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+# Ruby
+
+收集与翻译均来自reference
+
+
+
+## Variables
+
+
+
+### 基本变量类型
+
+```ruby
+# Nil: 空值 ，例如没有找到定义，下标所指的array对象超出array界限，hash下key不存在
+# string ： 字符型
+full_name = 'Ruby'
+# integer: 整数型
+count = 20
+# float ： 浮点数型
+book_price = 12.3
+#Array : 数组型
+fruits = ['Appel','Orange','Banana']
+#Hash : 哈系类型
+fruite_color = {apple:'read'}
+# Struct: 结构体
+Person = Struct.new(:name,:age)
+person1  = Person.new 'mike', 50
+
+```
+
+
+
+### 变量赋值
+
+```ruby
+# title赋值，默认为”Default title“。如果custom_title不为nil，则title是custom——title
+title =  custom_title || 'Default title'
+# search为空则将params[:search]赋值给search
+search ||= params[:search]
+
+```
+
+
+
+### 数组
+
+```ruby
+# 数组声明
+names = Array.new
+names = Array.new(20)
+# 直接定义一个数组给变量
+fruits = ['Apple', 'Orange', 'Banana']
+fruits = %w(Apple Orange Banana) 
+
+
+# 快速生成数组
+(1..10).to_a  # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+('a'..'e').to_a # ['a', 'b', 'c', 'd', 'e']
+
+fruits.length # 3
+
+fruits.first  # Apple
+fruits.last   # Banana
+
+fruits[0]     # Apple
+fruits[-2]    # Orange
+fruits[3]     # nil
+fruits[1..2]  # ['Orange', 'Banana']
+
+# 遍历
+fruits.each do { |fruit| puts fruit } 
+
+fruits.each_with_index do |fruit, index|
+  puts fruit  # Apple
+  puts index  # 0
+end
+```
+
+### Hash
+
+```ruby
+product = {}
+product['title'] = "Mac Book Pro"
+product[:price] = 1599.99
+product = { 'title' => 'Mac Book Pro', 'price' => 1599.99 }
+product = { title: 'Mac Book Pro', price: 1599.99 }
+puts product.fetch(:cost, 0)  # 返回默认值0
+product.keys   # [:title, :price]
+product.values # ['Mac Book Pro', 1599.99]
+
+product.each do |key, value| 
+  puts key
+  puts value
+end
+```
+
+
+
+
+
+## Function
+
+
+
+```ruby
+# 基本结构
+def func 
+    #some code
+end
+# 带参数的函数
+def greeting(name = 'John')  # = default argument value
+  "Hello #{name}"  # implicit return
+end
+puts greeting('Paul')  # Hello Paul
+
+# 可变参数的函数
+def greeting(*names)
+  names.each { |name| puts name }
+end
+
+# 传入指定命名的参数
+def display_product(price, options = {})
+  puts price, options[:hidden], options[:rounded]
+end
+display_product 1599, hidden: false, rounded: true
+```
+
+
+
+## Class&Modules
+
+class
+
+```ruby
+class ProductManager
+    #some code
+end
+```
+
+modules
+
+```ruby
+# 基础结构
+module CUstomerSupport
+    #some code
+end
+
+# 静态modules方法
+module Display
+  def self.hello
+    puts 'Hello'
+  end
+end
+Display.hello
+```
+
+## Control Flow & loop
+
+### 遍历
+
+```ruby
+loop do
+  puts "Stop loop by using 'break' statement"
+  puts "Skip one occurence by using 'next' statement"
+end
+
+while number < 100
+  puts number
+  number += 1
+end
+
+# Range
+# Range: 遍历表达方式1
+(1..10).each { |i| puts i } 
+# Range: 便利表达方式2
+(1..10).each do |i|  
+  puts i
+end
+
+# 进行计数范围的函数执行
+10.times { puts "Hello World" } # 输出10次Hello World
+
+```
+
+### 条件判断
+
+```ruby
+# Equal ==   And &&   Or ||   Not !
+if action == 1
+  puts "action 1"
+elsif action < 5
+  puts "action not 1 but less than 5"
+else
+  puts "action greater than 5"
+end
+
+#Unless (if的否定)
+puts 'The user is not active' unless active == true
+
+# 三目运算符
+active ? 'The user is active' : 'The user is not active'
+
+# case when else
+case points
+when 0
+  "Not good"
+when 1..50
+  "Better but not great"
+when 51..70
+  "Thats good!"
+when 71..99
+  "Great"
+when 100
+  "Perfect"
+else
+  "Score error"
+```
+
+
+
+## Common Methods
+
+### 输出
+
+```ruby
+# 带换行
+puts 'Hello World'
+# 不带换行
+print 'Hello World with no line break'
+
+```
+
+
+
+### 字符型
+
+```ruby
+# 字符串插值（格式化字符串输出用）
+name = 'Mike'
+message = 'Hello #{name}'
+
+# 获取字符串长度
+'This is a string'.length
+
+# 判断字符串是否为空
+''.empty? # true
+
+
+# 字符串字母大写
+'hello world'.upcase #HELLO WORLD
+
+
+# 字符串字母小写
+'HI'.downcase #hi
+
+
+# 首字母大写 剩余消协
+'miKE'.capitalize # Mike
+
+# 去除字符串空格
+' This is a string with space  '.strip
+
+# 左对齐并且增加字符填充
+'hello'.ljust(20, '.')  # 'hello...............'
