ruby map, reduce, select, reject, group_by

a = %w(a b cD)

map:
     针对每个element进行变换并返回整个修改后的map

     a.map do |item|
        a.upcase
     end

      a.map(&:upcase) 一样的效果， &:代表了item, 太简洁了，但格式难记。

reduce:
     把array变换为一个值后返回。 
   
     a.reduce(:+) #=> "abcD"

     (5..10).reduce(0) do |sum, value|
        sum + value
   end

      等于 (1..100).reduce(:+)

     reduct(0)里面的0是代表sum的初始值

select:
   根据条件返回一个子集   

   (1..8).select { |x| x % 2 == 0 } #=> [2, 4, 6, 8]

reject:
   根据条件剔除一个子集

   (1..8).reject { |x| x % 2 == 0 } #=> [1, 3, 5, 7] group_by:     根据条件组成Map      
 langs.group_by { |lang| lang[0] } #=> {"r"=>["ruby"], "p"=>["python", "perl"]}
 a=%w(chenxiao chenmin chensiheng xiaochen liyulong) 
 a.group_by{|item| item.index("chen") != nil} false=>["liyulong"], 
true=>["chenxiao", "chenmin", "chensiheng", "xiaochen"] 
Ruby you are the best friend of programmer!