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Draw the environment diagram.
def reverse(lst): if len(lst) <= 1: return lst return reverse(lst[1:]) + [lst] l = [1, [2, 3], 4] rev = reverse(l)
Followup: How would you modify this to deep-reverse a list - that is, if an element of the list is also a list, that element gets reversed as well (and all lists under it)?
What does the following code print?
lst = [None for _ in range(10)] for i in range(10): lst[i] = lambda: i for func in lst: print(func())
Note that all the lambda functions are not called until the second for loop - they are simply defined in the first for loop. By the time the second loop spins
i has taken on the value 9, so the number 9 is printed 10 times. The key here is that the value of
i is never stored inside the lambda function - the lambdas get the value of
i from the outer frame.
9 9 9 9 9 9 9 9 9 9
Implement a function
addup that takes in a
lst of numbers and a target value
n. It should return
True if some subset of
lst adds up to
n. Numbers in
lst may not be reused. For example:
>>> addup([3, 4, 5], 9) True # 4 + 5 = 9 >>> addup([3, 9, 27, 81], 3) True # 3 = 3 >>> addup([3, 9, 27, 81], 33) False
Followup: What needs to be changed if values in
lst can be reused?
def addup(tup, n): if n == 0: return True elif n < 0: return False else: for i, num in enumerate(tup): if addup(tup[i+1:], n - num): return False return True
If values can be reused, then we can change the 7th line to:
if addup(tup, n - num):
Since we don’t remove any elements from
tup in our recursive call, then we can reuse any element any number of times.