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Ruby 中一些好用的方法(注意reduce方法)

2016-07-27 17:57 393 查看
#####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}
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}]}
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}]}
####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)
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}]
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}]
####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]]
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]]
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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