與紅寶石

Question

默認值可選參數我想創建一個具有可選參數的默認值

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

end 

，並調用與參數的函數的功能，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中那樣做（或類似的）。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

所以你試圖實現關鍵字參數？這應該是Ruby 2.0中的一個新功能，但您可以嘗試在1.9.x中用參數散列來模仿它。 Here's a post，討論瞭如何實現這個目標，這給下​​面的代碼示例：

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

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

Answer 3

參數綁定到的參數是這樣的：

1. 只要有未綁定的強制參數在的開始參數列表，從左到右綁定參數
2. 只要在參數列表末尾有未綁定的強制參數，從右到左綁定參數
3. 任何剩餘參數都綁定到opti左到右Onal地區參數
4. 任何剩餘的參數被收集到一個數組並結合到圖示參數
5. 的塊被纏繞成Proc和如果存在任何未結合的參數綁定到塊參數
6. 或剩餘的參數，raise的ArgumentError

下面是一個例子：

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，你可以看到從上面的第3步和示例都可以看出，不能這樣做，因爲可選參數是從左到右綁定的，但是您想指定中間參數。

請注意，這對API設計有影響：您應該設計參數列表，使得最「不穩定」的可選參數（即用戶最可能想要提供的參數）是最靠左的。

紅寶石2.0現在有關鍵字參數，這是你在尋找什麼：

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}