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

1. The method reverse_append([],4) is called
2. Since 4 >= 0, the return statement does not get called.
3. The method reverse_append([],3) is called.
4. Since 3 >= 0, the return statement does not get called.
5. The method reverse_append([],2) is called.
6. Since 2 >= 0, the return statement does not get called.
7. The method reverse_append([],1) is called.
8. Since 1 >= 0, the return statement does not get called.
9. The method reverse_append([],0) is called.
10. Since 0 >= 0, the return statement does not get called.
11. The method reverse_append([],-1) is called.
12. Since -1 < 0, the array ([]) is returned.
13. We pop up one level in our call stack, to where n = 0 and arr = [].
14. arr << n and arr is returned, so now arr = [0].
15. We pop up one level in our call stack, to where n = 1 and arr = [0].
16. arr << n and arr is returned, so now arr = [0, 1].
17. We pop up one level in our call stack, to where n = 2 and arr = [0, 1].
18. arr << n and arr is returned, so now arr = [0, 1, 2].
19. We pop up one level in our call stack, to where n = 3 and arr = [0, 1, 2].
20. arr << n and arr is returned, so now arr = [0, 1, 2, 3].
21. We pop up one level in our call stack, to where n = 4 and arr = [0, 1, 2, 3].
22. arr << n and arr is returned, so now arr = [0, 1, 2, 3, 4].
23. Finally, the "top-level" method returns, and we have our final result.