Introduction to Programming Using Python, Y. Daniel Liang

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Chapter 15 Recursion


Section 15.2 Problem: Computing Factorials
15.1  Which of the following statements are true?
A. Every recursive function must have a base case or a stopping condition.
B. Every recursive call reduces the original problem, bringing it increasingly closer to a base case until it becomes that case.
C. Infinite recursion can occur if recursion does not reduce the problem in a manner that allows it to eventually converge into the base case.
D. Every recursive function must have a return value.
E. A recursive function is invoked differently from a non-recursive function.

15.2  Fill in the code to complete the following function for computing factorial.

def factorial(n):
    if n == 0: # Base case
        return 1
    else:
        return _____________________ # Recursive call
A. n * (n - 1)
B. n
C. n * factorial(n - 1)
D. factorial(n - 1) * n

15.3  What are the base cases in the following recursive function?

  def xfunction(n):
      if n > 0:
          print(n % 10)
          xfunction(n // 10)
A. n > 0
B. n <= 0
C. no base cases
D. n < 0

15.4  Analyze the following recursive function.

  def factorial(n):
      return n * factorial(n - 1)
  
A. Invoking factorial(0) returns 0.
B. Invoking factorial(1) returns 1.
C. Invoking factorial(2) returns 2.
D. Invoking factorial(3) returns 6.
E. The function runs infinitely and causes a StackOverflowError.

15.5  How many times is the factorial function in Listing 15.1 invoked for factorial(5)?
A. 3
B. 4
C. 5
D. 6

Section 15.3 Problem: Computing Fibonacci Numbers
15.6  Which of the following statements are true?
A. The Fibonacci series begins with 0 and 1, and each subsequent number is the sum of the preceding two numbers in the series.
B. The Fibonacci series begins with 1 and 1, and each subsequent number is the sum of the preceding two numbers in the series.
C. The Fibonacci series begins with 1 and 2, and each subsequent number is the sum of the preceding two numbers in the series.
D. The Fibonacci series begins with 2 and 3, and each subsequent number is the sum of the preceding two numbers in the series.

15.7  How many times is the fib function in Listing 15.2 invoked for fib(5)?
A. 14
B. 15
C. 25
D. 31
E. 32

15.8  Fill in the code to complete the following function for computing a Fibonacci number.

def fib(index):
    if index == 0: # Base case
        return 0
    elif index == 1: # Base case
        return 1
    else: # Reduction and recursive calls
        return _________________________
A. fib(index - 1)
B. fib(index - 2)
C. fib(index - 1) + fib(index - 2)
D. fib(index - 2) + fib(index - 1)

Section 15.4 Problem Solving Using Recursion
15.9  In the following function, what is the base case?

def xfunction(n):
    if n == 1:
        return 1
    else
        return n + xfunction(n - 1)
A. n is 1.
B. n is greater than 1.
C. n is less than 1.
D. no base case.

15.10  What is the return value for xfunction(4) after calling the following function?

def xfunction(n):
    if n == 1:
        return 1;
    else:
        return n + xfunction(n - 1)
A. 12
B. 11
C. 10
D. 9

15.11  Fill in the code to complete the following function for checking whether a string is a palindrome.

def isPalindrome(s):
    if len(s) <= 1: # Base case
        return True
    elif _____________________________
        return False
    else:
        return isPalindrome(s.substring(1, len(s) - 1))
A. s[0] != s[-1]: # Base case
B. s[0] != s[len(s)]: # Base case
C. s[1] != s[len(s) - 1]: # Base case
D. s[1] != s[len(s)]: # Base case

15.12  Analyze the following code:

def xfunction(x, length):
    print(x[length - 1], end = " ")
    xfunction(x, length - 1)

x = [12345]
xfunction(x, 5)
A. The program displays 1 2 3 4 6.
B. The program displays 1 2 3 4 5 and then raises an index out of range exception.
C. The program displays 5 4 3 2 1.
D. The program displays 5 4 3 2 1 and then raises an index out of range exception.

