Ruby: recursive method

Question

def reverse_append(arr, n)  
    return arr if n < 0 
    reverse_append(arr, n-1)
    arr << n
    arr
end 

reverse_append([],4) #=> [0, 1, 2, 3, 4]
         


        

Answer 1

There's a nice tool you can add to many editors called "Seeing Is Believing", which lets you see what is happening as code runs:

def reverse_append(arr, n)  
  return arr if n < 0 # => false, false, false, false, true
  reverse_append(arr, n-1) # => [], [0], [0, 1], [0, 1, 2]
  arr << n # => [0], [0, 1], [0, 1, 2], [0, 1, 2, 3]
  arr # => [0], [0, 1], [0, 1, 2], [0, 1, 2, 3]
end 

reverse_append([], 3) # => [0, 1, 2, 3]

However, with a name like "reverse_append" it seems like you should see a result that is descending in values:

def reverse_append(arr, n)  
  return arr if n < 0 # => false, false, false, false, true
  reverse_append(arr, n-1) # => [], [0], [1, 0], [2, 1, 0]
  arr.unshift n # => [0], [1, 0], [2, 1, 0], [3, 2, 1, 0]
  arr # => [0], [1, 0], [2, 1, 0], [3, 2, 1, 0]
end 

reverse_append([], 3) # => [3, 2, 1, 0]

In either case, there are a lot of easier ways to generate such an array without relying on recursion:

[*0..3] # => [0, 1, 2, 3]
(0..3).to_a # => [0, 1, 2, 3]

[*0..3].reverse # => [3, 2, 1, 0]
(0..3).to_a.reverse # => [3, 2, 1, 0]