, andhas access to the $n capturevariables

The replacement string canget the captures via escapednumbers (\1, \2, etc.)


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Numerics

Numbers are objects! Integer (Fixnum/Bignum) Float In standard library:

Complex, Rational, BigDecimal

Infix operators are methods e.g., 3 + 2 is really 3.+(2)


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Numeric methods

upto/downto/step3.upto(6) {|x| puts x * 10 }10.downto(1) {|x| puts -x + 10 }1.step(100,10) {|x| puts x }

divmod10.divmod(3) # [3,1]

abs (absolute value) floor, ceil, round, truncate


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Floats

Instances of Float "Wrapper"-style instances>> 10.1.object_id=> -604737798>> 10.1.object_id=> -604416838

Subject to the universal precision issues


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Time/date objects

Time, Date, DateTime classes Spread over several libraries:

you may have to "require 'time'" and/or "require 'date'"


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Time/date examples

=> Mon Nov 10 10:17:14 UTC 2008>> t = Time.now=> Mon Nov 10 05:17:31 -0500 2008>> t + 2000 # add seconds=> Mon Nov 10 05:50:51 -0500 2008>> t.day # likewise month, year, hour, min,

sec=> 10>> t.strftime("%y/%m/%d") # wraps strftime(3)=> "08/11/10">> t.utc? # UTC?=> false>> t.zone # timezone=> "EST"


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Date/DateTime

>> d = Date.today=> #>> puts d2008-11-10>> puts d + 1 # +/- days2008-11-11>> puts d >> 1 # >>/> puts d.julian # also gregorian2008-10-28

>> dt = DateTime.now=> #>> puts dt2008-11-10T05:25:54-05:00=> nil>> puts dt + 12008-11-11T05:25:54-05:00=> nil>> puts dt >> 12008-12-10T05:25:54-05:00


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File reading and writingRead in fle content as a string:str = File.read("filename")

Read in a fle as an array of lines: array = File.readlines("filename")

Open a fle for output: f = File.open("filename", "w")

Write to fle: f.puts("A line for the file")

Close fle: f.close

Use block to automate closing of flehandle: File.open("filename","w") do |f| f.puts("A line for the file") end

Iterate through lines of a fle: File.open("filename") do |f| f.each {|line| puts line.reverse } end

PowerPoint PresentationTopic overviewRubyView from outer spaceIdentifiersRuby identifiersVariablesVariable naming rulesConstantsSlide 10Method namesKeywordsBasic constructsLiterals and expressionsOperatorsOperators that are methodsConditionalsCase statementsObjects, messages, and methodsObject overviewObject derivationSending objects messagesMessages and methodsHow does an object know about a method?Three ways to teach an objectSlide 26Slide 27Slide 28Singleton methodsModulesClassesCreating an instance of a classInheritanceInheritance syntax:

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