Quiz 5 Study Guide

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You are responsible for all material covered through the end of Week 11 (recursion).

In addition to all concepts from Quiz 4, you should understand the following:

Python concepts

Searching: linear search and binary search
Analysis of algorithms/big-O notation:
- \$O(1)\$ — constant time
- \$O(\log N)\$ — logarithmic (ex: binary search)
- \$O(N)\$ — linear (ex: linear search)
- \$O(N^2)\$ — quadratic (ex: bubble sort)
Sorting: bubble vs selection sort
Swapping two values in a list
Recursion
- base case
- recursive call
- recursion on numbers, strings, and lists

You should be able to

compare different runtime analyses for algorithms
design a recursive function
draw the call stack for a recursive function

Practice problems

What would the output of the following code be?

for i in range(2):
    for j in range (3):
        print("i:%d j:%d" % (i,j))

True/False questions:
- Linear search requires a number of steps proportional to the size of the list being searched (T/F)
- The python in operator performs a binary search (T/F)
- Binary search is an \$N^2\$ algorithm (T/F)
- The number of times N can be divided by 2 (before reaching 1) is \$\log_2 N\$ (T/F)
How many steps would it take to do binary search on a list of size 1 million, when the item you are searching for is NOT in the list?
How many steps would it take to do linear search on a list of size 1 million, when the item you are searching for is NOT in the list?
Binary search is much faster than linear search, so why don’t we use it for every search problem?

For each iteration of the loop in binarySearch(x, L), show the index values for low, high, and mid, and the value stored at location mid. What will be returned by this function call?

x = 67
L = [10, 12, 14, 21, 37, 44, 45, 46, 58, 67, 99]
      0   1   2   3   4   5   6   7   8   9  10

low | high | mid | L[mid]
- - - - - - - - - - - - -
    |      |     |
    |      |     |
    |      |     |
    |      |     |

For each iteration of the loop in binarySearch(y, L), show the index values for low, high, and mid, and the value stored at location mid. What will be returned by this function call?

y = 4
L = [10, 12, 14, 21, 37, 44, 45, 46, 58, 67, 99]
      0   1   2   3   4   5   6   7   8   9  10

low | high | mid | L[mid]
- - - - - - - - - - - - -
    |      |     |
    |      |     |
    |      |     |
    |      |     |

Write a function to return the index of the smallest item in a given list.

def indexOfSmallest(ls):
   """
   Inputs:  A list of values
   Returns: The index of the smallest item in the list, -1 if the list
   is empty
   Purpose: To find the location of the smallest item in the given list
   """
   # complete this function

How many steps are required for each of the following?

for i in range(N):
    for j in range(N):
        print(i, j)

while N > 1:
    print(N)
    N = N//2

for i in range(N):
    for j in range(10):
        print(i*j)

Which algorithm requires time directly proportional to the size of the input list?
- linear search
- binary search
- bubble sort
- selection sort
Consider the following function, oneLoop(L), which is similar to the inner loop from bubble sort:
```
def oneLoop(L):
   for j in range(len(L)-1):
      if L[j] > L[j+1]:
         tmp = L[j+1]
         L[j+1] = L[j]
         L[j] = tmp
```
Write an expression for the asymptotic runtime of oneLoop, in terms of the length \$N\$ of the list L.

Show how the following list L=[17,4,19,3,11,8] would be changed after selection sort does its first 3 swaps:

For concreteness, here is some Python code for selection sort:

def selectionSort(L):
    for j in range(len(L)):
        ios = j       # index of smallest item so far
        for i in range(j+1,len(L)):
            if L[i] < L[ios]:
                ios = i
        # swap them
        L[j],L[ios] = L[ios],L[j]

Write a recursive function longestWord(words) that returns the longest word in words, a list of strings. You can assume that words contains at least one word.

Then, draw the stack as it would look at its largest (i.e, the deepest part of the recursion when it hits a base case), when the function is called on the list ['awesome', 'recursive', 'code'].