Question

PYTHON:

MUST NOT USE PYTHONS BUILT IN FUNCTIONS SUCH AS  result.append MUST BE SOLVED OUT WITHOUT SUCH FUNCTIONS

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1. Write a function interleave(vals1, vals2) that takes as inputs two lists vals1 and vals2 and uses recursion to construct and return a new string that is formed by interleaving the elements in the lists vals1 and vals2 to create a single list. In other words, the new list should alternate elements from the two input lists: the first element from vals1, followed by the first element from vals2, followed by the second element from vals1, followed by the second element from vals2, etc. If one of the lists is longer than the other, its “extra” elements – the ones with no counterparts in the shorter list – should appear immediately after the interleaved elements (if any) in the new list. For example:

>>> interleave([1, 1, 1], [2, 2, 2])
result: [1, 2, 1, 2, 1, 2]

>>> interleave([3, 4, 5, 6], [7, 8, 9, 0])
result: [3, 7, 4, 8, 5, 9, 6, 0]

>>> interleave([0, 0, 0, 0], [1, 1])  # two extra elements in first list
result: [0, 1, 0, 1, 0, 0]

>>> interleave([2, 1, 0], [3, 4, 5, 6])  # one extra element in second list
result: [2, 3, 1, 4, 0, 5, 6])

>>> interleave([1, 2], [])     # all of the first list's values are extra!
result: [1, 2]

>>> interleave([], [3, 4, 5])  # all of the second list's values are extra!
result: [3, 4, 5]

>>> interleave([], [])        
result: []

Hint: You will need more than one base case.

Answer 1

CODE

def interleave(a, b):
if len(a) == 0: # base case
return b
  
if len(b) == 0: # base case
return a
  
if not(a[-1] == "True" or a[-1] == "False"): # first case for non empty lists
a.append("True")
b.append("False")
  
if a[0] == "True": # base case
return b[:-1]
  
if b[0] == "True": # base case
return a[:-1]
  
if a[-1] == "True": # taking element from a
a[-1] = "False"
b[-1] = "True"
return [a[0]] + interleave(a[1:], b)
  
if b[-1] == "True": # taking element from b
b[-1] = "False"
a[-1] = "True"
return [b[0]] + interleave(a, b[1:])
  
  
# sample runs
print(interleave([1, 1, 1], [2, 2, 2]))
print(interleave([3, 4, 5, 6], [7, 8, 9, 0]))
print(interleave([0, 0, 0, 0], [1, 1]))
print(interleave([2, 1, 0], [3, 4, 5, 6]))
print(interleave([1, 2], []))
print(interleave([], [3, 4, 5]))

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