Transcript 7-ruby-features

Assorted Ruby Features
(Bet you can’t do this in Java!)
5-Apr-16
The command line
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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)
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ruby filename.rb executes the given file
ruby -e quotedString executes the quoted string
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Not actually very useful
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
Adding and removing methods
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def adds a method; undef removes a method
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To add an instance method, first “open” the class
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Example: class String; def nchars; length; end; end
There are several ways to add a class method to a class
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The only parameter to undef is the method name
def Person.species; 'human'; end
Within the Person class, you can use def self.species
You can add a method to individual objects
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def oscar.mood; 'grouchy' ; end
Numbers
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Numbers may be written in decimal, hexadecimal, octal, or
binary
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Numbers larger than four bytes are automatically treated as
Bignum objects
For readability, numbers may contain (but not begin or end with)
underscores
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Decimal: 3405691582
Hex: 0xCAFEBABE or 0XCAFEBABE
Octal: 031277535276 or 0o31277535276
Binary: 0b11001010111111101011101010111110 or 0Betc.
Examples: 3_405_691_582, 0b_111_101_101
Integers may be indexed to retrieve their bits
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Example: 5.step(0, -1) { |i| print 6[i] }  000110
Attributes (instance variables)
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Attributes (instance variables) of an object are written with an @
prefix: @name, @age, @hobbies, @favorite_language
By default, attributes are private
You can write getters:
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def name
@name
end
You can write setters:
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def name=(new_name)
@name = 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 = "Saj“
 Yes, we are calling the method name= !
Shorthand for getters and setters
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Writing code for routine getters and setters is tedious, so in Ruby
we don’t have to do it
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Shorthand for creating getters:
attr_reader :name, :age, :hobbies
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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
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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
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With arguments, they set the access of the named methods
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Example: private :dump, :swear
With no arguments, they set the default access for all
subsequent methods
eval
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eval executes a string
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Here’s how you don’t want to use this:
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Example: eval "puts x + 2"
eval gets
This can be a serious security risk
Here’s what Ruby does about this:
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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
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printf format_string, value, …, value
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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", 3.141592 
pi.=…3.1416
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
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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
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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) –
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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
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(filename) – returns the modification time of the file
Streams
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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
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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
limit), or nil, at which the string starts
Some more String methods
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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
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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
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Nondestructive methods can usually be chained
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Example: x = gets.chomp.strip.downcase
Many destructive methods return nil if they make no
changes in the receiver, hence cannot be chained
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Example: x = gets.chomp!.strip!.downcase! will result in
a runtime error
Regular expressions
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Ruby has regular expressions, almost identical to the
way they are done in Perl
Example (from the textbook):
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hamlet = "The slings and arrows of outrageous fortune."
hamlet.scan(/w+/)
["The", "slings", "and", "arrows", "of", "outrageous",
"fortune"]
Arrays
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An array literal can be written with brackets and commas
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a = [1, 1, 2, 3, 5, 8, 13, "hi"]
Arrays are zero based: a[2] == 2
Arrays can be expanded
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a = a + [21, 34]
pa
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[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.
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Before you write methods to manipulate arrays, you should look to see
whether the method you want has already been written
Hashes
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A hash (hash table) literal can be written with braces,
commas, and the => arrow
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h = {:apple => :red, :banana => :yellow,
:cherry => :red}
Element access is similar to that for arrays:
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h[:banana]
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h[:apple] = :green
ph
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:yellow
{: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
Loops in Ruby
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Ruby has several loops
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while condition do
statements
end
begin
statements
end while condition
until condition
statements
end
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begin
statements
end until condition
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for variable in range do
statements
end
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statement while condition
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statement until condition
However, loops are not used as
often in Ruby as in other
languages
Instead, Ruby programmers use
iterator methods
Iterators
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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
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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
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a = [1, 1, 2, 3, 5, 8, 13]
a.each { |i| print " #{i}" }
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a.each do |i|
print " #{i}"
end
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Output: 1 1 2 3 5 8 13
Output: 1 1 2 3 5 8 13
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 multiline blocks
Blocks
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A block is a chunk of code that can be passed as a
parameter to a method
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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
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def three_times
puts "---------- three_times“
yield
yield
yield
end
three_times { puts "hello" }
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---------- three_times
hello
hello
hello
My version of loop
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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"
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hi1385321Done
Fibonacci numbers
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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 }
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1 1 2 3 5 8 13 21 34 55 89
Passing a parameter to the block
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def count_to n
puts "---------- count_to #{n}"
for i in 1..n
yield
end
end
count_to 3 { puts "hello" }
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---------- count_to 3
hello
hello
hello
Returning a value from a coroutine
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def count_to_3
puts "---------- count_to_3“
yield 1
yield 2
yield 3
end
count_to_3 { |result| puts result }
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---------- count_to_3
1
2
3
Context
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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?
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Answer: The block carries its context along with it
A block is a closure
More iterators
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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
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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
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A proc is a procedure that is stored in a variable
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p = Proc.new { |x, y, z| puts 100 * x + 10 * y + z }
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Call a proc by using the variable’s call method
p.call 14, 9, 2
1492
max = Proc.new do |a, b|
if a > b then a else b end
end
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puts max.call(0.8, 0.12)
0.8
Procs are closures, too
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def scoper p
x=3
p.call
end
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x = 77
p = Proc.new { puts x }
x = 19
scoper p
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19
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Procs as parameters
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A block passed as a parameter to a function becomes a
Proc
The formal parameter must be last, and is prefixed with
an ampersand
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def foo &b
b.call
end
foo { puts "Hi!" }
Hi!
Reflection
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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
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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:
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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
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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, "; "}
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1; 3; 8;
Modules
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Classes in Ruby, as in Java, may extend only one other
class
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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
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Including a module is like copying the code into the class
Example:
class Enterprise < Starship
include Location
...
Metaprogramming
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Metaprogramming is using programs to write
programs
Example uses:
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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...
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In Ruby the program can make the changes to itself, as it
runs
Looking ahead
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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
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This is basically what my CIT597 course is all about
Rails uses metaprogramming to write your web
application for you
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It’s hard to convey just how much work this saves
The downside? You still have to understand the programs
that it writes
The End