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

Well step through the code with the supplied parameters. The first step is to check if n < 0 which its not. If it isn't 0 reverse append with [], 3 and appends the that array the number and then returns the array.

So it takes the array, adds 4 to it after it has gone through the step of dealing with [], 3, [], 2, [],1 and [], 0. So the first call that will succeed is just returning the array when it gets below 0, next is 0 gets appended, then one, then 2, then 3 and lastly the original call with 4 gets added arr << n.