Object & Class & Variable

This chapter will introduce Ruby basic knowledge about object, class and variable

Table of contents

1. Object
2. Class
3. Variable
4. Multiple assignment

Object

Every value in Ruby is an object, even the most primitive things below

1. Numeric: 1, -10, 3.14
2. String: 'foo', "bar"
3. Data collection: ['foo', 1, 'bar'], { name: 'Kodacamp', floor: 3}
4. Symbol: :foo, :bar

Class

All the objects created by Class, and have method class.

1. Numeric
2. String
3. Array
4. Hash
5. Symbol

irb(main):001:0> 1.class
=> Integer
irb(main):002:0> [].class
=> Array
irb(main):003:0> {}.class
=> Hash
irb(main):004:0> :foo.class
=> Symbol
irb(main):005:0> 'bar'.class
=> String

Variable

You can assign an object into variable, variable will record the object location in memory. We can use the variable to do data control.

Ruby has 4 kinds of variable

1. local variable
2. global variable
3. instance variable
4. class variable

local variable VS global variable

Create two files sub.rb and scope_test.rb sub.rb

$foo = 1
bar = 1

# scope_test.rb
$foo = 0
bar = 0

puts 'print data before request sub.rb'
puts $foo
puts bar

require_relative 'sub'
puts 'print data after request sub.rb'
puts $foo
puts bar

execute ruby scope_test.rb

 $~ ruby scope_test.rb
print data before request sub.rb
0    -> scope_test.rb global variable $foo
0    -> scope_test.rb local variable bar
print data after request sub.rb
1    -> sub.rb assign value 1 to global variable $foo
0    -> sub.rb assign value 1 to its local variable bar, scope_test.rb local variable bar still 0.

We do not recommend using global variable to handle data. It will let you code difficult to maintain. You will not know somewhere edit the globe variable.

We will introduce instance variables and class variables in another chapter.

Multiple assignment

1. Assign 1, 2, 3 to local variable a, b, c

    a = 1
    b = 2
    c = 3
   

   It can use multiple assignments.

    a, b, c = 1, 2, 3
   

   If you use * in local variable, Ruby will put extra elements to Array

    irb(main):001:0> a,b,*c = 1,2,3,4,5
    => [1, 2, 3, 4, 5] 
    irb(main):002:0> a
    => 1
    irb(main):003:0> b
    => 2
    irb(main):004:0> c
    => [3, 4, 5]
    irb(main):005:0> [a, b, c]
    => [1, 2, [3, 4, 5]]
   

    irb(main):001:0> a, *b, c = 1,2,3,4,5
    => [1, 2, 3, 4, 5] 
    irb(main):002:0> a
    => 1
    irb(main):003:0> b
    => [2, 3, 4]
    irb(main):004:0> c
    => 5
    irb(main):005:0> [a, b, c]
    => [1, [2, 3, 4], 5]
   

2. Exchange values We need to use a temporary variable to store the value then exchange

   irb(main):001:0> a, b = 0, 1
   => [0, 1]
   irb(main):002:0> tmp = a   -> assign local variable a's value to tmp
   => 0
   irb(main):003:0> a = b     -> assign local variable b's value to a
   => 1
   irb(main):004:0> b = tmp   -> assign local variable tmp value to b
   => 0
   irb(main):005:0> a
   => 1
   irb(main):006:0> b
   => 0
   

   If you use multiple assignments, only one line.

   irb(main):001:0> a, b = 0, 1
   => [0, 1]
   irb(main):002:0> a, b = b, a
   => [1, 0]
   irb(main):003:0> a
   => 1
   irb(main):004:0> b
   => 0