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# ruby

## 那些奇怪的语法

## 数组切片

> > a = \[1,2,3,4] => \[1, 2, 3, 4] a\[4] => nil a\[0,1] => \[1] a\[0..1] => \[1, 2] a\[0...1] => \[1] a\[0,0] => \[] a\[3,0] => \[] a\[3,4] => \[4] a\[2,10] => \[3, 4] a\[4] # 这一行+下几行会非常奇怪 => nil a\[4,10] => \[] # a\[4]明明已经nil了，居然还是\[] a\[5] => nil # a\[5]是nil,可和a\[4]不同得到的是nil a\[5,10] => nil

## hash(dict)默认值

> > d = Hash.new(\[]) => {} d\[:one] << 'a' => \["a"] d\[:two] << 'b' => \["a", "b"] d\[:one] => \["a", "b"] d\[:three] => \["a", "b"]

需要这样避免

```
hash = Hash.new {|hash, key| hash[key] = [] }
hash[:one] << "uno"
hash[:two] << "dos"

assert_equal ['uno'], hash[:one]
assert_equal ['dos'], hash[:two]
assert_equal [], hash[:three]
```