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Ruby on Rails内置方法之对象是否为空blank?    puts [ ].blank? #=> true      puts { 1 => 2}.blank? #=> false      puts " cat ".blank? #=> false      puts "".blank? #=> true      puts " ".blank? #自动去除前后空格=> true      puts nil.blank? #=> true Ruby on Rails内置方法之排序：group_by()groups = posts.group_by {|post| post.author_id}Ruby on Rails内置方法之组句：to_sentence()    puts [ "ant", "bat", "cat"].    to_sentence #=> "ant, bat, and cat"      puts [ "ant", "bat", "cat"].    to_sentence(:connector =>     "and not forgetting")      #=> "ant, bat, and not forgetting cat"      puts [ "ant", "bat", "cat"].    to_sentence(:skip_last_comma => true)      #=> "ant, bat and cat" Ruby on Rails内置方法之分组：in_groups_of()    [1,2,3,4,5,6,7].in_groups_of(3)     {|slice| p slice}      #=> [1, 2, 3]      [4, 5, 6]      [7,nil, nil]      [1,2,3,4,5,6,7].in_groups    _of(3,"X") {|slice| p slice}      #=> [1, 2, 3]      [4, 5, 6]      [7,"X", "X"] Ruby on Rails内置方法之字符串方法：    string = "Now is the time"     puts string.at(2) #=> "w"      puts string.from(8) #=> "he time"      puts string.to(8) #=> "Now is th"      puts string.first #=> "N"      puts string.first(3) #=> "Now"      puts string.last #=> "e"      puts string.last(4) #=> "time"      puts string.starts_with?("No") #=> true      puts string.ends_with?("ME") #=> false           count = Hash.new(0)      string.each_char {|ch| count[ch] += 1}      p count #=> {" "=>3, "w"=>1, "m"=>1,     "N"=>1, "o"=>1, "e"=>2, "h"=>1,     "s"=>1, "t"=>2, "i"=>2}

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