What's wrong with my selection sort algorithm?

Question

The answer might be obvious to the trained eye, but I've been hitting the books for a few hours now, my eyes are straining, and I can't seem to see the bug.

Below are two implementations of selection sort I wrote, and neither is sorting the input correctly. You can play with this code on an online interpreter.

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

Here's the test of the first implementation, selection_sort_enum:

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

Here's the test of the second implementation, selection_sort_loop:

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

Here's the output of the first implementation, selection_sort_enum:

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]

Here's the output of the second implementation, 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

In both the code snippets you are comparing with index i instead of index smallest.

This should work :

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

Output :

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]

Link to solution : 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