learning ruby 7, Expressions

[ 3, 1, 7, 0 ].sort.reverse   [7, 3, 1, 0]

Operator Expressions
a*b + c
(a.*(b)).+(c)

Miscellaneous Expressions
Command Expansion
`date`                   "Thu Aug 26 22:36:31 CDT 2004
"
`ls`.split[34]           "book.out"
%x{echo "Hello there"}   "Hello there
"
for i in 0..3
  status = `dbmanager status id=#{i}`
  # …
end

Assignment
a=b=1+2+3
a   →6
b   →6
a = (b = 1 + 2) + 3
a   →6
b   →3

attribute
de?ne a writable object attribute
class Song
  def duration=(new_duration)
    @duration = new_duration
  end
end
attribute assignment
song.duration = 234

Parallel Assignment
swap the values in two variables
a, b = b, a

x=0                               0
a, b, c = x, (x += 1), (x += 1)   [0, 1, 2]

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]

Nested Assignments
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

Other Forms of Assignment

Conditional Execution
                                                                       The number zero is not interpreted as a false value. Neither is a zero-length string.

De?ned?, And, Or, and Not
and and or have the same precedence,
&& has a higher precedence than ||.

defined? 1          "expression"
defined? dummy      nil
defined? printf     "method"
defined? String     "constant"
defined? $_         "global-variable"
defined? Math::PI   "constant"
defined? a=1        "assignment"
defined? 42.abs     "method"

Operator     Meaning
==     Test for equal value.
===       Used to compare the each of the items with the target in the when clause of a case statement.
<=>   General comparison operator. Returns −1, 0, or +1, depending on   whether its receiver is less than, equal to, or greater than its argument.
<, <=, >=, >    Comparison operators for less than, less than or equal, greater than or  equal, and greater than.
=~       Regular expression pattern match.
eql?     True if the receiver and argument have both the same type and equal values. 1 == 1.0 returns true, but 1.eql?(1.0) is false.
equal?   True if the receiver and argument have the same object ID.
== and =~ have negated forms, != and !~.

The value of Logical Expressions
nil   and true    nil
false and true    false
99    and false   false
99    and nil     nil
99    and "cat"   "cat"

false or nil     nil
nil   or false   false
99    or false   99

words[key] ||= []
words[key] << word
=====>
(words[key] ||= []) << word

If and Unless Expressions
then
if song.artist == "Gillespie" then handle = "Dizzy"
elsif song.artist == "Parker" then handle = "Bird"
else handle = "unknown"
end

:
if song.artist == "Gillespie": handle = "Dizzy"
elsif song.artist == "Parker": handle = "Bird"
else handle = "unknown"
end

if song.artist == "Gillespie"
  handle = "Dizzy"
elsif song.artist == "Parker"
  handle = "Bird"
else
  handle = "unknown"
end

unless song.duration > 180
  cost = 0.25
else
  cost = 0.35
end
==>
cost = song.duration > 180 ? 0.35 : 0.25

If and Unless Modi?ers
mon, day, year = $1, $2, $3 if date =~ /(dd)-(dd)-(dd)/
puts "a = #{a}" if debug
print total unless total.zero?

Case Expressions
leap years must be divisible by 400, or divisible by 4 and not by 100.
     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 /ps+(w+)/
  dump_variable($1)
when "quit", "exit"
  exit
else
  print "Illegal command: #{input_line}"
end

case shape
when Square, Rectangle
  # …
when Circle
  # …
when Triangle
  # …
else
  # …
end

Loops
while line = gets
  # …
end

until play_list.duration > 60
  play_list.add(song_list.pop)
end

a=1
a *= 2 while a < 100
a -= 10 until a < 100
a   → 98

    file = File.open("ordinal")
    while line = file.gets
      puts(line) if line =~ /third/ .. line =~ /fifth/
    end
produces:
    third
    fourth
    fifth

Iterators
    3.times do
      print "Ho! "
    end
produces:
    Ho! Ho! Ho!

    0.upto(9) do |x|
      print x, " "
    end
produces:
    0123456789

    0.step(12, 3) {|x| print x, " " }
produces:
    0 3 6 9 12

    [ 1, 1, 2, 3, 5 ].each {|val| print val, " " }
produces:
    11235

For . . . In
for song in songlist
  song.play
end

    for i in ['fee', 'fi', 'fo', 'fum']
      print i, " "
    end
    for i in 1..3
      print i, " "
    end
    for i in File.open("ordinal").find_all {|line| line =~ /d$/}
      print i.chomp, " "
    end
produces:
    fee fi fo fum 1 2 3 second third

Break, Redo, and Next
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

Retry
    for i in 1..100
      print "Now at #{i}. Restart? "
      retry if gets =~ /^y/i
    end
Running this interactively, you may see
    Now at 1. Restart? n
    Now at 2. Restart? y
    Now at 1. Restart? n
     …

Variable Scope, Loops, and Blocks
x = nil
y = nil
[ 1, 2, 3 ].each do |x|
  y=x+1
end
[ x, y ]   → [3, 4]

if false
  a=1
end
3.times {|i| a = i }
a        2