在Ruby的默认值可选参数(Optional arguments with default valu

我想创建一个具有默认值的可选参数的函数

def my_function(a = nil, b=nil, c=500)

end

并调用带有参数的功能，我想唯一指定

my_function(b=100)

如何做到这一点的Ruby 1.9.2？

Answer 1:

你不能这样做在Ruby中<2.0（或类似的东西）。 你可以做的最好的是：

def my_function(h = {})
  h[:c] ||= 500
  # Use h[:a], h[:b], h[:c]
  ...
end

my_function(b: 100)

Answer 2:

参数绑定到这样的参数：

1. 只要有在参数列表的开头绑定必选参数，绑定参数左到右
2. 只要有在参数列表的末尾绑定必选参数，绑定参数从右到左
3. 任何剩余的参数绑定到左到右的可选参数
4. 任何剩余的参数被收集到一个数组并结合于图示参数
5. 的块被缠绕成Proc和粘结在该块参数
6. 如果有任何未绑定的参数或吃剩的参数， raise了ArgumentError

下面是一个例子：

def foo(mand1, mand2, opt1=:opt1, opt2=:opt2, *splat, mand3, mand4, &block)
  p local_variables.map {|v| "#{v} = #{eval(v.to_s)}" }
end

foo 1, 2, 3
# ArgumentError: wrong number of arguments (3 for 4+)

foo 1, 2, 3, 4
# mand1 = 1
# mand2 = 2
# opt1 = opt1
# opt2 = opt2
# splat = []
# mand3 = 3
# mand4 = 4
# block = 

foo 1, 2, 3, 4, 5
# mand1 = 1
# mand2 = 2
# opt1 = 3
# opt2 = opt2
# splat = []
# mand3 = 4
# mand4 = 5
# block = 

foo 1, 2, 3, 4, 5, 6
# mand1 = 1
# mand2 = 2
# opt1 = 3
# opt2 = 4
# splat = []
# mand3 = 5
# mand4 = 6
# block = 

foo 1, 2, 3, 4, 5, 6, 7
# mand1 = 1
# mand2 = 2
# opt1 = 3
# opt2 = 4
# splat = [5]
# mand3 = 6
# mand4 = 7
# block = 

foo 1, 2, 3, 4, 5, 6, 7, 8 do end
# mand1 = 1
# mand2 = 2
# opt1 = 3
# opt2 = 4
# splat = [5, 6]
# mand3 = 7
# mand4 = 8
# block = #<Proc:0x007fdc732cb468@(pry):42>

所以，你既可以从上面和例子第3步看，你不能这样做，因为可选参数绑定左到右，但要指定中间的说法。

请注意，这对API的设计含义：你应该以这样的方式，最“不稳定”可选参数，即用户最有可能希望提供自己的那些，是最远的左侧设计您的参数列表。

红宝石2.0现在有关键字参数，这是你在寻找什么：

def foo(m1, m2, o1=:o1, o2=:o2, *s, m3, m4, key1: :key1, key2: :key2, **keys, &b)
  puts local_variables.map {|v| "#{v} = #{eval(v.to_s)}" }
end

foo 1, 2, 3
# ArgumentError: wrong number of arguments (3 for 4+)

foo 1, 2, 3, 4
# m1 = 1
# m2 = 2
# o1 = o1
# o2 = o2
# s = []
# m3 = 3
# m4 = 4
# key1 = key1
# key2 = key2
# b = 
# keys = {}

foo 1, 2, 3, 4, 5
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = o2
# s = []
# m3 = 4
# m4 = 5
# key1 = key1
# key2 = key2
# b = 
# keys = {}

foo 1, 2, 3, 4, 5, 6
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = 4
# s = []
# m3 = 5
# m4 = 6
# key1 = key1
# key2 = key2
# b = 
# keys = {}

foo 1, 2, 3, 4, 5, 6, 7
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = 4
# s = [5]
# m3 = 6
# m4 = 7
# key1 = key1
# key2 = key2
# b = 
# keys = {}

foo 1, 2, 3, 4, 5, 6, 7, 8
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = 4
# s = [5, 6]
# m3 = 7
# m4 = 8
# key1 = key1
# key2 = key2
# b = 
# keys = {}

foo 1, 2, 3, 4, 5, 6, 7, 8, key1: 9
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = 4
# s = [5, 6]
# m3 = 7
# m4 = 8
# key1 = 9
# key2 = key2
# b = 
# keys = {}

foo 1, 2, 3, 4, 5, 6, 7, 8, key1: 9, key2: 10
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = 4
# s = [5, 6]
# m3 = 7
# m4 = 8
# key1 = 9
# key2 = 10
# b = 
# keys = {}

foo 1, 2, 3, 4, 5, 6, 7, 8, key1: 9, key2: 10, key3: 11
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = 4
# s = [5, 6]
# m3 = 7
# m4 = 8
# key1 = 9
# key2 = 10
# b = 
# keys = {:key3=>11}

foo 1, 2, 3, 4, 5, 6, 7, 8, key1: 9, key2: 10, key3: 11, key4: 12 do end
# m1 = 1
# m2 = 2
# o1 = 3
# o2 = 4
# s = [5, 6]
# m3 = 7
# m4 = 8
# key1 = 9
# key2 = 10
# b = #<Proc:0x007fdc75135a48@(pry):77>
# keys = {:key3=>11, key4=>12}

Answer 3:

所以，你想实现的关键字参数？ 这应该是用Ruby 2.0中的新功能，但你可以尝试用参数的哈希，而不是模仿它在1.9.x的。 这里有一个帖子 ，讨论了如何实现这个目标，这给下面的代码示例：

def foo(options = {})
  options = {bar: 'bar'}.merge(options)
  puts "#{options[:bar]} #{options[:buz]}"
end

foo(buz: 'buz') # => 'bar buz'

文章来源: Optional arguments with default value in Ruby