Variables, expressions, standard types

"Variables, expressions, standard types"
или
то, что вы уже знаете, если делали коэны

$global_variables
Global variables begin with $
It is not recommended to use global variables. They make programs cryptic.

@instance_variables
class Customer
def initialize(id, name, addr)
@cust_id = id
@cust_name = name
@cust_addr = addr
end
def display_details()
puts "Customer id #@cust_id"
puts "Customer name #@cust_name"
puts "Customer address #@cust_addr"
end
end

@@class_variables
Class variables are shared among descendants of the class or module in which the class variables are
defined.
class Customer
@@no_of_customers = 0
…
def total_no_of_customers()
@@no_of_customers += 1
puts "Total number of customers: #@@no_of_customers"
end
end
cust1 = Customer.new("1", "John", "Wisdom Apartments, Ludhiya")
cust2 = Customer.new("2", "Poul", "New Empire road, Khandala")
cust1.total_no_of_customers() #=> Total number of customers: 1
cust2.total_no_of_customers() #=> Total number of customers: 2

local_variables
Local variables begin with a lowercase letter or _. The scope of a local variable ranges from class,
module, def, or do to the corresponding end or from a block's opening brace to its close brace {}.
irb(main):011:0> class TestClass
irb(main):012:1> test_local_variable = 1
irb(main):013:1> def test_method
irb(main):014:2> puts test_local_variable
irb(main):015:2> end
=> :test_method
irb(main):017:0> TestClass.new.test_method
NameError: undefined local variable or method `test_local_variable' for
#<TestClass:0x00007fc8be03a028>
from (irb):14:in `test_method'
from (irb):17
from /Users/yakavkrasnov/.rvm/rubies/ruby-2.4.2/bin/irb:11:in `<main>'

irb(main):018:0> class TestClass
irb(main):019:1> attr_reader :test_var
irb(main):020:1> def initialize(var)
irb(main):021:2> @test_var = var
irb(main):023:1> def smth
irb(main):024:2> puts test_var
=> :smth
irb(main):027:0> TestClass.new('haha').smth
haha
=> nil

CONSTANTS
irb(main):028:0> class Example
irb(main):029:1> VAR1 = 100
irb(main):030:1> VAR2 = 200
irb(main):031:1> def show
irb(main):032:2> puts "Value of first Constant is #{VAR1}"
irb(main):033:2> puts "Value of second Constant is #{VAR2}"
irb(main):036:0> object = Example.new()
=> #<Example:0x00007fc8be1d28e0>
irb(main):037:0> object.show
Value of first Constant is 100
Value of second Constant is 200

Ruby Pseudo-Variables
● self − The receiver object of the current method.
● true − Value representing true.
● false − Value representing false.
● nil − Value representing undefined.
● __FILE__ − The name of the current source file.
● __LINE__ − The current line number in the source file.

Ruby expressions
Expressions are constructed from operands and operators. The operators of an
expression indicate which operations to apply to the operands. The order of
evaluation of operators in an expression is determined by the precedence and
associativity of the operators.

Category Symbol
Resolution, access operators :: .
Array operators [ ] [ ]=
Exponentiation **
Not, complement, unary plus, minus ! ~ + -
Multiply, divide, modulo * / %
Addition, substraction + -
Shift operators << >>
Bitwise and &
Bitwise or, logical or ^ |
Relational operators > >= < <=
Bitwise or, logical or ^ |
Equality, pattern match operators <=> == === != =~ !~
Logical and &&
Logical or ||
Range operators .. ...
Ternary ?:
Assignment operators = += -= *= **= /= %= &= |= ^= <<= >>= ||= &&=
Alternate negation not
Alternate logical or, and or and

Ruby concatenating strings
"Return ".+"of ".+ "the ".+"King"
Under the hood, the + operator is a Ruby method. The string
literal is an object. We call a method of an object using
the access . operator.
Use interpolation instead of concatenation.
@user_name = 'Толик'
puts "Hello, #{@user_name}" #=> "Hello, Толик"

Ruby increment, decrement operators
Ruby has no such operators:
x++, ++x, x--, --x
why?
irb(main):031:0> --2
=> 2
use +=, -=, *=, /=

Ruby arithmetic operators
irb(main):048:0> 5.div 2.0
=> 2
irb(main):049:0> 5.fdiv 2
=> 2.5
irb(main):050:0> 5.quo 2
=> 5/2
irb(main):051:0> 5.0.quo 2.0
=> 2.5
irb(main):052:0> 10 % 4
=> 2
irb(main):057:0> 2 / 3
=> 0
irb(main):058:0> 2 / 3.0
=> 0.6666666666666666

Ruby Boolean operators
irb(main):064:0> true && true
=> true
irb(main):065:0> true && false
=> false
irb(main):066:0> false && true
=> false
irb(main):067:0> false && false
=> false
irb(main):071:0* false || true
=> true
irb(main):072:0> false || false
=> false
irb(main):073:0> true || true
=> true
irb(main):074:0> true || false
=> true

Ruby Boolean operators
irb(main):060:0> true && "Hello, #{@user_name}"
=> "Hello, Толик"
irb(main):061:0> 0 || 1
=> 0
irb(main):062:0> !0
=> false
irb(main):077:0> false || "Hello, #{@user_name}"
=> "Hello, Толик"

Ruby elational Operators
Symbol Meaning
< less than
<= less than or equal to
> greater than
>= greater than or equal to

Ruby associativity && Ruby range operators
a = b = c = d = 0
irb(main):089:0> print a, b, c, d
0000
irb(main):095:0> (1..3).to_a
=> [1, 2, 3]
irb(main):096:0> (1...3).to_a
=> [1, 2]

Ruby ternary operator
cond-exp ? exp1 : exp2
age = 32
adult = age >= 18 ? true : false

Variables, expressions, standard types

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Variables, expressions, standard types