#####inject
inject是我使用最频繁的方法了，它的强大之处在于可以方便的对嵌套的数组，哈希等混合数据结构进行合并或求和， 可以有效减少代码量。
例如最常见的数组套哈希：
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array = [{a:100}, {b:200}, {c:300}]
array.inject(0) { |sum, e| sum += e.values.first } #600
array.inject({}) { |sum, e| sum.merge e } #{:a=>100, :b=>200, :c=>300}
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- inject():括号中的是sum的初始值
- sum, e: 和在前，数组元素在后，中间必须以逗号隔开
#####group_by
group_by适用于对于数组和hash的分组. 在stackoverflow,我经常遇到这样的问题：
array根据相同的school_id进行分组
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array = [{"school_id"=>"1",
"plan_type"=>"All",
"view"=>"true",
"create"=>"true",
"approve"=>"true",
"grant"=>"true",
"region_id"=>nil},
{"school_id"=>"1",
"plan_type"=>"All",
"edit"=>"true",
"region_id"=>nil},
{"school_id"=>"2",
"plan_type"=>"All",
"edit"=>"true",
"grant"=>"true",
"region_id"=>nil}]
array.group_by { |e| e["school_id"] }
=> {"1"=>[{"school_id"=>"1", "plan_type"=>"All", "view"=>"true", "create"=>"true", "approve"=>"true", "grant"=>"true", "region_id"=>nil}, {"school_id"=>"1", "plan_type"=>"All", "edit"=>"true", "region_id"=>nil}], "2"=>[{"school_id"=>"2", "plan_type"=>"All", "edit"=>"true", "grant"=>"true", "region_id"=>nil}]}
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多条件分组：
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array.group_by { |e| [e["school_id"], e["plan_type"]] } #将多个条件放在数组当中
=> {["1", "All"]=>[{"school_id"=>"1", "plan_type"=>"All", "view"=>"true", "create"=>"true", "approve"=>"true", "grant"=>"true", "region_id"=>nil}, {"school_id"=>"1", "plan_type"=>"All", "edit"=>"true", "region_id"=>nil}], ["2", "All"]=>[{"school_id"=>"2", "plan_type"=>"All", "edit"=>"true", "grant"=>"true", "region_id"=>nil}]}
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####reduce
reduce作用和inject优点类似，但是它比inject还要简洁
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(5..10).inject {|sum, n| sum + n } # 45
(5..10).reduce(:+) # 45
(5..10).reduce(1, :+) # 46 (括号中第一个参数是初始值，第二个是方法名)
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以group_by中的array为例，将相同school_id的hash进行合并
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array.group_by { |e| e["school_id"] }.values.map { |i| i.inject({}) { |sum ,e| sum.merge e }}
=> [{"school_id"=>"1", "plan_type"=>"All", "view"=>"true", "create"=>"true", "approve"=>"true", "grant"=>"true", "region_id"=>nil, "edit"=>"true"}, {"school_id"=>"2", "plan_type"=>"All", "edit"=>"true", "grant"=>"true", "region_id"=>nil}]
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可以使用inject将hash合并，但是使用reduce效果会更好
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array.group_by { |e| e["school_id"] }.values.map { |i| i.reduce(:merge) }
=> [{"school_id"=>"1", "plan_type"=>"All", "view"=>"true", "create"=>"true", "approve"=>"true", "grant"=>"true", "region_id"=>nil, "edit"=>"true"}, {"school_id"=>"2", "plan_type"=>"All", "edit"=>"true", "grant"=>"true", "region_id"=>nil}]
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####zip
zip可以将两个数组合并为一个二维数组
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a= [1,2,3,4,5]
b=[6,7,8,9,10]
a.zip(b)
=> [[1, 6], [2, 7], [3, 8], [4, 9], [5, 10]]
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如果a.length > b.length，b中缺少的以nil代替 ，
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a=[1,2,3,4,5,6]
b=[6,7,8,9,10]
a.zip(b)
=> [[1, 6], [2, 7], [3, 8], [4, 9], [5, 10], [6, nil]]
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如果a.length < b.length，b中多余的直接被丢弃
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a= [1,2,3,4]
b=[6,7,8,9,10]
a.zip(b)
=> [[1, 6], [2, 7], [3, 8], [4, 9]]
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