smallsense - 2 years ago 85

Ruby Question

Given an array:

`array = [[:a,:b],[:a,:c],[:c,:b]]`

Return the following hash:

`hash = { :a => [:b,:c] , :c => [:b] }`

`hash = Hash[array]`

`hash = { :a => :c , :c => :b }`

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Answer Source

Using functional baby steps:

```
irb:01.0> array = [[:a,:b],[:a,:c],[:c,:b]]
#=> [[:a, :b], [:a, :c], [:c, :b]]
irb:02.0> array.group_by(&:first)
#=> {:a=>[[:a, :b], [:a, :c]], :c=>[[:c, :b]]}
irb:03.0> array.group_by(&:first).map{ |k,a| [k,a.map(&:last)] }
#=> [[:a, [:b, :c]], [:c, [:b]]]
irb:04.0> Hash[ array.group_by(&:first).map{ |k,a| [k,a.map(&:last)] } ]
#=> {:a=>[:b, :c], :c=>[:b]}
```

Using imperative style programming:

```
irb:10.0> h = Hash.new{ |h,k| h[k]=[] }
#=> {}
irb:11.0> array.each{ |k,v| h[k] << v }
#=> [[:a, :b], [:a, :c], [:c, :b]]
irb:12.0> h
#=> {:a=>[:b, :c], :c=>[:b]}
```

As an imperative one-liner:

```
irb:13.0> h = Hash.new{ |h,k| h[k]=[] }.tap{ |h| array.each{ |k,v| h[k] << v } }
#=> {:a=>[:b, :c], :c=>[:b]}
```

Or using everyone's favorite `inject`

:

```
irb:14.0> array.inject(Hash.new{ |h,k| h[k]=[] }){ |h,(k,v)| h[k] << v; h }
#=> {:a=>[:b, :c], :c=>[:b]}
```

If you really want to have single values not collided as an array, you can either un-array them as a post-processing step, or use a different hash accumulation strategy that only creates an array upon collision. Alternatively, wrap your head around this:

```
irb:17.0> hashes = array.map{ |pair| Hash[*pair] } # merge many mini hashes
#=> [{:a=>:b}, {:a=>:c}, {:c=>:b}]
irb:18.0> hashes.inject{ |h1,h2| h1.merge(h2){ |*a| a[1,2] } }
#=> {:a=>[:b, :c], :c=>:b}
```