+
+# 判断是否为子字符串
+'hello World'.include? 'World' # true
+
+#获取子字符串的下标
+'Welcome to this web site'.index('this') # 11
+
+
+# 获取字符串指定下标的内容（从0开始）
+'This is a string'[1]  # h
+'This is a string'[0..3]  # This
+'This is a string'[-1]  # g (-1代表最后）
+
+# 替换第一个子字符串
+'Hello dog my dog'.sub 'dog', 'cat'. # Hello cat my dog
+
+
+#字符串转换为array
+'Apple Orange Banana'.split ' '  #['Apple', 'Orange', 'Banana']
+
+
+#获取控制台键盘输入
+input = gets
+
+
+#获取命令行的参数（arguments)，例如 ruby main.rb arg1 arg2
+puts ARGV  # ['arg1', 'arg2']
+
+```
+
+### 数字型
+
+```ruby
+
+number.round 2.68  # 3
+number.floor 2.68  # 2
+number.ceil 2.68   # 3
+
+2.next  # 3
+
+# 随机数字
+random_number = rand(1..100)
+```
+
+### 数组
+
+```ruby
+
+fruits = ['Apple', 'Orange', 'Banana']
+
+# 数组是否包含
+fruits.include? 'Orange'  # true
+
+# 排序：正排和倒排
+[1, 5, 2, 4, 3].sort  # [1, 2, 3, 4, 5]
+[1, 2, 3].reverse  # [3, 2, 1]
+
+# 增加到数组末尾
+fruits.push 'Strawberry' # 方法1
+fruits <<  'Raspberry' # 方法2
+
+# 增加元素到数组开头
+fruits.unshift 'Strawberry'
+
+
+# 移除数组最后一个
+fruits.pop 
+# 移除一个
+fruits.delete_at(0) # 方法1 
+fruits.shift # 方法2
+
+
+# array转string
+fruits.join ', '  # 'apple, orange, banana'
+
+
+# 两个数组合并 - 合并赋值给新数组
+array1 = %w(dog cat bird)
+array2 = %w(fish hamster)
+array3 = array1 + array2 #['dog', 'cat', 'bird', 'fish', 'hamster']
+
+
+# 两个数组合并到原有数组基础上
+array1.concat array2 
+puts array1  #['dog', 'cat', 'bird', 'fish', 'hamster']
+
+
+
+# 使用*表达式来构造数组
+puts ['dog', *array2, 'bird']  #['dog', 'fish', 'hamster', bird']
+```
+
+### 类型转换
+
+```ruby
+
+123.to_s   # convert number to string "123"
+"123".to_i # convert string to integer 123
+"123".to_f # convert string to float 123.0
+
+
+#字符串转换为array（上面的例子）
+'Apple Orange Banana'.split ' '  #['Apple', 'Orange', 'Banana']
+# array转string （上面的例子）
+fruits.join ', '  
+```
+
+### 日期和时间
+
+```ruby
+Time.now.utc    # 2020-12-13 03:17:05 UTC
+Time.now.to_i   # Timestamp 1607829496
+
+christmas = Time.new(2020, 12, 25)  #
+puts christmas.wday # return 5 (Thursday)
+
+now = Time.now  # current time: 2020-12-13 03:08:15 +0000
+now.year    # 2020
+now.month   # 12
+now.day     # 13
+now.hour    # 3
+now.min     # 8
+now.sec     # 15
+now.sunday? # true
+
+past = Time.now - 20  # 返回当前时间减去20秒
+past_day = Time.now - 86400 # 60 secs * 60 mins * 24 hours
+past_day = Time.now - 1.day # 仅在Rail生效
+
+time = Time.new
+time.strftime("%d of %B, %Y")   # 格式化输出： "25 of December, 2020"
+```
+
+### 正则表达式
+
+```ruby
+zip_code = /\d{5}/
+"Hello".match zip_code  # nil
+"White House zip: 20500".match zip_code  # 20500
+"White House: 20500 and Air Force: 20330".scan zip_code # ['20500', '20330'] 
+"Apple Orange Banana".split(/\s+/) # ['Apple','Orange', 'Banana']
+```
+
+### I/O
+
+文件
+
+```ruby
+# 读文件
+text = File.read('exemple.txt')
+
+# 按行读取文件
+lines = File.readlines("exemple.txt")
+lines.each do |line|
+  puts "Line: #{line}"
+end
+
+# 写文件
+File.write('exemple.txt', 'text to write...')
+
+File.open(“index.html”, “w”) do | file | 
+  file.puts ‘text to write’ 
+end
+
+
+# 读取csv
+require 'csv'
+table = CSV.parse(File.read("products.csv"), headers: true)
+table[0]["id"] # 1000
+table[0]["name"] # "Mac Book Pro"
+
+# 写csv
+products = [
+  { name: "Mac Book Pro", price: 1599 },
+  { name: "IPad Pro", price: 799 }
+]
+CSV.open("products.csv", "w", headers: products.first.keys) do |csv|
+  products.each { |product| csv << product.values }
+end
+```
+
+
+
+## Errors/Exceptions Handling
+
+```ruby
+# 引发（raise）一个异常 并输出异常消息
+raise "This is an exception"
+
+
+```
+
+
+
+## Other Tips
+
+- `irb` 进入交互式ruby（interactive Ruby)模式中
+
+## Reference
+
+- https://dev.to/ericchapman/my-beloved-ruby-cheat-sheet-208o