basic syntax - ut · ruby string syntax • single quotes (only \' and \\) !'bill\'s...

$: Basic syntax - ut · Ruby string syntax • Single quotes (only \' and \\) !'Bill\'s "personal" book'" • Double quotes (many escape sequences) !"Found #{count} errors\nAborting$
Basic syntax

RUBY LUCIANO GARCÍA-‐BAÑUELOS

sum = 0 !i = 1 !while i <= 10 do! sum += i*i ! i = i + 1 !end!print "Sum of squares is #{sum}\n"!

No variable declara<ons

Newline is statement separator

do … end instead of { … }

Parenthesis are op<onal in method calls String interpola-on

Ruby string syntax •  Single quotes (only \' and \\)

!'Bill\'s "personal" book' "

•  Double quotes (many escape sequences) !"Found #{count} errors\nAborting job\n" "

• %q () – similar to simple quotes !%q<Nesting works: <b>Hello</b>> "

• %Q () – similar to double quotes !%Q|She said "#{greeting}"\n| "

•  “Here documents” !<<END "!First line "!Second line "!END "


Variable names and scopes


foo Local variable $foo Global variable @foo Instance variable in object @@foo Class variable MAX_USERS “Constant” (by conven<on)

Ruby statements


if x < 10 then! ... !elsif x < 20! ... !else! ... !end!

while x < 10 do ! ... !end!

array = [22, 34, 46, 92] !for value in array do! ... !end!

Literal arrays

Ruby classes


class Point ! def initialize(x, y) ! @x = x ! @y = y ! end! ! def x ! @x ! end! ! def x=(value) ! @x = value! end!end!

p = Point.new(3,4) !puts "p.x is #{p.x}"!p.x = 44 !

Another example: Binary tree


class BinaryTree!! def initialize(value = nil) ! @value = value ! @left = @right = nil! end!! def insert(value) ! ... ! end!! def traverse_in_order! @left.traverse_in_order unless @left.nil? ! puts @value unless @value.nil? ! @right.traverse_in_order unless @right.nil? ! end!!end!

Binary tree (cont.)


class BinaryTree!!

def insert(value) ! if @value.nil? ! @value = value! elsif value < @value! if @left.nil? ! @left = BinaryTree.new(value) ! else ! @left.insert(value) ! end! else! if @right.nil? ! @right = BinaryTree.new(value) ! else ! @right.insert(value) ! end! end! end!!

end!

Let’s play with our example


tree1 = BinaryTree.new(10) !tree1.insert(5) !tree1.insert(15) !tree1.insert(20) !!tree1.traverse_in_order !

tree2 = BinaryTree.new!input = %w{one two three four five six} !!for w in input !

"tree2.insert(w) !end!!tree2.traverse_in_order !

% notation


Modifier Meaning

%q[] Non-‐interpolated String (except for \\ \[ and \])

%Q[] Interpolated String (default)

%r[] Interpolated Regexp (flags can appear aYer the closing delimiter)

%i[] Non-‐interpolated Array of symbols, separated by whitespace (aYer Ruby 2.0)

%I[] Interpolated Array of symbols, separated by whitespace (aYer Ruby 2.0)

%w[] Non-‐interpolated Array of words, separated by whitespace

%W[] Interpolated Array of words, separated by whitespace

%x[] Interpolated shell command

Arrays


x = Array.new # same as x = []!x << 10 " " # same as x.push(10)!!x[0] = 99 !y = ["Alice", 23, 7.3] !x[1] = y[1] + y[-1] !

From the end and downwards

Iterating through arrays


a = [1,2,3,4,5] !i = 0 !while i < a.length! puts a[i] ! i = i + 1 !end! a = [1,2,3,4,5] !

a.each { |x| puts x } !

Code block Delimited by: { … } or do … end Parameter list body

More examples on code blocks

◦ n.-mes runs code block n <mes ◦ n.upto(m) runs code block for integers n..m ◦ a.find returns first element x of array such that the block returns true for x

◦ a.collect applies block to each element of array and returns new array (a.collect! modifies the original)‏


3.times { puts "hello"; puts "goodbye" } !5.upto(10) { |x| puts(x + 1) } ![1,2,3,4,5].find { |y| y % 2 == 0 } ![5,4,3].collect { |x| -x } !(1..10).inject(:*) !

Calling code blocks


def countx(x)‏! for i in (1..x)‏! puts i ! yield! end!end!!countx(4) { puts "foo" }!

>> countx(4) 1 foo 2 foo 3 foo 4 foo => 1..4

Let’s improve our binary tree class


class BinaryTree!! def traverse(mode = :in_order, &block) ! case mode ! when :in_order! @left.traverse mode, &block unless @left.nil? ! yield @value ! @right.traverse mode, &block unless @right.nil? ! when :pre_order! ... ! when :post_order! ... ! end! end !!end!

