Array#bsearch in Ruby returns nil 如果0索引值是唯一满足条件的值

Question

我在下面提供了测试用例，这只是一个满足我好奇心的问题。

Array#bsearch 的 ruby 文档说：

"这个方法returns第i个元素。如果i等于ary.size，它returns nil。"

我还没有完全理解这个解释。有人可以更深入地解释一下吗？

def greater_than_8(ary)                                                                        
  a = ary.bsearch { |x| x.to_f > 8 }                                                           
  a.nil? || a.to_f < 8 ? "No number greater than 8" : a                                        
end                                                                                            
                                                                                               
                                                                                               
ary = [0, 4, 7, 10, 12]                                                                        
puts greater_than_8(ary)                                                                       
#=> 10                                                                                         
                                                                                               
# Shouldn't this return 14?                                                                    
ary = [14, 3, 7]                                                                               
puts greater_than_8(ary)                                                                       
#=> "No number greater than 8" 

# THERE IT IS!                                                                
ary = [14, 13, 7]                                                                               
puts greater_than_8(ary)
#=> 14
                                                                                               
ary = [8, 8, 8, 8]                                                                             
puts greater_than_8(ary)                                                                       
#=> "No number greater than 8"                                                                 
                                                                                               
ary = [8.01, 123]                                                                              
puts greater_than_8(ary)                                                                       
#=> 8.01  

ary = ["one hundred", "151", "120", 9]                                                         
puts greater_than_8(ary)                                                                       
#=> 151

Answer 1

欢迎来到堆栈溢出！二分查找的前提是要查找的列表必须是有序的，而 [14, 3, 7] 不是。在排序版本 ([3, 7, 14]) 上调用您的函数应该会产生您期望的结果。

the elements of the array must be monotone (or sorted) with respect to the block.

Answer 2

Array#bsearch 的部分文档要求如下：

You can use this method in two modes: a find-minimum mode and a find-any mode. In either case, the elements of the array must be monotone (or sorted) with respect to the block.

在您的示例中，您使用的是查找最小值模式。从那里我们会查看您使用过的一些输入，看看它们是否单调。

# Monotone increasing, allowed
ary = [0, 4, 7, 10, 12]                                                                        

# Not monotone, not allowed                                                                                            
ary = [14, 3, 7]                                                                               

# Monotone decreasing, allowed
ary = [14, 13, 7]