与红宝石

Question

默认值可选参数我想创建一个具有可选参数的默认值

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

end 

，并调用与参数的函数的功能，我想只指定

my_function(b=100) 

如何在Ruby 1.9.2中完成此操作？

Answer 1

你不能在Ruby < 2.0中那样做（或类似的）。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

所以你试图实现关键字参数？这应该是Ruby 2.0中的一个新功能，但您可以尝试在1.9.x中用参数散列来模仿它。 Here's a post，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' 

Answer 3

参数绑定到的参数是这样的：

1. 只要有未绑定的强制参数在的开始参数列表，从左到右绑定参数
2. 只要在参数列表末尾有未绑定的强制参数，从右到左绑定参数
3. 任何剩余参数都绑定到opti左到右Onal地区参数
4. 任何剩余的参数被收集到一个数组并结合到图示参数
5. 的块被缠绕成Proc和如果存在任何未结合的参数绑定到块参数
6. 或剩余的参数，raise的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，你可以看到从上面的第3步和示例都可以看出，不能这样做，因为可选参数是从左到右绑定的，但是您想指定中间参数。

请注意，这对API设计有影响：您应该设计参数列表，使得最“不稳定”的可选参数（即用户最可能想要提供的参数）是最靠左的。

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}