Question:
This function reverses all the items found in the list. Even the inner lists get reversed. My problem is that I do not understand how the reversed_list keeps 7 and 6 after the recursion. Since once the deep_reverse is called, the reversed_list becomes empty and then it keeps the reverse of the inner list which is [5, 4, 3]. What I do not realize is that once this item is appended the reversed_list becomes [7, 6, [5, 4, 3]]def deep_reverse(arr):
# Terminaiton / Base condition
if len(arr) < 1:
return arr
reversed_items = [] # final list to be returned
for item in arr[::-1]:
# If this item is a list itself, invoke deep_reverse to reverse the items recursively.
if type(item) is list:
item = deep_reverse(item)
# append the item to the final list
reversed_items.append(item)
print(reversed_items)
return reversed_items ```
deep_reverse([1, 2, [3, 4, 5], 6, 7])
#printing the reversed_list
[7]
[7, 6]
[5]
[5, 4]
[5, 4, 3]
[7, 6, [5, 4, 3]]
[7, 6, [5, 4, 3], 2]
[7, 6, [5, 4, 3], 2, 1]
[/code]
Answer:
There’s more than one reversed_list
You say:
once the deep_reverse is called, the reversed_list becomes empty
But there isn’t just one
reversed_list. Every call creates a new list (and happens to bind it to a local variable of the same name), but the original call’s reversed_list continues to exist independently of the ones for each recursive call. This is the point of the program’s call stack: Each invocation of a function gets its own space, with its own local variables, separate from that of all other calls (including those to the same function).Each new call of
deep_reverse puts another call frame on top of the existing one, with its own reversed_list. After it finishes populating it, it returns it, and the caller (which may be deep_reverse itself, given the recursion) can do what it likes with it. In the case of your recursive calls, this means rebinding item, which is now bound to the list previously stored in the recursive call’s reversed_list, which can then be appended to the outer call’s own reversed_list.If you have better answer, please add a comment about this, thank you!