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Julia関数のデフォルト引数

Juliaの関数は引数を省略したときのデフォルト値を指定できる。

julia> function f(a, b=0)
           print("$a $b")
       end
f (generic function with 2 methods)

julia> f(1, 2)
1 2
julia> f(1)
1 0

まあ、これはこの手の言語としてはわりに普通の機能だ。でも、Juliaの場合、ちょっと面白いことに、このデフォルト値の部分に他の引数を参照した式がかける。

julia> function g(a::Vector{Int64}, b=length(a))
           print("$a $b")
       end
g (generic function with 2 methods)

julia> g([1,2,3])
[1, 2, 3] 3
julia> g([1,2,3], 10)
[1, 2, 3] 10

これは地味に便利。

ちなみにPythonでは書けない。

In [1]: def g(a, b=len(a)):
   ...:     print(a, b)
   ...:     
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-e17b8e13041f> in <module>()
----> 1 def g(a, b=len(a)):
      2     print(a, b)
      3 

NameError: name 'a' is not defined

こんな感じでエラーがでる。外部変数を参照して宣言することはできるが、宣言の際に評価されるだけなので挙動は全然違う

In [5]: x = [1,2]

In [6]: def g(a, b=len(x)):
   ...:     print(a,b)
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