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A quick Ruby Tutorial, Part 4

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Title: A quick Ruby Tutorial, Part 4

1
A quick Ruby Tutorial, Part 4

COMP313
Source Programming Ruby, The Pragmatic
Programmers Guide by Dave Thomas, Chad Fowler,
and Andy Hunt

2
2 forms of assignment

hard coded to variables and constants
instrument "piano"
MIDDLE_A 440
user defined for object attributes and other
element references
song.duration 234
instrument"ano" "ccolo"

3
problems

class BrokenAmplifier
attr_accessor left_channel, right_channel
def volume(vol)
left_channel self.right_channel vol
end
end
ba BrokenAmplifier.new
ba.left_channel ba.right_channel 99
ba.volume 5
ba.left_channel ? 99
ba.right_channel ? 5

4
Parallel assignment

a 1, 2, 3, 4
b, c a ? b 1, c 2
b, c a ? b 1, c 2, 3, 4
b, c 99, a ? b 99, c 1, 2, 3, 4
b, c 99, a ? b 99, c 1, 2, 3, 4
b, c 99, a ? b 99, c 1
b, c 99, a b 99, c 1, 2, 3, 4

5
Nested assignment

b, (c, d), e 1,2,3,4 ? b 1, c 2, d
nil, e 3
b, (c, d), e 1,2,3,4 ? b 1, c 2, d
nil, e 3
b, (c, d), e 1,2,3,4 ? b 1, c 2, d
3, e 4
b, (c, d), e 1,2,3,4,5 ? b 1, c 2, d
3, e 5
b, (c,d), e 1,2,3,4,5 ? b 1, c 2, d
3, 4, e 5

6
Overwriting operators like

class Bowdlerize
def initialize(string)
_at_value string.gsub(/aeiou/, '')
end
def (other)
Bowdlerize.new(self.to_s other.to_s)
end
def to_s
_at_value
end
end
a Bowdlerize.new("damn ") ? dmn
a "shame" ? dmn shm

7
defined? operator

defined? 1 ? "expression"
defined? dummy ? nil
defined? printf ? "method"
defined? String ? "constant"
defined? _ ? "global-variable"
defined? MathPI ? "constant"
defined? a 1 ? "assignment"
defined? 42.abs ? "method
defined? yield ? yield or nil

8
Case expressions

leap case
when year 400 0 true
when year 100 0 false
else year 4 0
end
case input_line
when "debug"
dump_debug_info
dump_symbols
when /p\s(\w)/
dump_variable(1)
when "quit", "exit"
exit
else print "Illegal command input_line"
end

9
another case examples

kind case year
when 1850..1889 then "Blues"
when 1890..1909 then "Ragtime"
when 1910..1929 then "New Orleans Jazz"
when 1930..1939 then "Swing"
when 1940..1950 then "Bebop"
else "Jazz"
end

10
case as instanceof test

case shape
when Square, Rectangle
...
when Circle
...
when Triangle
...
else
...
end

11
For .. in .. looping

based on each gt works for all classes defining
each
class Periods
def each
yield "Classical"
yield "Jazz"
yield "Rock"
end
end
periods Periods.new
for genre in periods
print genre, " "
end
produces Classical Jazz Rock

12
break, redo, next, retry

while line gets
next if line /\s/ skip comments
break if line /END/ stop at end
substitute stuff in backticks and try again
redo if line.gsub!(/(.?)/) eval(1)
process line ...
end
for i in 1..100
print "Now at i. Restart? "
retry if gets /y/
end

13
Loops, blocks, and scope of variables

while/until/for do NOT create a new scope (!
java), but explicit code blocks can
1, 2, 3 .each x y x 1
x, y --gt error, but
xnil
ynil
1, 2, 3 .each x y x 1
x, y --gt 3,4

14
Exceptions

op_file File.open(opfile_name, "w")
begin
Exceptions raised by this code will
be caught by the following rescue clause
while data socket.read(512)
op_file.write(data)
end
rescue
SystemCallError stderr.print "IO failed "
!
op_file.close File.delete(opfile_name)
raise
end

15
more on Exceptions

begin
eval string
rescue SyntaxError, NameError gt boom
print "String doesn't compile " boom
rescue StandardError gt bang
print "Error running script " bang
end
empty rescue catches StandardError instances,
may also use expression returning an Exception
class

16
ensure (cf. Javas finally)

f File.open("testfile")
begin
.. process
rescue
.. handle error
ensure
f.close unless f.nil?
end

17
retry

_at_esmtp true
begin
First try an extended login. If it fails
because the
server doesn't support it, fall back to a
normal login
if _at_esmtp then _at_command.ehlo(helodom)
else _at_command.helo(helodom)
end
rescue ProtocolError
if _at_esmtp then
_at_esmtp false
retry
else
raise
end
end

18
Raising exceptions

raise
raise "bad mp3 encoding"
raise InterfaceException, "Keyboard failure",
caller
raise "Missing name" if name.nil?
if i gt names.size
raise IndexError, "i gt size
(names.size)"
end
raise ArgumentError, "Name too big", caller1..-1

19
Retry example

class RetryException lt RuntimeError
attr ok_to_retry
def initialize(ok_to_retry)
_at_ok_to_retry ok_to_retry
end
end
def read_data(socket)
data socket.read(512)
raise RetryException.new(true), "transient read
error if data.nil?
.. normal processing
end

20
Retry example cont.

begin stuff read_data(socket)
.. process stuff
rescue RetryException gt detail
retry if detail.ok_to_retry
raise
end

21
Modules

provide namespace against clashes
implement mixin facility
module Trig
PI 3.141592654
def Trig.sin(x)
..
end
end
require 'trig'
y Trig.sin(TrigPI/4)

22
Module mixin

Modules are not classes, cannot have instances
module Debug
def who_am_i?
"self.class.name (\self.object_id)
self.to_s"
end
end
class Phonograph
include Debug
...
end
ph Phonograph.new("West End Blues")
ph.who_am_i? ? "Phonograph (937328) West End
Blues"

23
Mixin interaction

class Song
include Comparable
def initialize(duration)
_at_duration duration
end
def ltgt(other)
self.duration ltgt other.duration
end
end
song1, song2 Song.new(225), Song.new(260)
song1 ltgt song2 ? -1
song1 lt song2 ? true
song1 song1 ? true
song1 gt song2 ? false

24
Resolving name clashes

methods
immediate class, then
local mixins last one first, then
superclass, its mixins, etc
variable before method name
a 1
def a
2
end
a -gt 1
a() -gt 2

25
2 ways of IO

utility methods gets, open, print, printf, puts,
putc, readline, readlines, test
proper classes IO, File, BasicSocket,
endl \n"
STDOUT ltlt 99 ltlt " red balloons" ltlt endl

26
File IO examples