Let’s play again with our example


tree1 = BinaryTree.new![10,20,15,5].each {|x| tree1.insert(x)} !tree1.traverse { |x| puts x }!

tree2 = BinaryTree.new!%w{one two three four five six}.each{ |w| tree2.insert(w) } !!tree2.traverse(:in_order) { |x| print " #{x}" }; puts !tree2.traverse(:pre_order) { |x| print " #{x}" }; puts !tree2.traverse(:post_order) { |x| print " #{x}" }; puts !

Hashes


person = Hash.new"# same as person = {}!!person["last_name"] = "Rodriguez"!person[:first_name] = "Alice"!order = {"item" => "Corn Flakes", "weight" => 18} !order = {:item => "Corn Flakes", :weight => 18} !order = {item: "Corn Flakes", weight: 18} !

Ruby Luciano García-‐Bañuelos

Regular Expressions •  A way of describing pamerns or sets of strings ◦  Searching and matching ◦  Formally describing strings

◦  The symbols (lexemes or tokens) that make up a language

•  Common to lots of languages and tools ◦  awk, sed, perl, grep, Java, OCaml, C libraries, etc.

•  Based on some really elegant theory ◦  We’ll see that soon


Example Regular Expressions in Ruby

•  /Ruby/ ◦  Matches exactly the string "Ruby" ◦  Regular expressions can be delimited by /’s ◦  Use \ to escape /’s in regular expressions

•  /(Ruby|OCaml|Java)/ ◦  Matches either "Ruby", "OCaml", or "Java"

•  /(Ruby|Regular)/ or /R(uby|egular)/ ◦  Matches either "Ruby" or "Regular" ◦  Use ()’s for grouping; use \ to escape ()’s


Repetition in Regular Expressions

•  /(Ruby)*/ ◦  {"", "Ruby", "RubyRuby", "RubyRubyRuby", ...} ◦  * means zero or more occurrences

•  /(Ruby)+/ ◦  {"Ruby", "RubyRuby", "RubyRubyRuby", ... } ◦  + means one or more occurrence ◦  so /e+/ is the same as /ee*/


Repetition in Regular Expressions

• /(Ruby)?/ ◦  {"", "Ruby"} ◦  ? means op,onal, i.e., zero or one occurrence

• /(Ruby){3}/ ◦  {“RubyRubyRuby”, “RubyRubyRubyRuby”, …} ◦  {x} means repeat the search for at least x occurrences

• /(Ruby){3, 5}/ ◦  {“RubyRubyRuby”, “RubyRubyRubyRuby”, “RubyRubyRubyRubyRuby”} ◦  {x, y} means repeat the search for at least x occurrences and at most y occurrences


Watch Out for Precedence •  /(Ruby)*/ means {"", "Ruby", "RubyRuby", ...} ◦  But /Ruby*/ matches {"Rub", "Ruby", "Rubyy", ...}

•  In general ◦  * {n} and + bind most <ghtly ◦  Then concatena<on (adjacency of regular expressions)‏ ◦  Then |

•  Best to use parentheses to disambiguate


Character Classes •  /[abcd]/ ◦  {"a", "b", "c", "d"} (Can you write this another way?)‏

•  /[a-‐zA-‐Z0-‐9]/ ◦  Any upper or lower case lemer or digit

•  /[^0-‐9]/ ◦  Any character except 0-‐9 (the ^ is like not and must come first)‏

•  /[\t\n ]/ ◦  Tab, newline or space

•  /[a-‐zA-‐Z_\$][a-‐zA-‐Z_\$0-‐9]*/ ◦  Java iden<fiers ($ escaped...see next slide)‏


Special Characters . any character ^ beginning of line $ end of line \$ just a $ \d digit, [0-‐9] \s whitespace, [\t\r\n\f] \w word character, [A-‐Za-‐z0-‐9_] \D non-‐digit, [^0-‐9] \S non-‐space, [^\t\r\n\f] \W non-‐word, [^A-‐Za-‐z0-‐9_]


Potential Character Class Confusions ^ inside character classes: not outside character classes: beginning of line [] inside regular expressions: character class outside regular expressions: array note: [a-‐z] does not make a valid array, () inside character classes: literal characters ( ‏( /(0..2)/ does not mean 012 outside character classes: used for grouping -  inside character classes: range (e.g., a to z given by [a-‐z])‏ outside character classes: dash

Using regular expressions


line = gets # read line from standard input!if line =~ /Ruby/ then # returns nil if not found! puts "Found Ruby"!end!

gets =~ /^Min: (\d+) Max: (\d+)$/ !min, max = $1, $2 !

def m(s)‏! s =~ /(Foo)/ ! puts $1 # prints Foo!end!m("Foo")‏!puts $1 # prints nil!

Finally … metaprogramming


class Polyglot ! def initialize ! @@greetings = {estonian: "Tere hommikust", english: "Good morning"} ! end! def add_language(language, greeting) ! @@greetings[language] = greeting if language.is_a? Symbol ! end! def method_missing(name) ! unless @@greetings[name].nil? ! puts @@greetings[name] ! else! super! end! end!end!!a = Polyglot.new!!a.estonian!a.english!

basic syntax - ut · ruby string syntax • single quotes (only \' and \\) !'bill\'s...

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