#####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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4array = [{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}
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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19array = [{"school_id"=>"1",
"plan_type"=>"All",
"view"=>"true",
"create"=>"true",
"approve"=>"true",
"grant"=>"true",
"region_id"=>nil},
{"school_id"=>"1",
"edit"=>"true",
{"school_id"=>"2",
"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","edit"=>"true","region_id"=>nil}],"2"=>[{"school_id"=>"2","region_id"=>nil}]}
多条件分组：
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2array.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",["2","All"]=>[{"school_id"=>"2","region_id"=>nil}]}
####reduce
reduce作用和inject优点类似，但是它比inject还要简洁
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3(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)
以group_by中的array为例，将相同school_id的hash进行合并
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2array.group_by { |e| e["school_id"] }.values.map { |i| i.inject({}) { |sum,e| sum.merge e }}
=> [{"school_id"=>"1","region_id"=>nil,"edit"=>"true"},{"school_id"=>"2","region_id"=>nil}]
可以使用inject将hash合并，但是使用reduce效果会更好
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2array.group_by { |e| e["school_id"] }.values.map { |i| i.reduce(:merge) }
=> [{"school_id"=>"1","region_id"=>nil}]
####zip
zip可以将两个数组合并为一个二维数组
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4a= [1,2,3,4,5]
b=[6,7,8,9,10]
a.zip(b)
=> [[1,6],[2,7],[3,8],[4,9],[5,10]]
如果a.length > b.length，b中缺少的以nil代替
，
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4a=[1,5,6]
b=[6,10],[6,nil]]
如果a.length < b.length，b中多余的直接被丢弃
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4a= [1,4]
b=[6,9]]
