我的选择排序算法怎么了？

Question

这个答案对受过训练的人来说可能是显而易见的，但我已经读了几个小时了，我的眼睛很紧张，我似乎看不见虫子。

下面是我编写的两个选择排序的实现，它们都没有正确地对输入进行排序。您可以在在线解释器上使用播放这段代码。

def selection_sort_enum(array)
  n = array.length - 1

  0.upto(n - 1) do |i|
    smallest = i

    (i + 1).upto(n) do |j|
      smallest = j if array[j] < array[i]
    end

    array[i], array[smallest] = array[smallest], array[i] if i != smallest
  end
end

def selection_sort_loop(array)
  n = array.length - 1
  i = 0
  while i <= n - 1
    smallest = i
    j = i + 1
    while j <= n
      smallest = j if array[j] < array[i]
      j += 1
    end
    array[i], array[smallest] = array[smallest], array[i] if i != smallest
    i += 1
  end
end

下面是第一个实现的测试，selection_sort_enum

puts "Using enum:"
a1 = [*1..10].shuffle
puts "Before sort: #{a1.inspect}"
selection_sort_enum(a1)
puts "After sort: #{a1.inspect}"

下面是第二个实现的测试，selection_sort_loop

puts "Using while:"
a2 = [*1..10].shuffle
puts "Before sort: #{a2.inspect}"
selection_sort_enum(a2)
puts "After sort: #{a2.inspect}"

下面是第一个实现selection_sort_enum的输出

Using enum:
Before sort: [7, 5, 2, 10, 6, 1, 3, 4, 8, 9]
After sort:  [4, 3, 1, 9, 5, 2, 6, 7, 8, 10]

下面是第二个实现的输出，selection_sort_loop

Using while:
Before sort: [1, 10, 5, 3, 7, 4, 8, 9, 6, 2]
After sort:  [1, 2, 4, 3, 6, 5, 7, 8, 9, 10]

Answer 1

在这两个code snippets中，您都在与索引i而不是索引smallest进行比较。

这应该是可行的：

def selection_sort_enum(array)
  n = array.length - 1

  0.upto(n - 1) do |i|
    smallest = i

    (i + 1).upto(n) do |j|
      smallest = j if array[j] < array[smallest]
    end
    array[i], array[smallest] = array[smallest], array[i] if i != smallest
  end
end

def selection_sort_loop(array)
  n = array.length - 1
  i = 0
  while i <= n - 1
    smallest = i
    j = i + 1
    while j <= n
      smallest = j if array[j] < array[smallest]
      j += 1
    end
    array[i], array[smallest] = array[smallest], array[i] if i != smallest
    i += 1
  end
end

产出：

Using enum:
Before sort: [5, 6, 7, 9, 2, 4, 8, 1, 10, 3]
After sort: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Using while:
Before sort: [6, 5, 9, 2, 1, 3, 10, 4, 7, 8]
After sort: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

链接到解决方案：http://ideone.com/pKLriY

Answer 2

def selection_sort_enum(array)
  n = array.length - 1 

  0.upto(n) do |i| # n instead of (n - 1)
    smallest_index = i

    (i + 1).upto(n) do |j|
      smallest_index = j if array[j] < array[i]
    end

    puts "#{array}", smallest_index
    array[i], array[smallest_index] = array[smallest_index], array[i] if i != smallest_index
  end
end

Answer 3

你可能会对此感兴趣：

def selection_sort_enum(array)
  n = array.length - 1

  0.upto(n - 1) do |i|
    smallest = i

    (i + 1).upto(n) do |j|
      smallest = j if array[j] < array[i]
    end

    array[i], array[smallest] = array[smallest], array[i] if i != smallest
  end
  array # <-- added to return the modified array
end

def selection_sort_loop(array)
  n = array.length - 1
  i = 0
  while i <= n - 1
    smallest = i
    j = i + 1
    while j <= n
      smallest = j if array[j] < array[i]
      j += 1
    end
    array[i], array[smallest] = array[smallest], array[i] if i != smallest
    i += 1
  end
  array # <-- added to return the modified array
end

require 'fruity'

ARY = (1 .. 100).to_a.shuffle
compare do
  _enum { selection_sort_enum(ARY.dup) }
  _loop { selection_sort_loop(ARY.dup) }
end

其结果是：

# >> Running each test once. Test will take about 1 second.
# >> _enum is faster than _loop by 3x ± 1.0