Section 15.5 Recursive Helper functions
15.13  Fill in the code to complete the following function for checking whether a string is a palindrome.

def isPalindrome(s):
    return isPalindromeHelper(s, 0, len(s) - 1)

def isPalindromeHelper(s, low, high):
    if high <= low: # Base case
      return True
    elif s[low] != s[high]: # Base case
      return False
    else:
      return ____________________________
A. isPalindromeHelper(s)
B. isPalindromeHelper(s, low, high)
C. isPalindromeHelper(s, low + 1, high)
D. isPalindromeHelper(s, low, high - 1)
E. isPalindromeHelper(s, low + 1, high - 1)

15.14  Fill in the code to complete the following function for sorting a list.

def sort(lst):
    _________________________ # Sort the entire list

def sortHelper(lst, low, high):
    if low < high:
        # Find the smallest number and its index in lst[low .. high]
        indexOfMin = low
        min = lst[low]
        for i in range(low + 1, high + 1):
            if lst[i] < min:
                min = lst[i]
                indexOfMin = i

        # Swap the smallest in list(low .. high) with list(low)
        lst[indexOfMin] = lst[low]
        lst[low] = min

        # Sort the remaining list(low+1 .. high)
        sortHelper(lst, low + 1, high)
A. sortHelper(lst)
B. sortHelper(lst, len(lst) - 1)
C. sortHelper(lst, 0, len(lst) - 1)
D. sortHelper(lst, 0, len(lst) - 2)

15.15  Fill in the code to complete the following function for binary search.

def recursiveBinarySearch(lst, key):
    low = 0
    high = len(lst) - 1
    return ________________________________________

def recursiveBinarySearchHelper(lst, key, low, high):
    if low > high: # The list has been exhausted without a match
        return ?low - 1

    mid = (low + high) // 2
    if key < lst[mid]:
        return recursiveBinarySearchHelper(lst, key, low, mid - 1)
    elif key == lst[mid]:
        return mid
    else:
        return recursiveBinarySearchHelper(lst, key, mid + 1, high)
A. recursiveBinarySearchHelper(lst, key)
B. recursiveBinarySearchHelper(lst, key, low + 1, high - 1)
C. recursiveBinarySearchHelper(lst, key, low - 1, high + 1)
D. recursiveBinarySearchHelper(lst, key, low, high)

15.16  What will displayed by the following code?

def main():
    times = count("abcabc", 'a')
    print(ch + " appears " + str(times) + (" times " if times > 1 else " time ") + "in " + s)
    
def count(s, a):
    return countHelper(s, a, len(s) - 1)

def countHelper(s, a, high):
    result = 0;
    if high > 0:
        result = countHelper(s, a, high - 1) + (1 if s[high] == a else 0)

    return result;

main()
A. a appears 1 times in abcdabc
B. a appears 2 times in abcdabc
C. a appears 1 time in abcdabc
D. a appears 2 time in abcdabc

Section 15.7 Tower of Hanoi
15.17  How many times is the recursive moveDisks function invoked for 3 disks?
A. 3
B. 7
C. 10
D. 14

15.18  How many times is the recursive moveDisks function invoked for 4 disks?
A. 5
B. 10
C. 15
D. 20

15.19  Analyze the following two programs:

A:

def xfunction(length):
    if length > 1:
        print(length - 1, end = " ")
        xfunction(length - 1)

xfunction(5)

B:

def xfunction(length):
    while length > 1:
        print(length - 1, end = " ")
        xfunction(length - 1)

xfunction(5)
A. The two programs produce the same output 5 4 3 2 1.
B. The two programs produce the same output 1 2 3 4 5.
C. The two programs produce the same output 4 3 2 1.
D. The two programs produce the same output 1 2 3 4.
E. Program A produces the output 4 3 2 1 and Program B runs infinitely.

Section 15.10 Recursion versus Iteration
15.20  Which of the following statements are true?
A. Recursive functions run faster than non-recursive functions.
B. Recursive functions usually take more memory space than non-recursive functions.
C. A recursive function can always be replaced by a non-recursive function.
D. In some cases, however, using recursion enables you to give a natural, straightforward, simple solution to a program that would otherwise be difficult to solve.

Section 15.11 Tail Recursion
15.21  Analyze the following functions;

def f1(n):
    if n == 0:
        return 0
    else:
        return n + f1(n - 1)

def f2(n, result):
    if n == 0:
        return result
    else:
        return f2(n - 1, n + result)

print(f1(3))
print(f2(30))
A. f1 is tail recursion, but f2 is not
B. f2 is tail recursion, but f1 is not
C. f1 and f2 are both tail recursive
D. Neither f1 nor f2 is tail recursive

15.22  Show the output of the following code:

def f2(n, result):
    if n == 0:
        return 0
    else:
        return f2(n - 1, n + result)

print(f2(20))
A. 0
B. 1
C. 2
D. 3